1313 lines
36 KiB
Groff
1313 lines
36 KiB
Groff
.\" Copyright (c) 1999 Jeroen Ruigrok van der Werven
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd December 18, 2005
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.Dt ELF 5
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.Os
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.Sh NAME
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.Nm elf
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.Nd format of ELF executable binary files
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.Sh SYNOPSIS
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.In elf.h
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.Sh DESCRIPTION
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The header file
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.In elf.h
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defines the format of ELF executable binary files.
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Amongst these files are
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normal executable files, relocatable object files, core files and shared
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libraries.
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.Pp
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An executable file using the ELF file format consists of an ELF header,
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followed by a program header table or a section header table, or both.
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The ELF header is always at offset zero of the file.
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The program header
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table and the section header table's offset in the file are defined in the
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ELF header.
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The two tables describe the rest of the particularities of
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the file.
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.Pp
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Applications which wish to process ELF binary files for their native
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architecture only should include
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.In elf.h
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in their source code.
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These applications should need to refer to
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all the types and structures by their generic names
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.Dq Elf_xxx
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and to the macros by
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.Dq ELF_xxx .
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Applications written this way can be compiled on any architecture,
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regardless whether the host is 32-bit or 64-bit.
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.Pp
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Should an application need to process ELF files of an unknown
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architecture then the application needs to include both
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.In sys/elf32.h
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and
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.In sys/elf64.h
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instead of
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.In elf.h .
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Furthermore, all types and structures need to be identified by either
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.Dq Elf32_xxx
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or
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.Dq Elf64_xxx .
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The macros need to be identified by
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.Dq ELF32_xxx
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or
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.Dq ELF64_xxx .
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.Pp
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Whatever the system's architecture is, it will always include
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.In sys/elf_common.h
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as well as
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.In sys/elf_generic.h .
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.Pp
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These header files describe the above mentioned headers as C structures
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and also include structures for dynamic sections, relocation sections and
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symbol tables.
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.Pp
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The following types are being used for 32-bit architectures:
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.Bd -literal -offset indent
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Elf32_Addr Unsigned 32-bit program address
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Elf32_Half Unsigned 16-bit field
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Elf32_Lword Unsigned 64-bit field
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Elf32_Off Unsigned 32-bit file offset
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Elf32_Sword Signed 32-bit field or integer
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Elf32_Word Unsigned 32-bit field or integer
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.Ed
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.Pp
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For 64-bit architectures we have the following types:
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.Bd -literal -offset indent
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Elf64_Addr Unsigned 64-bit program address
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Elf64_Half Unsigned 16-bit field
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Elf64_Lword Unsigned 64-bit field
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Elf64_Off Unsigned 64-bit file offset
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Elf64_Sword Signed 32-bit field
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Elf64_Sxword Signed 64-bit field or integer
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Elf64_Word Unsigned 32-bit field
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Elf64_Xword Unsigned 64-bit field or integer
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.Ed
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.Pp
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All data structures that the file format defines follow the
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.Dq natural
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size and alignment guidelines for the relevant class.
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If necessary,
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data structures contain explicit padding to ensure 4-byte alignment
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for 4-byte objects, to force structure sizes to a multiple of 4, etc.
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.Pp
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The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
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.Bd -literal -offset indent
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typedef struct {
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unsigned char e_ident[EI_NIDENT];
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Elf32_Half e_type;
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Elf32_Half e_machine;
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Elf32_Word e_version;
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Elf32_Addr e_entry;
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Elf32_Off e_phoff;
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Elf32_Off e_shoff;
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Elf32_Word e_flags;
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Elf32_Half e_ehsize;
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Elf32_Half e_phentsize;
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Elf32_Half e_phnum;
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Elf32_Half e_shentsize;
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Elf32_Half e_shnum;
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Elf32_Half e_shstrndx;
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} Elf32_Ehdr;
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.Ed
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.Bd -literal -offset indent
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typedef struct {
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unsigned char e_ident[EI_NIDENT];
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Elf64_Half e_type;
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Elf64_Half e_machine;
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Elf64_Word e_version;
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Elf64_Addr e_entry;
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Elf64_Off e_phoff;
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Elf64_Off e_shoff;
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Elf64_Word e_flags;
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Elf64_Half e_ehsize;
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Elf64_Half e_phentsize;
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Elf64_Half e_phnum;
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Elf64_Half e_shentsize;
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Elf64_Half e_shnum;
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Elf64_Half e_shstrndx;
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} Elf64_Ehdr;
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.Ed
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.Pp
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The fields have the following meanings:
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.Pp
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.Bl -tag -width "e_phentsize" -compact -offset indent
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.It Dv e_ident
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This array of bytes specifies to interpret the file,
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independent of the processor or the file's remaining contents.
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Within this array everything is named by macros, which start with
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the prefix
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.Sy EI_
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and may contain values which start with the prefix
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.Sy ELF .
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The following macros are defined:
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.Pp
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.Bl -tag -width "EI_ABIVERSION" -compact
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.It Dv EI_MAG0
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The first byte of the magic number.
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It must be filled with
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.Sy ELFMAG0 .
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.It Dv EI_MAG1
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The second byte of the magic number.
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It must be filled with
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.Sy ELFMAG1 .
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.It Dv EI_MAG2
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The third byte of the magic number.
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It must be filled with
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.Sy ELFMAG2 .
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.It Dv EI_MAG3
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The fourth byte of the magic number.
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It must be filled with
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.Sy ELFMAG3 .
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.It Dv EI_CLASS
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The fifth byte identifies the architecture for this binary:
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.Pp
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.Bl -tag -width "ELFCLASSNONE" -compact
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.It Dv ELFCLASSNONE
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This class is invalid.
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.It Dv ELFCLASS32
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This defines the 32-bit architecture.
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It supports machines with files
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and virtual address spaces up to 4 Gigabytes.
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.It Dv ELFCLASS64
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This defines the 64-bit architecture.
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.El
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.It Dv EI_DATA
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The sixth byte specifies the data encoding of the processor-specific
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data in the file.
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Currently these encodings are supported:
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.Pp
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.Bl -tag -width "ELFDATA2LSB" -compact
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.It Dv ELFDATANONE
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Unknown data format.
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.It Dv ELFDATA2LSB
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Two's complement, little-endian.
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.It Dv ELFDATA2MSB
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Two's complement, big-endian.
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.El
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.It Dv EI_VERSION
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The version number of the ELF specification:
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.Pp
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.Bl -tag -width "EV_CURRENT" -compact
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.It Dv EV_NONE
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Invalid version.
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.It Dv EV_CURRENT
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Current version.
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.El
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.It Dv EI_OSABI
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This byte identifies the operating system
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and ABI to which the object is targeted.
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Some fields in other ELF structures have flags
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and values that have platform specific meanings;
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the interpretation of those fields is determined by the value of this byte.
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The following values are currently defined:
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.Pp
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.Bl -tag -width "ELFOSABI_STANDALONE" -compact
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.It Dv ELFOSABI_SYSV
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UNIX System V ABI.
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.It Dv ELFOSABI_HPUX
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HP-UX operating system ABI.
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.It Dv ELFOSABI_NETBSD
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.Nx
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operating system ABI.
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.It Dv ELFOSABI_LINUX
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GNU/Linux operating system ABI.
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.It Dv ELFOSABI_HURD
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GNU/Hurd operating system ABI.
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.It Dv ELFOSABI_86OPEN
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86Open Common IA32 ABI.
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.It Dv ELFOSABI_SOLARIS
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Solaris operating system ABI.
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.It Dv ELFOSABI_MONTEREY
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Monterey project ABI.
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.It Dv ELFOSABI_IRIX
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IRIX operating system ABI.
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.It Dv ELFOSABI_FREEBSD
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.Fx
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operating system ABI.
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.It Dv ELFOSABI_TRU64
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TRU64 UNIX operating system ABI.
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.It Dv ELFOSABI_ARM
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ARM architecture ABI.
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.It Dv ELFOSABI_STANDALONE
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Standalone (embedded) ABI.
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.El
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.It Dv EI_ABIVERSION
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This byte identifies the version of the ABI
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to which the object is targeted.
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This field is used to distinguish among incompatible versions of an ABI.
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The interpretation of this version number
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is dependent on the ABI identified by the EI_OSABI field.
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Applications conforming to this specification use the value 0.
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.It Dv EI_PAD
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Start of padding.
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These bytes are reserved and set to zero.
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Programs
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which read them should ignore them.
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The value for EI_PAD will change in
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the future if currently unused bytes are given meanings.
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.It Dv EI_BRAND
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Start of architecture identification.
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.It Dv EI_NIDENT
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The size of the e_ident array.
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.El
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.Pp
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.It Dv e_type
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This member of the structure identifies the object file type:
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.Pp
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.Bl -tag -width "ET_NONE" -compact
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.It Dv ET_NONE
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An unknown type.
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.It Dv ET_REL
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A relocatable file.
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.It Dv ET_EXEC
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An executable file.
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.It Dv ET_DYN
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A shared object.
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.It Dv ET_CORE
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A core file.
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.El
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.Pp
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.It Dv e_machine
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This member specifies the required architecture for an individual file:
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.Pp
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.Bl -tag -width "EM_MIPS_RS4_BE" -compact
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.It Dv EM_NONE
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An unknown machine.
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.It Dv EM_M32
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AT&T WE 32100.
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.It Dv EM_SPARC
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Sun Microsystems SPARC.
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.It Dv EM_386
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Intel 80386.
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.It Dv EM_68K
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Motorola 68000.
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.It Dv EM_88K
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Motorola 88000.
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.It Dv EM_486
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Intel 80486.
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.It Dv EM_860
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Intel 80860.
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.It Dv EM_MIPS
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MIPS RS3000 (big-endian only).
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.It Dv EM_MIPS_RS4_BE
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MIPS RS4000 (big-endian only).
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.It Dv EM_SPARC64
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SPARC v9 64-bit unofficial.
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.It Dv EM_PARISC
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HPPA.
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.It Dv EM_PPC
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PowerPC.
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.It Dv EM_ALPHA
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Compaq [DEC] Alpha.
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.El
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.Pp
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.It Dv e_version
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This member identifies the file version:
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.Pp
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.Bl -tag -width "EV_CURRENT" -compact
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.It Dv EV_NONE
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Invalid version
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.It Dv EV_CURRENT
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Current version
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.El
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.It Dv e_entry
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This member gives the virtual address to which the system first transfers
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control, thus starting the process.
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If the file has no associated entry
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point, this member holds zero.
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.It Dv e_phoff
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This member holds the program header table's file offset in bytes.
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If
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the file has no program header table, this member holds zero.
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.It Dv e_shoff
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This member holds the section header table's file offset in bytes.
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If the
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file has no section header table this member holds zero.
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.It Dv e_flags
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This member holds processor-specific flags associated with the file.
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Flag
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names take the form EF_`machine_flag'.
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Currently no flags have been defined.
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.It Dv e_ehsize
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This member holds the ELF header's size in bytes.
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.It Dv e_phentsize
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This member holds the size in bytes of one entry in the file's program header
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table; all entries are the same size.
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.It Dv e_phnum
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This member holds the number of entries in the program header
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table.
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If the file is using extended program header numbering, then the
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.Sy e_phnum
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member will contain the value
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.Dv PN_XNUM
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and the actual number of program header table entries will be stored
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in the
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.Sy sh_info
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member of the section header at index
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.Dv SHN_UNDEF .
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The product of
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.Sy e_phentsize
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and the number of program header table entries gives the program
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header table's size in bytes.
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If a file has no program header,
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.Sy e_phnum
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holds the value zero.
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.It Dv e_shentsize
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This member holds a sections header's size in bytes.
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A section header is one
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entry in the section header table; all entries are the same size.
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.It Dv e_shnum
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This member holds the number of entries in the section header table.
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If the file is using extended section numbering, then the
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.Sy e_shnum
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member will be zero and the actual section number will be stored in the
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.Sy sh_size
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member of the section header at index
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.Dv SHN_UNDEF .
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If a file has no section header table, both the
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.Sy e_shnum
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and the
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.Sy e_shoff
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fields of the ELF header will be zero.
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The product of
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.Sy e_shentsize
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and the number of sections in the file gives the section header
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table's size in bytes.
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.It Dv e_shstrndx
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This member holds the section header table index of the entry associated
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with the section name string table.
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If extended section numbering is being used, this field will hold the
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value
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.Sy SHN_XINDEX ,
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and the actual section header table index will be present in the
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.Sy sh_link
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field of the section header entry at index
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.Dv SHN_UNDEF .
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If the file has no section name string
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table, this member holds the value
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.Sy SHN_UNDEF .
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.El
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.Pp
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An executable or shared object file's program header table is an array of
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structures, each describing a segment or other information the system needs
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to prepare the program for execution.
|
|
An object file
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.Em segment
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contains one or more
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.Em sections .
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|
Program headers are meaningful only for executable and shared object files.
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A file specifies its own program header size with the ELF header's
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.Sy e_phentsize
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and
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.Sy e_phnum
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members.
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As with the Elf executable header, the program header
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also has different versions depending on the architecture:
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Word p_type;
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Elf32_Off p_offset;
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Elf32_Addr p_vaddr;
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Elf32_Addr p_paddr;
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Elf32_Word p_filesz;
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Elf32_Word p_memsz;
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Elf32_Word p_flags;
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Elf32_Word p_align;
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} Elf32_Phdr;
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|
.Ed
|
|
.Bd -literal -offset indent
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|
typedef struct {
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Elf64_Word p_type;
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Elf64_Word p_flags;
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Elf64_Off p_offset;
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Elf64_Addr p_vaddr;
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Elf64_Addr p_paddr;
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|
Elf64_Xword p_filesz;
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|
Elf64_Xword p_memsz;
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|
Elf64_Xword p_align;
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} Elf64_Phdr;
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|
.Ed
|
|
.Pp
|
|
The main difference between the 32-bit and the 64-bit program header lies
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|
only in the location of a
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.Sy p_flags
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|
member in the total struct.
|
|
.Pp
|
|
.Bl -tag -width "p_offset" -compact -offset indent
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|
.It Dv p_type
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|
This member of the Phdr struct tells what kind of segment this array
|
|
element describes or how to interpret the array element's information.
|
|
.Pp
|
|
.Bl -tag -width "PT_DYNAMIC" -compact
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|
.It Dv PT_NULL
|
|
The array element is unused and the other members' values are undefined.
|
|
This lets the program header have ignored entries.
|
|
.It Dv PT_LOAD
|
|
The array element specifies a loadable segment, described by
|
|
.Sy p_filesz
|
|
and
|
|
.Sy p_memsz .
|
|
The bytes from the file are mapped to the beginning of the memory
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|
segment.
|
|
If the segment's memory size
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|
.Pq Sy p_memsz
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|
is larger than the file size
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|
.Pq Sy p_filesz ,
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|
the
|
|
.Dq extra
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|
bytes are defined to hold the value 0 and to follow the segment's
|
|
initialized area.
|
|
The file size may not be larger than the memory size.
|
|
Loadable segment entries in the program header table appear in ascending
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|
order, sorted on the
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|
.Sy p_vaddr
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|
member.
|
|
.It Dv PT_DYNAMIC
|
|
The array element specifies dynamic linking information.
|
|
.It Dv PT_INTERP
|
|
The array element specifies the location and size of a null-terminated
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|
path name to invoke as an interpreter.
|
|
This segment type is meaningful
|
|
only for executable files (though it may occur for shared objects).
|
|
However
|
|
it may not occur more than once in a file.
|
|
If it is present it must precede
|
|
any loadable segment entry.
|
|
.It Dv PT_NOTE
|
|
The array element specifies the location and size for auxiliary information.
|
|
.It Dv PT_SHLIB
|
|
This segment type is reserved but has unspecified semantics.
|
|
Programs that
|
|
contain an array element of this type do not conform to the ABI.
|
|
.It Dv PT_PHDR
|
|
The array element, if present, specifies the location and size of the program
|
|
header table itself, both in the file and in the memory image of the program.
|
|
This segment type may not occur more than once in a file.
|
|
Moreover, it may
|
|
only occur if the program header table is part of the memory image of the
|
|
program.
|
|
If it is present it must precede any loadable segment entry.
|
|
.It Dv PT_LOPROC
|
|
This value up to and including
|
|
.Sy PT_HIPROC
|
|
are reserved for processor-specific semantics.
|
|
.It Dv PT_HIPROC
|
|
This value down to and including
|
|
.Sy PT_LOPROC
|
|
are reserved for processor-specific semantics.
|
|
.El
|
|
.Pp
|
|
.It Dv p_offset
|
|
This member holds the offset from the beginning of the file at which
|
|
the first byte of the segment resides.
|
|
.It Dv p_vaddr
|
|
This member holds the virtual address at which the first byte of the
|
|
segment resides in memory.
|
|
.It Dv p_paddr
|
|
On systems for which physical addressing is relevant, this member is
|
|
reserved for the segment's physical address.
|
|
Under
|
|
.Bx
|
|
this member is
|
|
not used and must be zero.
|
|
.It Dv p_filesz
|
|
This member holds the number of bytes in the file image of the segment.
|
|
It may be zero.
|
|
.It Dv p_memsz
|
|
This member holds the number of bytes in the memory image of the segment.
|
|
It may be zero.
|
|
.It Dv p_flags
|
|
This member holds flags relevant to the segment:
|
|
.Pp
|
|
.Bl -tag -width "PF_X" -compact
|
|
.It Dv PF_X
|
|
An executable segment.
|
|
.It Dv PF_W
|
|
A writable segment.
|
|
.It Dv PF_R
|
|
A readable segment.
|
|
.El
|
|
.Pp
|
|
A text segment commonly has the flags
|
|
.Sy PF_X
|
|
and
|
|
.Sy PF_R .
|
|
A data segment commonly has
|
|
.Sy PF_X ,
|
|
.Sy PF_W
|
|
and
|
|
.Sy PF_R .
|
|
.It Dv p_align
|
|
This member holds the value to which the segments are aligned in memory
|
|
and in the file.
|
|
Loadable process segments must have congruent values for
|
|
.Sy p_vaddr
|
|
and
|
|
.Sy p_offset ,
|
|
modulo the page size.
|
|
Values of zero and one mean no alignment is required.
|
|
Otherwise,
|
|
.Sy p_align
|
|
should be a positive, integral power of two, and
|
|
.Sy p_vaddr
|
|
should equal
|
|
.Sy p_offset ,
|
|
modulo
|
|
.Sy p_align .
|
|
.El
|
|
.Pp
|
|
An file's section header table lets one locate all the file's sections.
|
|
The
|
|
section header table is an array of Elf32_Shdr or Elf64_Shdr structures.
|
|
The
|
|
ELF header's
|
|
.Sy e_shoff
|
|
member gives the byte offset from the beginning of the file to the section
|
|
header table.
|
|
.Sy e_shnum
|
|
holds the number of entries the section header table contains.
|
|
.Sy e_shentsize
|
|
holds the size in bytes of each entry.
|
|
.Pp
|
|
A section header table index is a subscript into this array.
|
|
Some section
|
|
header table indices are reserved.
|
|
An object file does not have sections for
|
|
these special indices:
|
|
.Pp
|
|
.Bl -tag -width "SHN_LORESERVE" -compact
|
|
.It Dv SHN_UNDEF
|
|
This value marks an undefined, missing, irrelevant, or otherwise meaningless
|
|
section reference.
|
|
For example, a symbol
|
|
.Dq defined
|
|
relative to section number
|
|
.Sy SHN_UNDEF
|
|
is an undefined symbol.
|
|
.It Dv SHN_LORESERVE
|
|
This value specifies the lower bound of the range of reserved indices.
|
|
.It Dv SHN_LOPROC
|
|
This value up to and including
|
|
.Sy SHN_HIPROC
|
|
are reserved for processor-specific semantics.
|
|
.It Dv SHN_HIPROC
|
|
This value down to and including
|
|
.Sy SHN_LOPROC
|
|
are reserved for processor-specific semantics.
|
|
.It Dv SHN_ABS
|
|
This value specifies absolute values for the corresponding reference.
|
|
For
|
|
example, symbols defined relative to section number
|
|
.Sy SHN_ABS
|
|
have absolute values and are not affected by relocation.
|
|
.It Dv SHN_COMMON
|
|
Symbols defined relative to this section are common symbols, such as FORTRAN
|
|
COMMON or unallocated C external variables.
|
|
.It Dv SHN_HIRESERVE
|
|
This value specifies the upper bound of the range of reserved indices.
|
|
The
|
|
system reserves indices between
|
|
.Sy SHN_LORESERVE
|
|
and
|
|
.Sy SHN_HIRESERVE ,
|
|
inclusive.
|
|
The section header table does not contain entries for the
|
|
reserved indices.
|
|
.El
|
|
.Pp
|
|
The section header has the following structure:
|
|
.Bd -literal -offset indent
|
|
typedef struct {
|
|
Elf32_Word sh_name;
|
|
Elf32_Word sh_type;
|
|
Elf32_Word sh_flags;
|
|
Elf32_Addr sh_addr;
|
|
Elf32_Off sh_offset;
|
|
Elf32_Word sh_size;
|
|
Elf32_Word sh_link;
|
|
Elf32_Word sh_info;
|
|
Elf32_Word sh_addralign;
|
|
Elf32_Word sh_entsize;
|
|
} Elf32_Shdr;
|
|
.Ed
|
|
.Bd -literal -offset indent
|
|
typedef struct {
|
|
Elf64_Word sh_name;
|
|
Elf64_Word sh_type;
|
|
Elf64_Xword sh_flags;
|
|
Elf64_Addr sh_addr;
|
|
Elf64_Off sh_offset;
|
|
Elf64_Xword sh_size;
|
|
Elf64_Word sh_link;
|
|
Elf64_Word sh_info;
|
|
Elf64_Xword sh_addralign;
|
|
Elf64_Xword sh_entsize;
|
|
} Elf64_Shdr;
|
|
.Ed
|
|
.Pp
|
|
.Bl -tag -width "sh_addralign" -compact
|
|
.It Dv sh_name
|
|
This member specifies the name of the section.
|
|
Its value is an index
|
|
into the section header string table section, giving the location of
|
|
a null-terminated string.
|
|
.It Dv sh_type
|
|
This member categorizes the section's contents and semantics.
|
|
.Pp
|
|
.Bl -tag -width "SHT_PROGBITS" -compact
|
|
.It Dv SHT_NULL
|
|
This value marks the section header as inactive.
|
|
It does not
|
|
have an associated section.
|
|
Other members of the section header
|
|
have undefined values.
|
|
.It Dv SHT_PROGBITS
|
|
The section holds information defined by the program, whose
|
|
format and meaning are determined solely by the program.
|
|
.It Dv SHT_SYMTAB
|
|
This section holds a symbol table.
|
|
Typically,
|
|
.Sy SHT_SYMTAB
|
|
provides symbols for link editing, though it may also be used
|
|
for dynamic linking.
|
|
As a complete symbol table, it may contain
|
|
many symbols unnecessary for dynamic linking.
|
|
An object file can
|
|
also contain a
|
|
.Sy SHN_DYNSYM
|
|
section.
|
|
.It Dv SHT_STRTAB
|
|
This section holds a string table.
|
|
An object file may have multiple
|
|
string table sections.
|
|
.It Dv SHT_RELA
|
|
This section holds relocation entries with explicit addends, such
|
|
as type
|
|
.Sy Elf32_Rela
|
|
for the 32-bit class of object files.
|
|
An object may have multiple
|
|
relocation sections.
|
|
.It Dv SHT_HASH
|
|
This section holds a symbol hash table.
|
|
All object participating in
|
|
dynamic linking must contain a symbol hash table.
|
|
An object file may
|
|
have only one hash table.
|
|
.It Dv SHT_DYNAMIC
|
|
This section holds information for dynamic linking.
|
|
An object file may
|
|
have only one dynamic section.
|
|
.It Dv SHT_NOTE
|
|
This section holds information that marks the file in some way.
|
|
.It Dv SHT_NOBITS
|
|
A section of this type occupies no space in the file but otherwise
|
|
resembles
|
|
.Sy SHN_PROGBITS .
|
|
Although this section contains no bytes, the
|
|
.Sy sh_offset
|
|
member contains the conceptual file offset.
|
|
.It Dv SHT_REL
|
|
This section holds relocation offsets without explicit addends, such
|
|
as type
|
|
.Sy Elf32_Rel
|
|
for the 32-bit class of object files.
|
|
An object file may have multiple
|
|
relocation sections.
|
|
.It Dv SHT_SHLIB
|
|
This section is reserved but has unspecified semantics.
|
|
.It Dv SHT_DYNSYM
|
|
This section holds a minimal set of dynamic linking symbols.
|
|
An
|
|
object file can also contain a
|
|
.Sy SHN_SYMTAB
|
|
section.
|
|
.It Dv SHT_LOPROC
|
|
This value up to and including
|
|
.Sy SHT_HIPROC
|
|
are reserved for processor-specific semantics.
|
|
.It Dv SHT_HIPROC
|
|
This value down to and including
|
|
.Sy SHT_LOPROC
|
|
are reserved for processor-specific semantics.
|
|
.It Dv SHT_LOUSER
|
|
This value specifies the lower bound of the range of indices reserved for
|
|
application programs.
|
|
.It Dv SHT_HIUSER
|
|
This value specifies the upper bound of the range of indices reserved for
|
|
application programs.
|
|
Section types between
|
|
.Sy SHT_LOUSER
|
|
and
|
|
.Sy SHT_HIUSER
|
|
may be used by the application, without conflicting with current or future
|
|
system-defined section types.
|
|
.El
|
|
.Pp
|
|
.It Dv sh_flags
|
|
Sections support one-bit flags that describe miscellaneous attributes.
|
|
If a flag bit is set in
|
|
.Sy sh_flags ,
|
|
the attribute is
|
|
.Dq on
|
|
for the section.
|
|
Otherwise, the attribute is
|
|
.Dq off
|
|
or does not apply.
|
|
Undefined attributes are set to zero.
|
|
.Pp
|
|
.Bl -tag -width "SHF_EXECINSTR" -compact
|
|
.It Dv SHF_WRITE
|
|
This section contains data that should be writable during process
|
|
execution.
|
|
.It Dv SHF_ALLOC
|
|
The section occupies memory during process execution.
|
|
Some control
|
|
sections do not reside in the memory image of an object file.
|
|
This
|
|
attribute is off for those sections.
|
|
.It Dv SHF_EXECINSTR
|
|
The section contains executable machine instructions.
|
|
.It Dv SHF_MASKPROC
|
|
All bits included in this mask are reserved for processor-specific
|
|
semantics.
|
|
.El
|
|
.Pp
|
|
.It Dv sh_addr
|
|
If the section will appear in the memory image of a process, this member
|
|
holds the address at which the section's first byte should reside.
|
|
Otherwise, the member contains zero.
|
|
.It Dv sh_offset
|
|
This member's value holds the byte offset from the beginning of the file
|
|
to the first byte in the section.
|
|
One section type,
|
|
.Sy SHT_NOBITS ,
|
|
occupies no space in the file, and its
|
|
.Sy sh_offset
|
|
member locates the conceptual placement in the file.
|
|
.It Dv sh_size
|
|
This member holds the section's size in bytes.
|
|
Unless the section type
|
|
is
|
|
.Sy SHT_NOBITS ,
|
|
the section occupies
|
|
.Sy sh_size
|
|
bytes in the file.
|
|
A section of type
|
|
.Sy SHT_NOBITS
|
|
may have a non-zero size, but it occupies no space in the file.
|
|
.It Dv sh_link
|
|
This member holds a section header table index link, whose interpretation
|
|
depends on the section type.
|
|
.It Dv sh_info
|
|
This member holds extra information, whose interpretation depends on the
|
|
section type.
|
|
.It Dv sh_addralign
|
|
Some sections have address alignment constraints.
|
|
If a section holds a
|
|
doubleword, the system must ensure doubleword alignment for the entire
|
|
section.
|
|
That is, the value of
|
|
.Sy sh_addr
|
|
must be congruent to zero, modulo the value of
|
|
.Sy sh_addralign .
|
|
Only zero and positive integral powers of two are allowed.
|
|
Values of zero
|
|
or one mean the section has no alignment constraints.
|
|
.It Dv sh_entsize
|
|
Some sections hold a table of fixed-sized entries, such as a symbol table.
|
|
For such a section, this member gives the size in bytes for each entry.
|
|
This member contains zero if the section does not hold a table of
|
|
fixed-size entries.
|
|
.El
|
|
.Pp
|
|
Various sections hold program and control information:
|
|
.Bl -tag -width ".shstrtab" -compact
|
|
.It .bss
|
|
(Block Started by Symbol)
|
|
This section holds uninitialized data that contributes to the program's
|
|
memory image.
|
|
By definition, the system initializes the data with zeros
|
|
when the program begins to run.
|
|
This section is of type
|
|
.Sy SHT_NOBITS .
|
|
The attributes types are
|
|
.Sy SHF_ALLOC
|
|
and
|
|
.Sy SHF_WRITE .
|
|
.It .comment
|
|
This section holds version control information.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
No attribute types are used.
|
|
.It .data
|
|
This section holds initialized data that contribute to the program's
|
|
memory image.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attribute types are
|
|
.Sy SHF_ALLOC
|
|
and
|
|
.Sy SHF_WRITE .
|
|
.It .data1
|
|
This section holds initialized data that contribute to the program's
|
|
memory image.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attribute types are
|
|
.Sy SHF_ALLOC
|
|
and
|
|
.Sy SHF_WRITE .
|
|
.It .debug
|
|
This section holds information for symbolic debugging.
|
|
The contents
|
|
are unspecified.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
No attribute types are used.
|
|
.It .dynamic
|
|
This section holds dynamic linking information.
|
|
The section's attributes
|
|
will include the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Whether the
|
|
.Sy SHF_WRITE
|
|
bit is set is processor-specific.
|
|
This section is of type
|
|
.Sy SHT_DYNAMIC .
|
|
See the attributes above.
|
|
.It .dynstr
|
|
This section holds strings needed for dynamic linking, most commonly
|
|
the strings that represent the names associated with symbol table entries.
|
|
This section is of type
|
|
.Sy SHT_STRTAB .
|
|
The attribute type used is
|
|
.Sy SHF_ALLOC .
|
|
.It .dynsym
|
|
This section holds the dynamic linking symbol table.
|
|
This section is of type
|
|
.Sy SHT_DYNSYM .
|
|
The attribute used is
|
|
.Sy SHF_ALLOC .
|
|
.It .fini
|
|
This section holds executable instructions that contribute to the process
|
|
termination code.
|
|
When a program exits normally the system arranges to
|
|
execute the code in this section.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attributes used are
|
|
.Sy SHF_ALLOC
|
|
and
|
|
.Sy SHF_EXECINSTR .
|
|
.It .got
|
|
This section holds the global offset table.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attributes are processor-specific.
|
|
.It .hash
|
|
This section holds a symbol hash table.
|
|
This section is of type
|
|
.Sy SHT_HASH .
|
|
The attribute used is
|
|
.Sy SHF_ALLOC .
|
|
.It .init
|
|
This section holds executable instructions that contribute to the process
|
|
initialization code.
|
|
When a program starts to run the system arranges to
|
|
execute the code in this section before calling the main program entry point.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attributes used are
|
|
.Sy SHF_ALLOC
|
|
and
|
|
.Sy SHF_EXECINSTR .
|
|
.It .interp
|
|
This section holds the pathname of a program interpreter.
|
|
If the file has
|
|
a loadable segment that includes the section, the section's attributes will
|
|
include the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Otherwise, that bit will be off.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
.It .line
|
|
This section holds line number information for symbolic debugging, which
|
|
describes the correspondence between the program source and the machine code.
|
|
The contents are unspecified.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
No attribute types are used.
|
|
.It .note
|
|
This section holds information in the
|
|
.Dq Note Section
|
|
format described below.
|
|
This section is of type
|
|
.Sy SHT_NOTE .
|
|
No attribute types are used.
|
|
.It .plt
|
|
This section holds the procedure linkage table.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attributes are processor-specific.
|
|
.It .relNAME
|
|
This section holds relocation information as described below.
|
|
If the file
|
|
has a loadable segment that includes relocation, the section's attributes
|
|
will include the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Otherwise the bit will be off.
|
|
By convention,
|
|
.Dq NAME
|
|
is supplied by the section to which the relocations apply.
|
|
Thus a relocation
|
|
section for
|
|
.Sy .text
|
|
normally would have the name
|
|
.Sy .rel.text .
|
|
This section is of type
|
|
.Sy SHT_REL .
|
|
.It .relaNAME
|
|
This section holds relocation information as described below.
|
|
If the file
|
|
has a loadable segment that includes relocation, the section's attributes
|
|
will include the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Otherwise the bit will be off.
|
|
By convention,
|
|
.Dq NAME
|
|
is supplied by the section to which the relocations apply.
|
|
Thus a relocation
|
|
section for
|
|
.Sy .text
|
|
normally would have the name
|
|
.Sy .rela.text .
|
|
This section is of type
|
|
.Sy SHT_RELA .
|
|
.It .rodata
|
|
This section holds read-only data that typically contributes to a
|
|
non-writable segment in the process image.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attribute used is
|
|
.Sy SHF_ALLOC .
|
|
.It .rodata1
|
|
This section hold read-only data that typically contributes to a
|
|
non-writable segment in the process image.
|
|
This section is of type
|
|
.Sy SHT_PROGBITS .
|
|
The attribute used is
|
|
.Sy SHF_ALLOC .
|
|
.It .shstrtab
|
|
This section holds section names.
|
|
This section is of type
|
|
.Sy SHT_STRTAB .
|
|
No attribute types are used.
|
|
.It .strtab
|
|
This section holds strings, most commonly the strings that represent the
|
|
names associated with symbol table entries.
|
|
If the file has a loadable
|
|
segment that includes the symbol string table, the section's attributes
|
|
will include the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Otherwise the bit will be off.
|
|
This section is of type
|
|
.Sy SHT_STRTAB .
|
|
.It .symtab
|
|
This section holds a symbol table.
|
|
If the file has a loadable segment
|
|
that includes the symbol table, the section's attributes will include
|
|
the
|
|
.Sy SHF_ALLOC
|
|
bit.
|
|
Otherwise the bit will be off.
|
|
This section is of type
|
|
.Sy SHT_SYMTAB .
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.It .text
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This section holds the
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.Dq text ,
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or executable instructions, of a program.
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This section is of type
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.Sy SHT_PROGBITS .
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The attributes used are
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.Sy SHF_ALLOC
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and
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.Sy SHF_EXECINSTR .
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.It .jcr
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This section holds information about Java classes that must
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be registered.
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.It .eh_frame
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This section holds information used for C++ exception-handling.
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.El
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.Pp
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String table sections hold null-terminated character sequences, commonly
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called strings.
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The object file uses these strings to represent symbol
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and section names.
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One references a string as an index into the string
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table section.
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The first byte, which is index zero, is defined to hold
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a null character.
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Similarly, a string table's last byte is defined to
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hold a null character, ensuring null termination for all strings.
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.Pp
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An object file's symbol table holds information needed to locate and
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relocate a program's symbolic definitions and references.
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A symbol table
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index is a subscript into this array.
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Word st_name;
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Elf32_Addr st_value;
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Elf32_Word st_size;
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unsigned char st_info;
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unsigned char st_other;
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Elf32_Half st_shndx;
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} Elf32_Sym;
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.Ed
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.Bd -literal -offset indent
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typedef struct {
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Elf64_Word st_name;
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unsigned char st_info;
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unsigned char st_other;
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Elf64_Half st_shndx;
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Elf64_Addr st_value;
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Elf64_Xword st_size;
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} Elf64_Sym;
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.Ed
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.Pp
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.Bl -tag -width "st_value" -compact
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.It Dv st_name
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This member holds an index into the object file's symbol string table,
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which holds character representations of the symbol names.
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If the value
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is non-zero, it represents a string table index that gives the symbol
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name.
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Otherwise, the symbol table has no name.
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.It Dv st_value
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This member gives the value of the associated symbol.
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.It Dv st_size
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Many symbols have associated sizes.
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This member holds zero if the symbol
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has no size or an unknown size.
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.It Dv st_info
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This member specifies the symbol's type and binding attributes:
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.Pp
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.Bl -tag -width "STT_SECTION" -compact
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.It Dv STT_NOTYPE
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The symbol's type is not defined.
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.It Dv STT_OBJECT
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The symbol is associated with a data object.
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.It Dv STT_FUNC
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The symbol is associated with a function or other executable code.
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.It Dv STT_SECTION
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The symbol is associated with a section.
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Symbol table entries of
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this type exist primarily for relocation and normally have
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.Sy STB_LOCAL
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bindings.
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.It Dv STT_FILE
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By convention the symbol's name gives the name of the source file
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associated with the object file.
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A file symbol has
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.Sy STB_LOCAL
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bindings, its section index is
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.Sy SHN_ABS ,
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and it precedes the other
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.Sy STB_LOCAL
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symbols of the file, if it is present.
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.It Dv STT_LOPROC
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This value up to and including
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.Sy STT_HIPROC
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are reserved for processor-specific semantics.
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.It Dv STT_HIPROC
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This value down to and including
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.Sy STT_LOPROC
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are reserved for processor-specific semantics.
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.El
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.Pp
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.Bl -tag -width "STB_GLOBAL" -compact
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.It Dv STB_LOCAL
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Local symbols are not visible outside the object file containing their
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definition.
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Local symbols of the same name may exist in multiple file
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without interfering with each other.
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.It Dv STB_GLOBAL
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Global symbols are visible to all object files being combined.
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One file's
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definition of a global symbol will satisfy another file's undefined
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reference to the same symbol.
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.It Dv STB_WEAK
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Weak symbols resemble global symbols, but their definitions have lower
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precedence.
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.It Dv STB_LOPROC
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This value up to and including
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.Sy STB_HIPROC
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are reserved for processor-specific semantics.
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.It Dv STB_HIPROC
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This value down to and including
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.Sy STB_LOPROC
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are reserved for processor-specific semantics.
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.Pp
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There are macros for packing and unpacking the binding and type fields:
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.Pp
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.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact
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.It Xo
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.Fn ELF32_ST_BIND info
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.Xc
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or
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.Fn ELF64_ST_BIND info
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extract a binding from an st_info value.
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.It Xo
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.Fn ELF64_ST_TYPE info
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.Xc
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or
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.Fn ELF32_ST_TYPE info
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extract a type from an st_info value.
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.It Xo
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.Fn ELF32_ST_INFO bind type
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.Xc
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or
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.Fn ELF64_ST_INFO bind type
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convert a binding and a type into an st_info value.
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.El
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.El
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.Pp
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.It Dv st_other
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This member currently holds zero and has no defined meaning.
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.It Dv st_shndx
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Every symbol table entry is
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.Dq defined
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in relation to some section.
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This member holds the relevant section
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header table index.
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.El
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.Pp
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Relocation is the process of connecting symbolic references with
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symbolic definitions.
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Relocatable files must have information that
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describes how to modify their section contents, thus allowing executable
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and shared object files to hold the right information for a process'
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program image.
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Relocation entries are these data.
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.Pp
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Relocation structures that do not need an addend:
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Addr r_offset;
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Elf32_Word r_info;
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} Elf32_Rel;
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.Ed
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.Bd -literal -offset indent
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typedef struct {
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Elf64_Addr r_offset;
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Elf64_Xword r_info;
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} Elf64_Rel;
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.Ed
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.Pp
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Relocation structures that need an addend:
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.Bd -literal -offset indent
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typedef struct {
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Elf32_Addr r_offset;
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Elf32_Word r_info;
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Elf32_Sword r_addend;
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} Elf32_Rela;
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.Ed
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.Bd -literal -offset indent
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typedef struct {
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Elf64_Addr r_offset;
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Elf64_Xword r_info;
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Elf64_Sxword r_addend;
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} Elf64_Rela;
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.Ed
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.Pp
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.Bl -tag -width "r_offset" -compact
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.It Dv r_offset
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This member gives the location at which to apply the relocation action.
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For a relocatable file, the value is the byte offset from the beginning
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of the section to the storage unit affected by the relocation.
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For an
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executable file or shared object, the value is the virtual address of
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the storage unit affected by the relocation.
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.It Dv r_info
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This member gives both the symbol table index with respect to which the
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relocation must be made and the type of relocation to apply.
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Relocation
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types are processor-specific.
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When the text refers to a relocation
|
|
entry's relocation type or symbol table index, it means the result of
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applying
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.Sy ELF_[32|64]_R_TYPE
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or
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.Sy ELF[32|64]_R_SYM ,
|
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respectively to the entry's
|
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.Sy r_info
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member.
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.It Dv r_addend
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This member specifies a constant addend used to compute the value to be
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stored into the relocatable field.
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.El
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.Sh SEE ALSO
|
|
.Xr as 1 ,
|
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.Xr gdb 1 ,
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.Xr ld 1 ,
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.Xr objdump 1 ,
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.Xr execve 2 ,
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.Xr ar 5 ,
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.Xr core 5
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.Rs
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.%A Hewlett Packard
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.%B Elf-64 Object File Format
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.Re
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.Rs
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.%A Santa Cruz Operation
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.%B System V Application Binary Interface
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.Re
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.Rs
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.%A Unix System Laboratories
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.%T Object Files
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.%B "Executable and Linking Format (ELF)"
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.Re
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.Sh HISTORY
|
|
The ELF header files made their appearance in
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.Fx 2.2.6 .
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ELF in itself first appeared in
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.At V .
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The ELF format is an adopted standard.
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.Sh AUTHORS
|
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This manual page was written by
|
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.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org
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with inspiration from BSDi's
|
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.Bsx
|
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.Xr elf 5
|
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manpage.
|