freebsd-dev/libexec/rtld-elf/rtld.1

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.\" Copyright (c) 1995 Paul Kranenburg
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgment:
.\" This product includes software developed by Paul Kranenburg.
.\" 3. The name of the author may not be used to endorse or promote products
.\" derived from this software without specific prior written permission
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
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.\" $FreeBSD$
.\"
.Dd May 20, 2017
.Dt RTLD 1
.Os
.Sh NAME
.Nm ld-elf.so.1 ,
.Nm ld.so ,
.Nm rtld
.Nd run-time link-editor
.Sh DESCRIPTION
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The
.Nm
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utility is a self-contained shared object providing run-time
support for loading and link-editing shared objects into a process'
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address space.
It is also commonly known as the dynamic linker.
It uses the data structures
contained within dynamically linked programs to determine which shared
libraries are needed and loads them using the
.Xr mmap 2
system call.
.Pp
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After all shared libraries have been successfully loaded,
.Nm
proceeds to resolve external references from both the main program and
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all objects loaded.
A mechanism is provided for initialization routines
to be called on a per-object basis, giving a shared object an opportunity
to perform any extra set-up before execution of the program proper begins.
This is useful for C++ libraries that contain static constructors.
.Pp
When resolving dependencies for the loaded objects,
.Nm
translates dynamic token strings in rpath and soname.
If the
.Fl "z origin"
option of the static linker was set when linking the binary,
the token expansion is performed at the object load time, see
.Xr ld 1 .
The following strings are recognized now:
.Bl -tag -width ".Pa $PLATFORM"
.It Pa $ORIGIN
Translated to the full path of the loaded object.
.It Pa $OSNAME
Translated to the name of the operating system implementation.
.It Pa $OSREL
Translated to the release level of the operating system.
.It Pa $PLATFORM
Translated to the machine hardware platform.
.El
.Pp
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The
.Nm
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utility itself is loaded by the kernel together with any dynamically-linked
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program that is to be executed.
The kernel transfers control to the
dynamic linker.
After the dynamic linker has finished loading,
relocating, and initializing the program and its required shared
objects, it transfers control to the entry point of the program.
Import the DragonFly BSD commit 4f0bc915b65fcf5a23214f6d221d65c80be68ad4 by John Marino <draco@marino.st>, with the following (edited) commit message Date: Sat, 24 Mar 2012 06:40:50 +0100 Subject: [PATCH 1/1] rtld: Implement DT_RUNPATH and -z nodefaultlib DT_RUNPATH is incorrectly being considered as an alias of DT_RPATH. The purpose of DT_RUNPATH is to have two different types of rpath: one that can be overridden by the environment variable LD_LIBRARY_PATH and one that can't. With the currently implementation, LD_LIBRARY_PATH will always trump any embedded rpath or runpath tags. Current path search order by rtld: ================================== LD_LIBRARY_PATH DT_RPATH / DT_RUNPATH (always the same) ldconfig hints file (default: /var/run/ld-elf.so.hints) /usr/lib New path search order by rtld: ============================== DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file /usr/lib The new path search matches how the linux runtime loader works. The other major added feature is support for linker flag "-z nodefaultlib". When this flag is passed to the linker, rtld will skip all references to the standard library search path ("/usr/lib" in this case but it could handle more color delimited paths) except in DT_RPATH and DT_RUNPATH. New path search order by rtld with -z nodefaultlib flag set: ============================================================ DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file (skips all references to /usr/lib) FreeBSD notes: - we fixed some bugs which were submitted to DragonFly and merged there as commit 1ff8a2bd3eb6e5587174c6a983303ea3a79e0002; - we added LD_LIBRARY_PATH_RPATH environment variable to switch to the previous behaviour of considering DT_RPATH a synonym for DT_RUNPATH; - the FreeBSD default search path is /lib:/usr/lib and not /usr/lib. Reviewed by: kan MFC after: 1 month MFC note: flip the ld_library_path_rpath default value for stable/9
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The following search order is used to locate required shared objects:
.Pp
Import the DragonFly BSD commit 4f0bc915b65fcf5a23214f6d221d65c80be68ad4 by John Marino <draco@marino.st>, with the following (edited) commit message Date: Sat, 24 Mar 2012 06:40:50 +0100 Subject: [PATCH 1/1] rtld: Implement DT_RUNPATH and -z nodefaultlib DT_RUNPATH is incorrectly being considered as an alias of DT_RPATH. The purpose of DT_RUNPATH is to have two different types of rpath: one that can be overridden by the environment variable LD_LIBRARY_PATH and one that can't. With the currently implementation, LD_LIBRARY_PATH will always trump any embedded rpath or runpath tags. Current path search order by rtld: ================================== LD_LIBRARY_PATH DT_RPATH / DT_RUNPATH (always the same) ldconfig hints file (default: /var/run/ld-elf.so.hints) /usr/lib New path search order by rtld: ============================== DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file /usr/lib The new path search matches how the linux runtime loader works. The other major added feature is support for linker flag "-z nodefaultlib". When this flag is passed to the linker, rtld will skip all references to the standard library search path ("/usr/lib" in this case but it could handle more color delimited paths) except in DT_RPATH and DT_RUNPATH. New path search order by rtld with -z nodefaultlib flag set: ============================================================ DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file (skips all references to /usr/lib) FreeBSD notes: - we fixed some bugs which were submitted to DragonFly and merged there as commit 1ff8a2bd3eb6e5587174c6a983303ea3a79e0002; - we added LD_LIBRARY_PATH_RPATH environment variable to switch to the previous behaviour of considering DT_RPATH a synonym for DT_RUNPATH; - the FreeBSD default search path is /lib:/usr/lib and not /usr/lib. Reviewed by: kan MFC after: 1 month MFC note: flip the ld_library_path_rpath default value for stable/9
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.Bl -enum -offset indent -compact
.It
.Dv DT_RPATH
of the referencing object unless that object also contains a
.Dv DT_RUNPATH
tag
.It
.Dv DT_RPATH
of the program unless the referencing object contains a
.Dv DT_RUNPATH
tag
.It
Path indicated by
.Ev LD_LIBRARY_PATH
environment variable
.It
.Dv DT_RUNPATH
of the referencing object
.It
Hints file produced by the
.Xr ldconfig 8
Import the DragonFly BSD commit 4f0bc915b65fcf5a23214f6d221d65c80be68ad4 by John Marino <draco@marino.st>, with the following (edited) commit message Date: Sat, 24 Mar 2012 06:40:50 +0100 Subject: [PATCH 1/1] rtld: Implement DT_RUNPATH and -z nodefaultlib DT_RUNPATH is incorrectly being considered as an alias of DT_RPATH. The purpose of DT_RUNPATH is to have two different types of rpath: one that can be overridden by the environment variable LD_LIBRARY_PATH and one that can't. With the currently implementation, LD_LIBRARY_PATH will always trump any embedded rpath or runpath tags. Current path search order by rtld: ================================== LD_LIBRARY_PATH DT_RPATH / DT_RUNPATH (always the same) ldconfig hints file (default: /var/run/ld-elf.so.hints) /usr/lib New path search order by rtld: ============================== DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file /usr/lib The new path search matches how the linux runtime loader works. The other major added feature is support for linker flag "-z nodefaultlib". When this flag is passed to the linker, rtld will skip all references to the standard library search path ("/usr/lib" in this case but it could handle more color delimited paths) except in DT_RPATH and DT_RUNPATH. New path search order by rtld with -z nodefaultlib flag set: ============================================================ DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file (skips all references to /usr/lib) FreeBSD notes: - we fixed some bugs which were submitted to DragonFly and merged there as commit 1ff8a2bd3eb6e5587174c6a983303ea3a79e0002; - we added LD_LIBRARY_PATH_RPATH environment variable to switch to the previous behaviour of considering DT_RPATH a synonym for DT_RUNPATH; - the FreeBSD default search path is /lib:/usr/lib and not /usr/lib. Reviewed by: kan MFC after: 1 month MFC note: flip the ld_library_path_rpath default value for stable/9
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utility
.It
The
.Pa /lib
and
.Pa /usr/lib
directories, unless the referencing object was linked using the
.Dq Fl z Ar nodefaultlib
option
.El
.Pp
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The
.Nm
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utility
recognizes a number of environment variables that can be used to modify
its behaviour.
On 64-bit architectures, the linker for 32-bit objects recognizes
all the environment variables listed below, but is being prefixed with
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.Ev LD_32_ ,
for example:
.Ev LD_32_TRACE_LOADED_OBJECTS .
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.Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
.It Ev LD_DUMP_REL_POST
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If set,
.Nm
will print a table containing all relocations after symbol
binding and relocation.
.It Ev LD_DUMP_REL_PRE
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If set,
.Nm
will print a table containing all relocations before symbol
binding and relocation.
.It Ev LD_LIBMAP
A library replacement list in the same format as
.Xr libmap.conf 5 .
For convenience, the characters
.Ql =
and
.Ql \&,
can be used instead of a space and a newline.
This variable is parsed after
.Xr libmap.conf 5 ,
and will override its entries.
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_LIBMAP_DISABLE
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If set, disables the use of
.Xr libmap.conf 5
and
.Ev LD_LIBMAP .
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_ELF_HINTS_PATH
This variable will override the default location of
.Dq hints
file.
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_LIBRARY_PATH
A colon separated list of directories, overriding the default search path
for shared libraries.
This variable is unset for set-user-ID and set-group-ID programs.
Import the DragonFly BSD commit 4f0bc915b65fcf5a23214f6d221d65c80be68ad4 by John Marino <draco@marino.st>, with the following (edited) commit message Date: Sat, 24 Mar 2012 06:40:50 +0100 Subject: [PATCH 1/1] rtld: Implement DT_RUNPATH and -z nodefaultlib DT_RUNPATH is incorrectly being considered as an alias of DT_RPATH. The purpose of DT_RUNPATH is to have two different types of rpath: one that can be overridden by the environment variable LD_LIBRARY_PATH and one that can't. With the currently implementation, LD_LIBRARY_PATH will always trump any embedded rpath or runpath tags. Current path search order by rtld: ================================== LD_LIBRARY_PATH DT_RPATH / DT_RUNPATH (always the same) ldconfig hints file (default: /var/run/ld-elf.so.hints) /usr/lib New path search order by rtld: ============================== DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file /usr/lib The new path search matches how the linux runtime loader works. The other major added feature is support for linker flag "-z nodefaultlib". When this flag is passed to the linker, rtld will skip all references to the standard library search path ("/usr/lib" in this case but it could handle more color delimited paths) except in DT_RPATH and DT_RUNPATH. New path search order by rtld with -z nodefaultlib flag set: ============================================================ DT_RPATH of the calling object if no DT_RUNPATH DT_RPATH of the main binary if no DT_RUNPATH and binary isn't calling obj LD_LIBRARY_PATH DT_RUNPATH ldconfig hints file (skips all references to /usr/lib) FreeBSD notes: - we fixed some bugs which were submitted to DragonFly and merged there as commit 1ff8a2bd3eb6e5587174c6a983303ea3a79e0002; - we added LD_LIBRARY_PATH_RPATH environment variable to switch to the previous behaviour of considering DT_RPATH a synonym for DT_RUNPATH; - the FreeBSD default search path is /lib:/usr/lib and not /usr/lib. Reviewed by: kan MFC after: 1 month MFC note: flip the ld_library_path_rpath default value for stable/9
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.It Ev LD_LIBRARY_PATH_RPATH
If the variable is specified and has a value starting with
any of \'y\', \'Y\' or \'1\' symbols, the path specified by
.Ev LD_LIBRARY_PATH
variable is allowed to override the path from
.Dv DT_RPATH
for binaries which does not contain
.Dv DT_RUNPATH
tag.
For such binaries, when the variable
.Ev LD_LIBRARY_PATH_RPATH
is set,
.Dq Fl z Ar nodefaultlib
link-time option is ignored as well.
.It Ev LD_PRELOAD
A list of shared libraries, separated by colons and/or white space,
to be linked in before any
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other shared libraries.
If the directory is not specified then
the directories specified by
.Ev LD_LIBRARY_PATH
will be searched first
followed by the set of built-in standard directories.
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_LIBRARY_PATH_FDS
A colon separated list of file descriptor numbers for library directories.
This is intended for use within
.Xr capsicum 4
sandboxes, when global namespaces such as the filesystem are unavailable.
It is consulted just after LD_LIBRARY_PATH.
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_BIND_NOT
When set to a nonempty string, prevents modifications of the PLT slots when
doing bindings.
As result, each call of the PLT-resolved function is resolved.
In combination with debug output, this provides complete account of
all bind actions at runtime.
This variable is unset for set-user-ID and set-group-ID programs.
.It Ev LD_BIND_NOW
When set to a nonempty string, causes
.Nm
to relocate all external function calls before starting execution of the
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program.
Normally, function calls are bound lazily, at the first call
of each function.
.Ev LD_BIND_NOW
increases the start-up time of a program, but it avoids run-time
surprises caused by unexpectedly undefined functions.
.It Ev LD_TRACE_LOADED_OBJECTS
When set to a nonempty string, causes
.Nm
to exit after loading the shared objects and printing a summary which includes
the absolute pathnames of all objects, to standard output.
.It Ev LD_TRACE_LOADED_OBJECTS_ALL
When set to a nonempty string, causes
.Nm
to expand the summary to indicate which objects caused each object to
be loaded.
.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
When set, these variables are interpreted as format strings a la
.Xr printf 3
to customize the trace output and are used by
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.Xr ldd 1 Ns 's
.Fl f
option and allows
.Xr ldd 1
to be operated as a filter more conveniently.
If the dependency name starts with string
.Pa lib ,
.Ev LD_TRACE_LOADED_OBJECTS_FMT1
is used, otherwise
.Ev LD_TRACE_LOADED_OBJECTS_FMT2
is used.
The following conversions can be used:
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.Bl -tag -width 4n
.It Li %a
The main program's name
(also known as
.Dq __progname ) .
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.It Li \&%A
The value of the environment variable
.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
Typically used to print both the names of programs and shared libraries
being inspected using
.Xr ldd 1 .
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.It Li %o
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The library name.
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.It Li %p
The full pathname as determined by
.Nm rtld Ns 's
library search rules.
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.It Li %x
The library's load address.
.El
.Pp
Additionally,
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.Ql \en
and
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.Ql \et
are recognized and have their usual meaning.
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.It Ev LD_UTRACE
If set,
.Nm
will log events such as the loading and unloading of shared objects via
.Xr utrace 2 .
.It Ev LD_LOADFLTR
If set,
.Nm
will process the filtee dependencies of the loaded objects immediately,
instead of postponing it until required.
Normally, the filtees are opened at the time of the first symbol resolution
from the filter object.
.El
.Sh DIRECT EXECUTION MODE
.Nm
is typically used implicitly, loaded by the kernel as requested by the
.Dv PT_INTERP
program header of the executed binary.
.Fx
also supports a direct execution mode for the dynamic linker.
In this mode, the user explicitly executes
.Nm
and provides the path of the program to be linked and executed as
an argument.
This mode allows use of a non-standard dynamic linker for a program
activation without changing the binary or without changing
the installed dynamic linker.
Execution options may be specified.
.Pp
The syntax of the direct invocation is
.Bd -ragged -offset indent
.Pa /libexec/ld-elf.so.1
.Op Fl f Ar fd
.Op Fl p
.Op Fl -
.Pa image_path
.Op Ar image arguments
.Ed
.Pp
The options are:
.Bl -tag -width indent
.It Fl f Ar fd
File descriptor
.Ar fd
references the binary to be activated by
.Nm .
It must already be opened in the process when executing
.Nm .
If this option is specified,
.Ar image_path
is only used to provide the
.Va argv[0]
value to the program.
.It Fl p
If the
.Pa image_path
argument specifies a name which does not contain a slash
.Dq Li /
character,
.Nm
uses the search path provided by the environment variable
.Dv PATH
to find the binary to execute.
.It Fl -
Ends the
.Nm
options.
The argument following
.Fl -
is interpreted as the path of the binary to execute.
.El
.Pp
In the direct execution mode,
.Nm
emulates verification of the binary execute permission for the
current user.
This is done to avoid breaking user expectations in naively restricted
execution environments.
The verification only uses Unix
.Dv DACs ,
ignores
.Dv ACLs ,
and is naturally prone to race conditions.
Environments which rely on such restrictions are weak
and breakable on their own.
.Sh FILES
.Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact
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.It Pa /var/run/ld-elf.so.hints
Hints file.
.It Pa /var/run/ld-elf32.so.hints
Hints file for 32-bit binaries on 64-bit system.
.It Pa /etc/libmap.conf
The libmap configuration file.
.It Pa /etc/libmap32.conf
The libmap configuration file for 32-bit binaries on 64-bit system.
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.El
.Sh SEE ALSO
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.Xr ld 1 ,
.Xr ldd 1 ,
.Xr capsicum 4 ,
.Xr elf 5 ,
.Xr libmap.conf 5 ,
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.Xr ldconfig 8