It dumps auxv as seen by interpreter, right before starting any user code. Copied from: glibc Sponsored by: The FreeBSD Foundation MFC after: 1 week
491 lines
14 KiB
Groff
491 lines
14 KiB
Groff
.\" Copyright (c) 1995 Paul Kranenburg
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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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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgment:
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.\" This product includes software developed by Paul Kranenburg.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd November 13, 2021
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.Dt RTLD 1
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.Os
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.Sh NAME
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.Nm ld-elf.so.1 ,
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.Nm ld.so ,
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.Nm rtld
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.Nd run-time link-editor
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.Sh DESCRIPTION
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The
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.Nm
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utility is a self-contained shared object providing run-time
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support for loading and link-editing shared objects into a process'
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address space.
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It is also commonly known as the dynamic linker.
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It uses the data structures
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contained within dynamically linked programs to determine which shared
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libraries are needed and loads them using the
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.Xr mmap 2
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system call.
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.Pp
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After all shared libraries have been successfully loaded,
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.Nm
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proceeds to resolve external references from both the main program and
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all objects loaded.
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A mechanism is provided for initialization routines
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to be called on a per-object basis, giving a shared object an opportunity
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to perform any extra set-up before execution of the program proper begins.
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This is useful for C++ libraries that contain static constructors.
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.Pp
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When resolving dependencies for the loaded objects,
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.Nm
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translates dynamic token strings in rpath and soname.
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If the
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.Fl "z origin"
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option of the static linker was set when linking the binary,
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the token expansion is performed at the object load time, see
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.Xr ld 1 .
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The following strings are recognized now:
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.Bl -tag -width ".Pa $PLATFORM"
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.It Pa $ORIGIN
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Translated to the full path of the loaded object.
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.It Pa $OSNAME
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Translated to the name of the operating system implementation.
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.It Pa $OSREL
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Translated to the release level of the operating system.
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.It Pa $PLATFORM
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Translated to the machine hardware platform.
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.El
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.Pp
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The
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.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.
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The kernel transfers control to the
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dynamic linker.
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After the dynamic linker has finished loading,
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relocating, and initializing the program and its required shared
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objects, it transfers control to the entry point of the program.
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The following search order is used to locate required shared objects:
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.Pp
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.Bl -enum -offset indent -compact
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.It
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.Dv DT_RPATH
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of the referencing object unless that object also contains a
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.Dv DT_RUNPATH
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tag
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.It
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.Dv DT_RPATH
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of the program unless the referencing object contains a
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.Dv DT_RUNPATH
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tag
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.It
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Path indicated by
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.Ev LD_LIBRARY_PATH
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environment variable
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.It
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.Dv DT_RUNPATH
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of the referencing object
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.It
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Hints file produced by the
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.Xr ldconfig 8
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utility
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.It
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The
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.Pa /lib
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and
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.Pa /usr/lib
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directories, unless the referencing object was linked using the
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.Dq Fl z Ar nodefaultlib
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option
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.El
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.Pp
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The
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.Nm
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utility
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recognizes a number of environment variables that can be used to modify
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its behaviour.
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On 64-bit architectures, the linker for 32-bit objects recognizes
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all the environment variables listed below, but is being prefixed with
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.Ev LD_32_ ,
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for example:
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.Ev LD_32_TRACE_LOADED_OBJECTS .
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If the activated image is setuid or setgid, the variables are ignored.
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.Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
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.It Ev LD_DUMP_REL_POST
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If set,
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.Nm
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will print a table containing all relocations after symbol
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binding and relocation.
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.It Ev LD_DUMP_REL_PRE
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If set,
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.Nm
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will print a table containing all relocations before symbol
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binding and relocation.
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.It Ev LD_DYNAMIC_WEAK
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If set, use the ELF standard-compliant symbol lookup behavior:
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resolve to the first found symbol definition.
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.Pp
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By default,
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.Fx
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provides the non-standard symbol lookup behavior:
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when a weak symbol definition is found, remember the definition and
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keep searching in the remaining shared objects for a non-weak definition.
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If found, the non-weak definition is preferred, otherwise the remembered
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weak definition is returned.
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.Pp
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Symbols exported by dynamic linker itself (see
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.Xr dlfcn 3 )
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are always resolved using
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.Fx
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rules regardless of the presence of the variable.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_LIBMAP
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A library replacement list in the same format as
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.Xr libmap.conf 5 .
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For convenience, the characters
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.Ql =
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and
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.Ql \&,
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can be used instead of a space and a newline.
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This variable is parsed after
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.Xr libmap.conf 5 ,
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and will override its entries.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_LIBMAP_DISABLE
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If set, disables the use of
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.Xr libmap.conf 5
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and
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.Ev LD_LIBMAP .
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_ELF_HINTS_PATH
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This variable will override the default location of
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.Dq hints
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file.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_LIBRARY_PATH
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A colon separated list of directories, overriding the default search path
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for shared libraries.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_LIBRARY_PATH_RPATH
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If the variable is specified and has a value starting with
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any of \'y\', \'Y\' or \'1\' symbols, the path specified by
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.Ev LD_LIBRARY_PATH
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variable is allowed to override the path from
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.Dv DT_RPATH
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for binaries which does not contain
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.Dv DT_RUNPATH
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tag.
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For such binaries, when the variable
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.Ev LD_LIBRARY_PATH_RPATH
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is set,
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.Dq Fl z Ar nodefaultlib
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link-time option is ignored as well.
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.It Ev LD_PRELOAD
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A list of shared libraries, separated by colons and/or white space,
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to be linked in before any
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other shared libraries.
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If the directory is not specified then
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the directories specified by
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.It Ev LD_PRELOAD_FDS
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A colon separated list of file descriptor numbers for libraries.
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This is intended for preloading libraries in which we already have a file
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descriptor.
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This may optimize the process of loading libraries because we do not have to
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look for them in directories.
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It may also be useful in a capability base system where we do not have access to
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global namespaces such as the filesystem.
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.Ev LD_LIBRARY_PATH
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will be searched first
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followed by the set of built-in standard directories.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_LIBRARY_PATH_FDS
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A colon separated list of file descriptor numbers for library directories.
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This is intended for use within
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.Xr capsicum 4
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sandboxes, when global namespaces such as the filesystem are unavailable.
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It is consulted just after LD_LIBRARY_PATH.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_BIND_NOT
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When set to a nonempty string, prevents modifications of the PLT slots when
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doing bindings.
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As result, each call of the PLT-resolved function is resolved.
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In combination with debug output, this provides complete account of
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all bind actions at runtime.
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This variable is unset for set-user-ID and set-group-ID programs.
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.It Ev LD_BIND_NOW
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When set to a nonempty string, causes
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.Nm
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to relocate all external function calls before starting execution of the
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program.
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Normally, function calls are bound lazily, at the first call
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of each function.
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.Ev LD_BIND_NOW
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increases the start-up time of a program, but it avoids run-time
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surprises caused by unexpectedly undefined functions.
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.It Ev LD_TRACE_LOADED_OBJECTS
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When set to a nonempty string, causes
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.Nm
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to exit after loading the shared objects and printing a summary which includes
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the absolute pathnames of all objects, to standard output.
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.It Ev LD_TRACE_LOADED_OBJECTS_ALL
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When set to a nonempty string, causes
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.Nm
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to expand the summary to indicate which objects caused each object to
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be loaded.
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.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
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.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
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When set, these variables are interpreted as format strings a la
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.Xr printf 3
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to customize the trace output and are used by
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.Xr ldd 1 Ns 's
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.Fl f
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option and allows
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.Xr ldd 1
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to be operated as a filter more conveniently.
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If the dependency name starts with string
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.Pa lib ,
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.Ev LD_TRACE_LOADED_OBJECTS_FMT1
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is used, otherwise
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.Ev LD_TRACE_LOADED_OBJECTS_FMT2
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is used.
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The following conversions can be used:
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.Bl -tag -width 4n
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.It Li %a
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The main program's name
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(also known as
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.Dq __progname ) .
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.It Li \&%A
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The value of the environment variable
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.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
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Typically used to print both the names of programs and shared libraries
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being inspected using
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.Xr ldd 1 .
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.It Li %o
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The library name.
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.It Li %p
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The full pathname as determined by
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.Nm rtld Ns 's
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library search rules.
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.It Li %x
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The library's load address.
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.El
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.Pp
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Additionally,
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.Ql \en
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and
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.Ql \et
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are recognized and have their usual meaning.
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.It Ev LD_UTRACE
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If set,
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.Nm
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will log events such as the loading and unloading of shared objects via
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.Xr utrace 2 .
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.It Ev LD_LOADFLTR
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If set,
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.Nm
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will process the filtee dependencies of the loaded objects immediately,
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instead of postponing it until required.
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Normally, the filtees are opened at the time of the first symbol resolution
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from the filter object.
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.It Ev LD_SHOW_AUXV
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If set, causes
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.Nm
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to dump content of the aux vector to standard output, before passing
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control to any user code.
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.El
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.Sh DIRECT EXECUTION MODE
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.Nm
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is typically used implicitly, loaded by the kernel as requested by the
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.Dv PT_INTERP
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program header of the executed binary.
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.Fx
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also supports a direct execution mode for the dynamic linker.
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In this mode, the user explicitly executes
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.Nm
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and provides the path of the program to be linked and executed as
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an argument.
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This mode allows use of a non-standard dynamic linker for a program
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activation without changing the binary or without changing
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the installed dynamic linker.
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Execution options may be specified.
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.Pp
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The syntax of the direct invocation is
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.Bd -ragged -offset indent
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.Pa /libexec/ld-elf.so.1
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.Op Fl b Ar exe
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.Op Fl d
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.Op Fl f Ar fd
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.Op Fl p
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.Op Fl u
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.Op Fl v
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.Op Fl -
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.Pa image_path
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.Op Ar image arguments
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.Ed
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.Pp
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The options are:
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.Bl -tag -width indent
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.It Fl b Ar exe
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Use the executable
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.Fa exe
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instead of
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.Fa image_path
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for activation.
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If this option is specified,
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.Ar image_path
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is only used to provide the
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.Va argv[0]
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value to the program.
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.It Fl d
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Turn off the emulation of the binary execute permission.
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.It Fl f Ar fd
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File descriptor
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.Ar fd
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references the binary to be activated by
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.Nm .
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It must already be opened in the process when executing
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.Nm .
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If this option is specified,
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.Ar image_path
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is only used to provide the
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.Va argv[0]
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value to the program.
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.It Fl p
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If the
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.Pa image_path
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argument specifies a name which does not contain a slash
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.Dq Li /
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character,
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.Nm
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uses the search path provided by the environment variable
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.Dv PATH
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to find the binary to execute.
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.It Fl u
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Ignore all
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.Ev LD_
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environment variables that otherwise affect the dynamic
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linker behavior.
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.It Fl v
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Display information about this run-time linker binary, then exit.
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.It Fl -
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Ends the
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.Nm
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options.
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The argument following
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.Fl -
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is interpreted as the path of the binary to execute.
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.El
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.Pp
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In the direct execution mode,
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.Nm
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emulates verification of the binary execute permission for the
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current user.
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This is done to avoid breaking user expectations in naively restricted
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execution environments.
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The verification only uses Unix
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.Dv DACs ,
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ignores
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.Dv ACLs ,
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and is naturally prone to race conditions.
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Environments which rely on such restrictions are weak
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and breakable on their own.
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It can be turned off with the
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.Fl d
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option.
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.Sh VERSIONING
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Newer
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.Nm
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might provide some features or changes in runtime behavior that cannot be
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easily detected at runtime by checking of the normal exported symbols.
|
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Note that it is almost always wrong to verify
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.Dv __FreeBSD_version
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in userspace to detect features, either at compile or at run time,
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because either kernel, or libc, or environment variables could not
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match the running
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.Nm .
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.Pp
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To solve the problem,
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.Nm
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exports some feature indicators in the
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.Fx
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private symbols namespace
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.Dv FBSDprivate_1.0 .
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Symbols start with the
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.Dv _rtld_version
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prefix.
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|
Current list of defined symbols and corresponding features is:
|
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.Bl -tag -width indent
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.It Dv _rtld_version__FreeBSD_version
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Symbol exports the value of the
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.Dv __FreeBSD_version
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definition as it was provided during the
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.Nm
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build.
|
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The symbol is always present since the
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.Dv _rtld_version
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facility was introduced.
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.It Dv _rtld_version_laddr_offset
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The
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.Va l_addr
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member of the
|
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.Vt link_map
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structure contains the load offset of the shared object.
|
|
Before that,
|
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.Va l_addr
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|
contained the base address of the library.
|
|
See
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.Xr dlinfo 3 .
|
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.Pp
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|
Also it indicates the presence of
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.Va l_refname
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member of the structure.
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.It Dv _rtld_version_dlpi_tls_data
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The
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.Va dlpi_tls_data
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member of the structure
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.Vt dl_phdr_info
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|
contains the address of the module TLS segment for the calling thread,
|
|
and not the address of the initialization segment.
|
|
.El
|
|
.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
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|
Hints file.
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|
.It Pa /var/run/ld-elf32.so.hints
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|
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.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr ld 1 ,
|
|
.Xr ldd 1 ,
|
|
.Xr dlinfo 3 ,
|
|
.Xr capsicum 4 ,
|
|
.Xr elf 5 ,
|
|
.Xr libmap.conf 5 ,
|
|
.Xr ldconfig 8
|