libexec/rtld-elf/rtld.1

.\" Copyright (c) 1995 Paul Kranenburg
.\" All rights reserved.
.\"
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.\" modification, are permitted provided that the following conditions
.\" are met:
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.\"    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
.\"    must display the following acknowledgment:
.\"      This product includes software developed by Paul Kranenburg.
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.\"    derived from this software without specific prior written permission
.\"
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.Dd November 10, 2022
.Dt RTLD 1
.Os
.Sh NAME
.Nm ld-elf.so.1 ,
.Nm ld.so ,
.Nm rtld
.Nd run-time link-editor
.Sh DESCRIPTION
The
.Nm
utility is a self-contained shared object providing run-time
support for loading and link-editing shared objects into a process'
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
After all shared libraries have been successfully loaded,
.Nm
proceeds to resolve external references from both the main program and
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.
.It Pa $LIB
Translated to the system library path component on the platform.
It is
.Pa lib
for native binaries, and typically
.Pa lib32
for compat32 binaries.
Other translations might exist for other ABIs supported on the platform.
.El
.Pp
The
.Nm
utility itself is loaded by the kernel together with any dynamically-linked
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.
The following search order is used to locate required shared objects:
.Pp
.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
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
The
.Nm
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
.Ev LD_32_ ,
for example:
.Ev LD_32_TRACE_LOADED_OBJECTS .
If the activated image is setuid or setgid, the variables are ignored.
.Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
.It Ev LD_DUMP_REL_POST
If set,
.Nm
will print a table containing all relocations after symbol
binding and relocation.
.It Ev LD_DUMP_REL_PRE
If set,
.Nm
will print a table containing all relocations before symbol
binding and relocation.
.It Ev LD_DYNAMIC_WEAK
If set, use the ELF standard-compliant symbol lookup behavior:
resolve to the first found symbol definition.
.Pp
By default,
.Fx
provides the non-standard symbol lookup behavior:
when a weak symbol definition is found, remember the definition and
keep searching in the remaining shared objects for a non-weak definition.
If found, the non-weak definition is preferred, otherwise the remembered
weak definition is returned.
.Pp
Symbols exported by dynamic linker itself (see
.Xr dlfcn 3 )
are always resolved using
.Fx
rules regardless of the presence of the variable.
This variable is unset for set-user-ID and set-group-ID programs.
.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
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.
.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
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_PRELOAD_FDS
A colon separated list of file descriptor numbers for libraries.
This is intended for preloading libraries in which we already have a file
descriptor.
This may optimize the process of loading libraries because we do not have to
look for them in directories.
It may also be useful in a capability base system where we do not have access to
global namespaces such as the filesystem.
.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
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
.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:
.Bl -tag -width 4n
.It Li %a
The main program's name
(also known as
.Dq __progname ) .
.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 .
.It Li %o
The library name.
.It Li %p
The full pathname as determined by
.Nm rtld Ns 's
library search rules.
.It Li %x
The library's load address.
.El
.Pp
Additionally,
.Ql \en
and
.Ql \et
are recognized and have their usual meaning.
.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.
.It Ev LD_SHOW_AUXV
If set, causes
.Nm
to dump content of the aux vector to standard output, before passing
control to any user code.
.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 b Ar exe
.Op Fl d
.Op Fl f Ar fd
.Op Fl p
.Op Fl u
.Op Fl v
.Op Fl -
.Pa image_path
.Op Ar image arguments
.Ed
.Pp
The options are:
.Bl -tag -width indent
.It Fl b Ar exe
Use the executable
.Fa exe
instead of
.Fa image_path
for activation.
If this option is specified,
.Ar image_path
is only used to provide the
.Va argv[0]
value to the program.
.It Fl d
Turn off the emulation of the binary execute permission.
.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 u
Ignore all
.Ev LD_
environment variables that otherwise affect the dynamic
linker behavior.
.It Fl v
Display information about this run-time linker binary, then exit.
.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.
It can be turned off with the
.Fl d
option.
.Sh VERSIONING
Newer
.Nm
might provide some features or changes in runtime behavior that cannot be
easily detected at runtime by checking of the normal exported symbols.
Note that it is almost always wrong to verify
.Dv __FreeBSD_version
in userspace to detect features, either at compile or at run time,
because either kernel, or libc, or environment variables could not
match the running
.Nm .
.Pp
To solve the problem,
.Nm
exports some feature indicators in the
.Fx
private symbols namespace
.Dv FBSDprivate_1.0 .
Symbols start with the
.Dv _rtld_version
prefix.
Current list of defined symbols and corresponding features is:
.Bl -tag -width indent
.It Dv _rtld_version__FreeBSD_version
Symbol exports the value of the
.Dv __FreeBSD_version
definition as it was provided during the
.Nm
build.
The symbol is always present since the
.Dv _rtld_version
facility was introduced.
.It Dv _rtld_version_laddr_offset
The
.Va l_addr
member of the
.Vt link_map
structure contains the load offset of the shared object.
Before that,
.Va l_addr
contained the base address of the library.
See
.Xr dlinfo 3 .
.Pp
Also it indicates the presence of
.Va l_refname
member of the structure.
.It Dv _rtld_version_dlpi_tls_data
The
.Va dlpi_tls_data
member of the structure
.Vt dl_phdr_info
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
.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.
.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