RTLD(1) FreeBSD General Commands Manual RTLD(1)
NAME
ld-elf.so.1, ld.so, rtld - run-time link-editor
DESCRIPTION
The ld-elf.so.1 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
mmap(2) system call.
After all shared libraries have been successfully loaded, ld-elf.so.1
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.
When resolving dependencies for the loaded objects, ld-elf.so.1
translates dynamic token strings in rpath and soname. If the -z origin
option of the static linker was set when linking the binary, the token
expansion is performed at the object load time, see ld(1). The following
strings are recognized now:
$ORIGIN Translated to the full path of the loaded object.
$OSNAME Translated to the name of the operating system implementation.
$OSREL Translated to the release level of the operating system.
$PLATFORM Translated to the machine hardware platform.
$LIB Translated to the system library path component on the
platform. It is lib for native binaries, and typically lib32
for compat32 binaries. Other translations might exist for
other ABIs supported on the platform.
The ld-elf.so.1 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:
1. DT_RPATH of the referencing object unless that object also
contains a DT_RUNPATH tag
2. DT_RPATH of the program unless the referencing object contains
a DT_RUNPATH tag
3. Path indicated by LD_LIBRARY_PATH environment variable
4. DT_RUNPATH of the referencing object
5. Hints file produced by the ldconfig(8) utility
6. The /lib and /usr/lib directories, unless the referencing
object was linked using the "-z nodefaultlib" option
The ld-elf.so.1 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 LD_32_, for example:
LD_32_TRACE_LOADED_OBJECTS. If the activated image is setuid or setgid,
the variables are ignored.
The run-time linker is able to access the environment provided at process
startup. After startup, environment variables are maintained by higher-
level libraries and are not accessible by the run-time linker. At run-
time, effective settings can be queried using rtld_get_var(3), and some
of them can be changed with rtld_set_var(3).
LD_DUMP_REL_POST If set, ld-elf.so.1 will print a table containing all
relocations after symbol binding and relocation.
LD_DUMP_REL_PRE If set, ld-elf.so.1 will print a table containing all
relocations before symbol binding and relocation.
LD_DYNAMIC_WEAK If set, use the ELF standard-compliant symbol lookup
behavior: resolve to the first found symbol
definition.
By default, FreeBSD 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.
Symbols exported by dynamic linker itself (see
dlfcn(3)) are always resolved using FreeBSD rules
regardless of the presence of the variable. This
variable is unset for set-user-ID and set-group-ID
programs.
LD_LIBMAP A library replacement list in the same format as
libmap.conf(5). For convenience, the characters `='
and `,' can be used instead of a space and a newline.
This variable is parsed after libmap.conf(5), and will
override its entries. This variable is unset for set-
user-ID and set-group-ID programs.
LD_LIBMAP_DISABLE If set, disables the use of libmap.conf(5) and
LD_LIBMAP. This variable is unset for set-user-ID and
set-group-ID programs.
LD_ELF_HINTS_PATH This variable will override the default location of
"hints" file. This variable is unset for set-user-ID
and set-group-ID programs.
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.
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 LD_LIBRARY_PATH variable is allowed to
override the path from DT_RPATH for binaries which
does not contain DT_RUNPATH tag. For such binaries,
when the variable LD_LIBRARY_PATH_RPATH is set, "-z
nodefaultlib" link-time option is ignored as well.
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 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.
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.
LD_LIBRARY_PATH_FDS
A colon separated list of file descriptor numbers for
library directories. This is intended for use within
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.
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.
LD_BIND_NOW When set to a nonempty string, causes ld-elf.so.1 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.
LD_BIND_NOW increases the start-up time of a program,
but it avoids run-time surprises caused by
unexpectedly undefined functions.
LD_TRACE_LOADED_OBJECTS
When set to a nonempty string, causes ld-elf.so.1 to
exit after loading the shared objects and printing a
summary which includes the absolute pathnames of all
objects, to standard output.
LD_TRACE_LOADED_OBJECTS_ALL
When set to a nonempty string, causes ld-elf.so.1 to
expand the summary to indicate which objects caused
each object to be loaded.
LD_TRACE_LOADED_OBJECTS_FMT1
LD_TRACE_LOADED_OBJECTS_FMT2
When set, these variables are interpreted as format
strings a la printf(3) to customize the trace output
and are used by ldd(1)'s -f option and allows ldd(1)
to be operated as a filter more conveniently. If the
dependency name starts with string lib,
LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise
LD_TRACE_LOADED_OBJECTS_FMT2 is used. The following
conversions can be used:
%a The main program's name (also known as
"__progname").
%A The value of the environment variable
LD_TRACE_LOADED_OBJECTS_PROGNAME. Typically
used to print both the names of programs and
shared libraries being inspected using ldd(1).
%o The library name.
%p The full pathname as determined by rtld's
library search rules.
%x The library's load address.
Additionally, `\n' and `\t' are recognized and have
their usual meaning.
LD_UTRACE If set, ld-elf.so.1 will log events such as the
loading and unloading of shared objects via utrace(2).
LD_LOADFLTR If set, ld-elf.so.1 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.
LD_SHOW_AUXV If set, causes ld-elf.so.1 to dump content of the aux
vector to standard output, before passing control to
any user code.
LD_STATIC_TLS_EXTRA
If the variable is specified and has a numeric value,
ld-elf.so.1 will set the size of the static TLS extra
space to the specified number of bytes. The static
TLS extra space is used when loading objects compiled
for initial-exec TLS code model with dlopen(3). The
minimum value that can be specified is '128'.
LD_NO_DL_ITERATE_PHDR_AFTER_FORK
Allow dl_iterate_phdr(3) to block in callback, without
causing deadlock with the fork(2). The drawback is
that the image started in this mode cannot use
dl_iterate_phdr(3) after fork.
DIRECT EXECUTION MODE
ld-elf.so.1 is typically used implicitly, loaded by the kernel as
requested by the PT_INTERP program header of the executed binary.
FreeBSD also supports a direct execution mode for the dynamic linker. In
this mode, the user explicitly executes ld-elf.so.1 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.
The syntax of the direct invocation is
/libexec/ld-elf.so.1 [-b exe] [-d] [-f fd] [-o OPT=VALUE] [-p] [-u]
[-v] [--] image_path [image arguments]
The options are:
-b exe Use the executable exe instead of image_path for activation. If
this option is specified, image_path is only used to provide the
argv[0] value to the program.
-d Turn off the emulation of the binary execute permission.
-f fd File descriptor fd references the binary to be activated by
ld-elf.so.1. It must already be opened in the process when
executing ld-elf.so.1. If this option is specified, image_path
is only used to provide the argv[0] value to the program.
-o OPT=VALUE
Set the OPT configuration variable to the value VALUE. The
possible variable names are listed above as LD_ prefixed
environment variables, but here are referenced without the LD_
prefix. A configuration variable set this way does not leak into
the activated image's environment.
The option can be repeated as many times as needed to set all
configuration parameters. The parameters set using this option
have priority over the same parameters assigned via environment.
-p If the image_path argument specifies a name which does not
contain a slash "/" character, ld-elf.so.1 uses the search path
provided by the environment variable PATH to find the binary to
execute.
-u Ignore all LD_ environment variables and previous command line -o
options that otherwise affect the dynamic linker behavior.
-v Display information about this run-time linker binary, then exit.
-- Ends the ld-elf.so.1 options. The argument following -- is
interpreted as the path of the binary to execute.
In the direct execution mode, ld-elf.so.1 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 DACs, ignores 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 -d
option.
VERSIONING
Newer ld-elf.so.1 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
__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 ld-elf.so.1.
To solve the problem, ld-elf.so.1 exports some feature indicators in the
FreeBSD private symbols namespace FBSDprivate_1.0. Symbols start with
the _rtld_version prefix. Current list of defined symbols and
corresponding features is:
_rtld_version__FreeBSD_version
Symbol exports the value of the __FreeBSD_version definition as
it was provided during the ld-elf.so.1 build. The symbol is
always present since the _rtld_version facility was introduced.
_rtld_version_laddr_offset
The l_addr member of the link_map structure contains the load
offset of the shared object. Before that, l_addr contained the
base address of the library. See dlinfo(3).
Also it indicates the presence of l_refname member of the
structure.
_rtld_version_dlpi_tls_data
The dlpi_tls_data member of the structure dl_phdr_info contains
the address of the module TLS segment for the calling thread, and
not the address of the initialization segment.
FILES
/var/run/ld-elf.so.hints Hints file.
/var/run/ld-elf32.so.hints Hints file for 32-bit binaries on 64-bit
system.
/etc/libmap.conf The libmap configuration file.
/etc/libmap32.conf The libmap configuration file for 32-bit
binaries on 64-bit system.
SEE ALSO
ld(1), ldd(1), dlinfo(3), rtld_get_var(3), capsicum(4), elf(5),
libmap.conf(5), ldconfig(8)
FreeBSD 15.1-STABLE-HBSD July 24, 2024 RTLD(1)