ld.so, ld-linux.so - dynamic linker/loader
The dynamic linker can be run either indirectly by running some dynamically
linked program or shared object (in which case no command-line options to the
dynamic linker can be passed and, in the ELF case, the dynamic linker which is
stored in the
.interp section of the program is executed) or directly
by running:
/lib/ld-linux.so.* [OPTIONS] [PROGRAM [ARGUMENTS]]
The programs
ld.so and
ld-linux.so* find and load the shared
objects (shared libraries) needed by a program, prepare the program to run,
and then run it.
Linux binaries require dynamic linking (linking at run time) unless the
-static option was given to
ld(1) during compilation.
The program
ld.so handles a.out binaries, a binary format used long ago.
The program
ld-linux.so* (
/lib/ld-linux.so.1 for libc5,
/lib/ld-linux.so.2 for glibc2) handles binaries that are in the more
modern ELF format. Both programs have the same behavior, and use the same
support files and programs (
ldd(1),
ldconfig(8), and
/etc/ld.so.conf).
When resolving shared object dependencies, the dynamic linker first inspects
each dependency string to see if it contains a slash (this can occur if a
shared object pathname containing slashes was specified at link time). If a
slash is found, then the dependency string is interpreted as a (relative or
absolute) pathname, and the shared object is loaded using that pathname.
If a shared object dependency does not contain a slash, then it is searched for
in the following order:
- (1)
- Using the directories specified in the DT_RPATH dynamic
section attribute of the binary if present and DT_RUNPATH attribute does
not exist. Use of DT_RPATH is deprecated.
- (2)
- Using the environment variable LD_LIBRARY_PATH,
unless the executable is being run in secure-execution mode (see below),
in which case this variable is ignored.
- (3)
- Using the directories specified in the DT_RUNPATH dynamic
section attribute of the binary if present. Such directories are searched
only to find those objects required by DT_NEEDED (direct dependencies)
entries and do not apply to those objects' children, which must themselves
have their own DT_RUNPATH entries. This is unlike DT_RPATH, which is
applied to searches for all children in the dependency tree.
- (4)
- From the cache file /etc/ld.so.cache, which contains
a compiled list of candidate shared objects previously found in the
augmented library path. If, however, the binary was linked with the -z
nodeflib linker option, shared objects in the default paths are
skipped. Shared objects installed in hardware capability directories (see
below) are preferred to other shared objects.
- (5)
- In the default path /lib, and then /usr/lib.
(On some 64-bit architectures, the default paths for 64-bit shared objects
are /lib64, and then /usr/lib64.) If the binary was linked
with the -z nodeflib linker option, this step is skipped.
In several places, the dynamic linker expands dynamic string tokens:
- •
- In the environment variables LD_LIBRARY_PATH,
LD_PRELOAD, and LD_AUDIT,
- •
- inside the values of the dynamic section tags
DT_NEEDED, DT_RPATH, DT_RUNPATH, DT_AUDIT, and
DT_DEPAUDIT of ELF binaries,
- •
- in the arguments to the ld.so command line options
--audit, --library-path, and --preload (see below),
and
- •
- in the filename arguments to the dlopen(3) and
dlmopen(3) functions.
The substituted tokens are as follows:
-
$ORIGIN (or equivalently ${ORIGIN})
- This expands to the directory containing the program or
shared object. Thus, an application located in somedir/app could be
compiled with
-
gcc -Wl,-rpath,'$ORIGIN/../lib'
- so that it finds an associated shared object in
somedir/lib no matter where somedir is located in the
directory hierarchy. This facilitates the creation of "turn-key"
applications that do not need to be installed into special directories,
but can instead be unpacked into any directory and still find their own
shared objects.
-
$LIB (or equivalently ${LIB})
- This expands to lib or lib64 depending on the
architecture (e.g., on x86-64, it expands to lib64 and on x86-32,
it expands to lib).
-
$PLATFORM (or equivalently ${PLATFORM})
- This expands to a string corresponding to the processor
type of the host system (e.g., "x86_64"). On some architectures,
the Linux kernel doesn't provide a platform string to the dynamic linker.
The value of this string is taken from the AT_PLATFORM value in the
auxiliary vector (see getauxval(3)).
Note that the dynamic string tokens have to be quoted properly when set from a
shell, to prevent their expansion as shell or environment variables.
-
--argv0 string (since glibc 2.33)
- Set argv[0] to the value string before
running the program.
-
--audit list
- Use objects named in list as auditors. The objects
in list are delimited by colons.
- --inhibit-cache
- Do not use /etc/ld.so.cache.
-
--library-path path
- Use path instead of LD_LIBRARY_PATH
environment variable setting (see below). The names ORIGIN,
LIB, and PLATFORM are interpreted as for the
LD_LIBRARY_PATH environment variable.
-
--inhibit-rpath list
- Ignore RPATH and RUNPATH information in object names in
list. This option is ignored when running in secure-execution mode
(see below). The objects in list are delimited by colons or
spaces.
- --list
- List all dependencies and how they are resolved.
-
--list-tunables (since glibc 2.33)
- Print the names and values of all tunables, along with the
minimum and maximum allowed values.
-
--preload list (since glibc 2.30)
- Preload the objects specified in list. The objects
in list are delimited by colons or spaces. The objects are
preloaded as explained in the description of the LD_PRELOAD
environment variable below.
- By contrast with LD_PRELOAD, the --preload
option provides a way to perform preloading for a single executable
without affecting preloading performed in any child process that executes
a new program.
- --verify
- Verify that program is dynamically linked and this dynamic
linker can handle it.
Various environment variables influence the operation of the dynamic linker.
For security reasons, if the dynamic linker determines that a binary should be
run in secure-execution mode, the effects of some environment variables are
voided or modified, and furthermore those environment variables are stripped
from the environment, so that the program does not even see the definitions.
Some of these environment variables affect the operation of the dynamic linker
itself, and are described below. Other environment variables treated in this
way include:
GCONV_PATH,
GETCONF_DIR,
HOSTALIASES,
LOCALDOMAIN,
LOCPATH,
MALLOC_TRACE,
NIS_PATH,
NLSPATH,
RESOLV_HOST_CONF,
RES_OPTIONS,
TMPDIR,
and
TZDIR.
A binary is executed in secure-execution mode if the
AT_SECURE entry in
the auxiliary vector (see
getauxval(3)) has a nonzero value. This entry
may have a nonzero value for various reasons, including:
- •
- The process's real and effective user IDs differ, or the
real and effective group IDs differ. This typically occurs as a result of
executing a set-user-ID or set-group-ID program.
- •
- A process with a non-root user ID executed a binary that
conferred capabilities to the process.
- •
- A nonzero value may have been set by a Linux Security
Module.
Among the more important environment variables are the following:
-
LD_ASSUME_KERNEL (since glibc 2.2.3)
- Each shared object can inform the dynamic linker of the
minimum kernel ABI version that it requires. (This requirement is encoded
in an ELF note section that is viewable via readelf -n as a
section labeled NT_GNU_ABI_TAG.) At run time, the dynamic linker
determines the ABI version of the running kernel and will reject loading
shared objects that specify minimum ABI versions that exceed that ABI
version.
-
LD_ASSUME_KERNEL can be used to cause the dynamic
linker to assume that it is running on a system with a different kernel
ABI version. For example, the following command line causes the dynamic
linker to assume it is running on Linux 2.2.5 when loading the shared
objects required by myprog:
-
$ LD_ASSUME_KERNEL=2.2.5 ./myprog
- On systems that provide multiple versions of a shared
object (in different directories in the search path) that have different
minimum kernel ABI version requirements, LD_ASSUME_KERNEL can be
used to select the version of the object that is used (dependent on the
directory search order).
- Historically, the most common use of the
LD_ASSUME_KERNEL feature was to manually select the older
LinuxThreads POSIX threads implementation on systems that provided both
LinuxThreads and NPTL (which latter was typically the default on such
systems); see pthreads(7).
-
LD_BIND_NOW (since glibc 2.1.1)
- If set to a nonempty string, causes the dynamic linker to
resolve all symbols at program startup instead of deferring function call
resolution to the point when they are first referenced. This is useful
when using a debugger.
- LD_LIBRARY_PATH
- A list of directories in which to search for ELF libraries
at execution time. The items in the list are separated by either colons or
semicolons, and there is no support for escaping either separator. A
zero-length directory name indicates the current working directory.
- This variable is ignored in secure-execution mode.
- Within the pathnames specified in LD_LIBRARY_PATH,
the dynamic linker expands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the names) as
described above in Dynamic string tokens. Thus, for example, the
following would cause a library to be searched for in either the
lib or lib64 subdirectory below the directory containing the
program to be executed:
-
$ LD_LIBRARY_PATH='$ORIGIN/$LIB' prog
- (Note the use of single quotes, which prevent expansion of
$ORIGIN and $LIB as shell variables!)
- LD_PRELOAD
- A list of additional, user-specified, ELF shared objects to
be loaded before all others. This feature can be used to selectively
override functions in other shared objects.
- The items of the list can be separated by spaces or colons,
and there is no support for escaping either separator. The objects are
searched for using the rules given under DESCRIPTION. Objects are searched
for and added to the link map in the left-to-right order specified in the
list.
- In secure-execution mode, preload pathnames containing
slashes are ignored. Furthermore, shared objects are preloaded only from
the standard search directories and only if they have set-user-ID mode bit
enabled (which is not typical).
- Within the names specified in the LD_PRELOAD list,
the dynamic linker understands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the names) as
described above in Dynamic string tokens. (See also the discussion
of quoting under the description of LD_LIBRARY_PATH.)
- There are various methods of specifying libraries to be
preloaded, and these are handled in the following order:
- (1)
- The LD_PRELOAD environment variable.
- (2)
- The --preload command-line option when invoking the
dynamic linker directly.
- (3)
- The /etc/ld.so.preload file (described below).
- LD_TRACE_LOADED_OBJECTS
- If set (to any value), causes the program to list its
dynamic dependencies, as if run by ldd(1), instead of running
normally.
Then there are lots of more or less obscure variables, many obsolete or only for
internal use.
-
LD_AUDIT (since glibc 2.4)
- A list of user-specified, ELF shared objects to be loaded
before all others in a separate linker namespace (i.e., one that does not
intrude upon the normal symbol bindings that would occur in the process)
These objects can be used to audit the operation of the dynamic linker.
The items in the list are colon-separated, and there is no support for
escaping the separator.
-
LD_AUDIT is ignored in secure-execution mode.
- The dynamic linker will notify the audit shared objects at
so-called auditing checkpoints—for example, loading a new shared
object, resolving a symbol, or calling a symbol from another shared
object—by calling an appropriate function within the audit shared
object. For details, see rtld-audit(7). The auditing interface is
largely compatible with that provided on Solaris, as described in its
Linker and Libraries Guide, in the chapter Runtime Linker
Auditing Interface.
- Within the names specified in the LD_AUDIT list, the
dynamic linker understands the tokens $ORIGIN, $LIB, and
$PLATFORM (or the versions using curly braces around the names) as
described above in Dynamic string tokens. (See also the discussion
of quoting under the description of LD_LIBRARY_PATH.)
- Since glibc 2.13, in secure-execution mode, names in the
audit list that contain slashes are ignored, and only shared objects in
the standard search directories that have the set-user-ID mode bit enabled
are loaded.
-
LD_BIND_NOT (since glibc 2.1.95)
- If this environment variable is set to a nonempty string,
do not update the GOT (global offset table) and PLT (procedure linkage
table) after resolving a function symbol. By combining the use of this
variable with LD_DEBUG (with the categories bindings and
symbols), one can observe all run-time function bindings.
-
LD_DEBUG (since glibc 2.1)
- Output verbose debugging information about operation of the
dynamic linker. The content of this variable is one of more of the
following categories, separated by colons, commas, or (if the value is
quoted) spaces:
- help
- Specifying help in the value of this variable does
not run the specified program, and displays a help message about which
categories can be specified in this environment variable.
- all
- Print all debugging information (except statistics
and unused; see below).
- bindings
- Display information about which definition each symbol is
bound to.
- files
- Display progress for input file.
- libs
- Display library search paths.
- reloc
- Display relocation processing.
- scopes
- Display scope information.
- statistics
- Display relocation statistics.
- symbols
- Display search paths for each symbol look-up.
- unused
- Determine unused DSOs.
- versions
- Display version dependencies.
- Since glibc 2.3.4, LD_DEBUG is ignored in
secure-execution mode, unless the file /etc/suid-debug exists (the
content of the file is irrelevant).
-
LD_DEBUG_OUTPUT (since glibc 2.1)
- By default, LD_DEBUG output is written to standard
error. If LD_DEBUG_OUTPUT is defined, then output is written to the
pathname specified by its value, with the suffix "." (dot)
followed by the process ID appended to the pathname.
-
LD_DEBUG_OUTPUT is ignored in secure-execution
mode.
-
LD_DYNAMIC_WEAK (since glibc 2.1.91)
- By default, when searching shared libraries to resolve a
symbol reference, the dynamic linker will resolve to the first definition
it finds.
- Old glibc versions (before glibc 2.2), provided a different
behavior: if the linker found a symbol that was weak, it would remember
that symbol and keep searching in the remaining shared libraries. If it
subsequently found a strong definition of the same symbol, then it would
instead use that definition. (If no further symbol was found, then the
dynamic linker would use the weak symbol that it initially found.)
- The old glibc behavior was nonstandard. (Standard practice
is that the distinction between weak and strong symbols should have effect
only at static link time.) In glibc 2.2, the dynamic linker was modified
to provide the current behavior (which was the behavior that was provided
by most other implementations at that time).
- Defining the LD_DYNAMIC_WEAK environment variable
(with any value) provides the old (nonstandard) glibc behavior, whereby a
weak symbol in one shared library may be overridden by a strong symbol
subsequently discovered in another shared library. (Note that even when
this variable is set, a strong symbol in a shared library will not
override a weak definition of the same symbol in the main program.)
- Since glibc 2.3.4, LD_DYNAMIC_WEAK is ignored in
secure-execution mode.
-
LD_HWCAP_MASK (since glibc 2.1)
- Mask for hardware capabilities.
-
LD_ORIGIN_PATH (since glibc 2.1)
- Path where the binary is found.
- Since glibc 2.4, LD_ORIGIN_PATH is ignored in
secure-execution mode.
-
LD_POINTER_GUARD (from glibc 2.4 to glibc 2.22)
- Set to 0 to disable pointer guarding. Any other value
enables pointer guarding, which is also the default. Pointer guarding is a
security mechanism whereby some pointers to code stored in writable
program memory (return addresses saved by setjmp(3) or function
pointers used by various glibc internals) are mangled semi-randomly to
make it more difficult for an attacker to hijack the pointers for use in
the event of a buffer overrun or stack-smashing attack. Since glibc 2.23,
LD_POINTER_GUARD can no longer be used to disable pointer guarding,
which is now always enabled.
-
LD_PROFILE (since glibc 2.1)
- The name of a (single) shared object to be profiled,
specified either as a pathname or a soname. Profiling output is appended
to the file whose name is: "
$LD_PROFILE_OUTPUT/$LD_PROFILE.profile".
- Since glibc 2.2.5, LD_PROFILE is ignored in
secure-execution mode.
-
LD_PROFILE_OUTPUT (since glibc 2.1)
- Directory where LD_PROFILE output should be written.
If this variable is not defined, or is defined as an empty string, then
the default is /var/tmp.
-
LD_PROFILE_OUTPUT is ignored in secure-execution
mode; instead /var/profile is always used. (This detail is relevant
only before glibc 2.2.5, since in later glibc versions, LD_PROFILE
is also ignored in secure-execution mode.)
-
LD_SHOW_AUXV (since glibc 2.1)
- If this environment variable is defined (with any value),
show the auxiliary array passed up from the kernel (see also
getauxval(3)).
- Since glibc 2.3.4, LD_SHOW_AUXV is ignored in
secure-execution mode.
-
LD_TRACE_PRELINKING (since glibc 2.4)
- If this environment variable is defined, trace prelinking
of the object whose name is assigned to this environment variable. (Use
ldd(1) to get a list of the objects that might be traced.) If the
object name is not recognized, then all prelinking activity is
traced.
-
LD_USE_LOAD_BIAS (since glibc 2.3.3)
- By default (i.e., if this variable is not defined),
executables and prelinked shared objects will honor base addresses of
their dependent shared objects and (nonprelinked) position-independent
executables (PIEs) and other shared objects will not honor them. If
LD_USE_LOAD_BIAS is defined with the value 1, both executables and
PIEs will honor the base addresses. If LD_USE_LOAD_BIAS is defined
with the value 0, neither executables nor PIEs will honor the base
addresses.
- Since glibc 2.3.3, this variable is ignored in
secure-execution mode.
-
LD_VERBOSE (since glibc 2.1)
- If set to a nonempty string, output symbol versioning
information about the program if the LD_TRACE_LOADED_OBJECTS
environment variable has been set.
-
LD_WARN (since glibc 2.1.3)
- If set to a nonempty string, warn about unresolved
symbols.
-
LD_PREFER_MAP_32BIT_EXEC (x86-64 only; since glibc
2.23)
- According to the Intel Silvermont software optimization
guide, for 64-bit applications, branch prediction performance can be
negatively impacted when the target of a branch is more than 4 GB
away from the branch. If this environment variable is set (to any value),
the dynamic linker will first try to map executable pages using the
mmap(2) MAP_32BIT flag, and fall back to mapping without
that flag if that attempt fails. NB: MAP_32BIT will map to the low
2 GB (not 4 GB) of the address space.
- Because MAP_32BIT reduces the address range
available for address space layout randomization (ASLR),
LD_PREFER_MAP_32BIT_EXEC is always disabled in secure-execution
mode.
- /lib/ld.so
- a.out dynamic linker/loader
-
/lib/ld-linux.so.{1,2}
- ELF dynamic linker/loader
- /etc/ld.so.cache
- File containing a compiled list of directories in which to
search for shared objects and an ordered list of candidate shared objects.
See ldconfig(8).
- /etc/ld.so.preload
- File containing a whitespace-separated list of ELF shared
objects to be loaded before the program. See the discussion of
LD_PRELOAD above. If both LD_PRELOAD and
/etc/ld.so.preload are employed, the libraries specified by
LD_PRELOAD are preloaded first. /etc/ld.so.preload has a
system-wide effect, causing the specified libraries to be preloaded for
all programs that are executed on the system. (This is usually
undesirable, and is typically employed only as an emergency remedy, for
example, as a temporary workaround to a library misconfiguration
issue.)
- lib*.so*
- shared objects
Some shared objects are compiled using hardware-specific instructions which do
not exist on every CPU. Such objects should be installed in directories whose
names define the required hardware capabilities, such as
/usr/lib/sse2/. The dynamic linker checks these directories against the
hardware of the machine and selects the most suitable version of a given
shared object. Hardware capability directories can be cascaded to combine CPU
features. The list of supported hardware capability names depends on the CPU.
The following names are currently recognized:
- Alpha
- ev4, ev5, ev56, ev6, ev67
- MIPS
- loongson2e, loongson2f, octeon, octeon2
- PowerPC
- 4xxmac, altivec, arch_2_05, arch_2_06, booke, cellbe, dfp,
efpdouble, efpsingle, fpu, ic_snoop, mmu, notb, pa6t, power4, power5,
power5+, power6x, ppc32, ppc601, ppc64, smt, spe, ucache, vsx
- SPARC
- flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
- s390
- dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa,
stfle, z900, z990, z9-109, z10, zarch
- x86 (32-bit only)
- acpi, apic, clflush, cmov, cx8, dts, fxsr, ht, i386, i486,
i586, i686, mca, mmx, mtrr, pat, pbe, pge, pn, pse36, sep, ss, sse, sse2,
tm
ld(1),
ldd(1),
pldd(1),
sprof(1),
dlopen(3),
getauxval(3),
elf(5),
capabilities(7),
rtld-audit(7),
ldconfig(8),
sln(8)