NAME¶
ld.so, ld-linux.so* - dynamic linker/loader
SYNOPSIS¶
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]]
DESCRIPTION¶
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 format used long ago;
ld-linux.so* (
/lib/ld-linux.so.1 for libc5,
/lib/ld-linux.so.2 for glibc2) handles ELF, which everybody has been
using for years now. Otherwise, both 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:
- o
- 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.
- o
- Using the environment variable LD_LIBRARY_PATH (unless the
executable is being run in secure-execution mode; see below). in which
case it is ignored.
- o
- Using the directories specified in the DT_RUNPATH dynamic section
attribute of the binary if present.
- o
- 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.
- o
- 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.
Rpath token expansion¶
ld.so understands certain strings in an rpath specification (DT_RPATH or
DT_RUNPATH); those strings are substituted 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)).
OPTIONS¶
- --list
- List all dependencies and how they are resolved.
- --verify
- Verify that program is dynamically linked and this dynamic linker can
handle it.
- --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).
- --audit list
- Use objects named in list as auditors.
ENVIRONMENT¶
Various environment variables influence the operation of the dynamic linker.
Secure-execution mode¶
For security reasons, the effects of some environment variables are voided or
modified if the dynamic linker determines that the binary should be run in
secure-execution mode. This determination is made by checking whether 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
permitted or effective capabilities.
- *
- A nonzero value may have been set by a Linux Security Module.
Environment variables¶
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. Similar to the PATH environment variable. This variable
is ignored in secure-execution mode.
Within the pathnames specified in LD_LIBRARY_PATH, the dynamic linker
expands the strings $ORIGIN, $LIB, and $PLATFORM (or
the versions using curly braces around the names) as described above in
Rpath token expansion. 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. The items of the list can be separated by spaces or
colons. This can be used to selectively override functions in other shared
objects. The objects are searched for using the rules given under
DESCRIPTION.
In secure-execution mode, preload pathnames containing slashes are ignored,
and only shared objects in the standard search directories that have the
set-user-ID mode bit enabled are loaded.
Within the names specified in the LD_PRELOAD list, the dynamic linker
understands the strings $ORIGIN, $LIB, and $PLATFORM
(or the versions using curly braces around the names) as described above
in Rpath token expansion.
- 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 colon-separated 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.
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 strings $ORIGIN, $LIB, and $PLATFORM
(or the versions using curly braces around the names) as described above
in Rpath token expansion.
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 the dynamic linker. If set to
all, print all debugging information, Setting this variable to
help does not run the specified program, and displays a help
message about which categories can be specified in this environment
variable. The categories are:
- 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.
- The value in LD_DEBUG can specify multiple categories, separated by
colons, commas, or (if the value is quoted) spaces.
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)
- File in which LD_DEBUG output should be written. The default is
standard error. LD_DEBUG_OUTPUT is ignored in secure-execution
mode.
- LD_DYNAMIC_WEAK (since glibc 2.1.91)
- If this environment variable is defined (with any value), allow weak
symbols to be overridden (reverting to old glibc behavior). 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 (glibc from 2.4 to 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".
- 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.
- 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 2.3.5, 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. 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 4GB away from the branch. If this
environment variable is set (to any value), ld.so 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 2GB (not 4GB) 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.
FILES¶
- /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.
- /etc/ld.so.preload
- File containing a whitespace-separated list of ELF shared objects to be
loaded before the program.
- lib*.so*
- shared objects
NOTES¶
Hardware capabilities¶
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
SEE ALSO¶
ld(1),
ldd(1),
pldd(1),
sprof(1),
dlopen(3),
getauxval(3),
capabilities(7),
rtld-audit(7),
ldconfig(8),
sln(8)
COLOPHON¶
This page is part of release 4.10 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.