ld.so(8) — Linux manual pages

Linux manual pages	Section 8

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* handles ELF (/lib/ld-linux.so.1 for libc5, /lib/ld-linux.so.2 for glibc2), 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:

(ELF only) 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.
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.
(ELF only) Using the directories specified in the DT_RUNPATH dynamic section attribute of the binary if present.
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.
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 (glibc since 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

(libc5; glibc since 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 shared objects in the standard search directories are loaded only if the set-user-ID mode bit is enabled on the shared object file.

Within the pathnames specified in LD_PRELOAD, 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

(ELF only) 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_AOUT_LIBRARY_PATH

(libc5) Version of LD_LIBRARY_PATH for a.out binaries only. Old versions of ld−linux.so.1 also supported LD_ELF_LIBRARY_PATH.

LD_AOUT_PRELOAD

(libc5) Version of LD_PRELOAD for a.out binaries only. Old versions of ld−linux.so.1 also supported LD_ELF_PRELOAD.

LD_AUDIT

(glibc since 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.

LD_BIND_NOT

(glibc since 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 symbol.

LD_DEBUG

(glibc since 2.1) Output verbose debugging information about the dynamic linker. If set to all prints all debugging information it has, if set to help prints a help message about which categories can be specified in this environment variable. 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

(glibc since 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

(glibc since 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

(glibc since 2.1) Mask for hardware capabilities.

LD_KEEPDIR

(a.out only)(libc5) Don't ignore the directory in the names of a.out libraries to be loaded. Use of this option is strongly discouraged.

LD_NOWARN

(a.out only)(libc5) Suppress warnings about a.out libraries with incompatible minor version numbers.

LD_ORIGIN_PATH

(glibc since 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

(glibc since 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

(glibc since 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

(glibc since 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

(glibc since 2.4) If this environment variable is defined (with any value), 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

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

(glibc since 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

(ELF only)(glibc since 2.1.3) If set to a nonempty string, warn about unresolved symbols.

LD_PREFER_MAP_32BIT_EXEC

(x86-64 only)(glibc since 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.

LDD_ARGV0

(libc5) argv[0] to be used by ldd(1) when none is present.

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

The ld.so functionality is available for executables compiled using libc version 4.4.3 or greater. ELF functionality is available since Linux 1.1.52 and libc5.

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-bitonly): 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

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