ld-linux

The dynamic linker understands certain token 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)).

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.

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 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. 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 Rpath token expansion. (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 Rpath token expansion. (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 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 (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".

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.

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