Ubuntu Manpage: ld.so, ld-linux.so* - dynamic linker/loader

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* 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:

o (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.

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 (ELF only) 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).

       --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.

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.

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 (for non-set-user-ID programs).
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-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

COLOPHON

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