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*
(/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 as 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. 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.

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
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)).

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.
(For details, see the discussion of individual environment variables below.) 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.

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
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. 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.
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.)

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
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.

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.  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

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

COLOPHON

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