|
NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENVIRONMENT | FILES | NOTES | SEE ALSO | COLOPHON |
|
LD.SO(8) Linux Programmer's Manual LD.SO(8)
ld.so, ld-linux.so - dynamic linker/loader
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]]
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)).
--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.
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 dif‐
ferent minimum kernel ABI version requirements, LD_ASSUME_KER‐
NEL 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 fea‐
ture was to manually select the older LinuxThreads POSIX
threads implementation on systems that provided both Linux‐
Threads 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 ref‐
erenced. 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 vari‐
able.
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 fol‐
lowing 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 sepa‐
rated 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 nor‐
mally.
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 bind‐
ings 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 combin‐
ing the use of this variable with LD_DEBUG (with the cate‐
gories 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 sym‐
bol reference, the dynamic linker will resolve to the first
definition it finds.
Old glibc versions (before 2.2), provided a different behav‐
ior: 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 defini‐
tion of the same symbol, then it would instead use that defi‐
nition. (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 imple‐
mentations 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-execu‐
tion 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 inter‐
nals) 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_OUT‐
PUT/$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 vari‐
able. (Use ldd(1) to get a list of the objects that might be
traced.) If the object name is not recognized, then all pre‐
linking activity is traced.
LD_USE_LOAD_BIAS (since glibc 2.3.3)
By default (i.e., if this variable is not defined), executa‐
bles 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 exe‐
cutables nor PIEs will honor the base addresses.
Since glibc 2.3.3, this variable is ignored in secure-execu‐
tion mode.
LD_VERBOSE (since glibc 2.1)
If set to a nonempty string, output symbol versioning informa‐
tion about the program if the LD_TRACE_LOADED_OBJECTS environ‐
ment 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_PRE‐
FER_MAP_32BIT_EXEC is always disabled in secure-execution
mode.
/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
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
ld(1), ldd(1), pldd(1), sprof(1), dlopen(3), getauxval(3), elf(5),
capabilities(7), rtld-audit(7), ldconfig(8), sln(8)
This page is part of release 4.15 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/.
GNU 2017-09-15 LD.SO(8)
Pages that refer to this page: ldd(1), memusage(1), pldd(1), sprof(1), execve(2), prctl(2), uselib(2), dladdr(3), dlinfo(3), dl_iterate_phdr(3), dlopen(3), dlsym(3), getauxval(3), lttng-ust(3), lttng-ust-cyg-profile(3), environ(7), rtld-audit(7), execstack(8), ldconfig(8), prelink(8), sln(8)
Copyright and license for this manual page
|
HTML rendering created 2018-02-02 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH.
|
|