Various environment variables influence the operation of the
dynamic linker.
Secure-execution mode
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.
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, and there is no support for
escaping either separator. A zero-length directory name
indicates the current working directory.
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 Dynamic string tokens. 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 Dynamic string tokens. (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 Dynamic string tokens. (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.
