ld.so.8: Note some further details of secure-execution mode

[thirdparty/man-pages.git] / man8 / ld.so.8

.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" This is in the public domain
.\" %%%LICENSE_END
.\"
.TH LD.SO 8 2019-02-27 "GNU" "Linux Programmer's Manual"
.SH NAME
ld.so, ld\-linux.so \- dynamic linker/loader
.SH 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
.B .interp
section of the program is executed) or directly by running:
.PP
.I /lib/ld\-linux.so.*
[OPTIONS] [PROGRAM [ARGUMENTS]]
.SH DESCRIPTION
The programs
.B ld.so
and
.B ld\-linux.so*
find and load the shared objects (shared libraries) needed by a program,
prepare the program to run, and then run it.
.PP
Linux binaries require dynamic linking (linking at run time)
unless the
.B \-static
option was given to
.BR ld (1)
during compilation.
.PP
The program
.B ld.so
handles a.out binaries, a format used long ago;
.B ld\-linux.so*
(\fI/lib/ld\-linux.so.1\fP for libc5, \fI/lib/ld\-linux.so.2\fP 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
.BR ldd (1),
.BR ldconfig (8),
and
.IR /etc/ld.so.conf .
.PP
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.
.PP
If a shared object dependency does not contain a slash,
then it is searched for in the following order:
.IP o 3
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.
.IP o
Using the environment variable
.BR LD_LIBRARY_PATH ,
unless the executable is being run in secure-execution mode (see below),
in which case this variable is ignored.
.IP 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.
.IP o
From the cache file
.IR /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
.B \-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.
.IP o
In the default path
.IR /lib ,
and then
.IR /usr/lib .
(On some 64-bit architectures, the default paths for 64-bit shared objects are
.IR /lib64 ,
and then
.IR /usr/lib64 .)
If the binary was linked with the
.B \-z nodeflib
linker option, this step is skipped.
.SS Rpath token expansion
.PP
The dynamic linker
understands certain token strings in an rpath specification
(DT_RPATH or DT_RUNPATH).
Those strings are substituted as follows:
.TP
.IR $ORIGIN " (or equivalently " ${ORIGIN} )
This expands to
the directory containing the program or shared object.
Thus, an application located in
.I somedir/app
could be compiled with
.IP
    gcc \-Wl,\-rpath,\(aq$ORIGIN/../lib\(aq
.IP
so that it finds an associated shared object in
.I somedir/lib
no matter where
.I 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.
.TP
.IR $LIB " (or equivalently " ${LIB} )
This expands to
.I lib
or
.I lib64
depending on the architecture
(e.g., on x86-64, it expands to
.IR lib64
and
on x86-32, it expands to
.IR lib ).
.TP
.IR $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
.BR AT_PLATFORM
value in the auxiliary vector (see
.BR getauxval (3)).
.\" To get an idea of the places that $PLATFORM would match,
.\" look at the output of the following:
.\"
.\"     mkdir /tmp/d
.\"     LD_LIBRARY_PATH=/tmp/d strace -e open /bin/date 2>&1 | grep /tmp/d
.\"
.\" ld.so lets names be abbreviated, so $O will work for $ORIGIN;
.\" Don't do this!!
.SH OPTIONS
.TP
.BI \-\-audit " list"
Use objects named in
.I list
as auditors.
The objects in
.I list
are delimited by colons.
.TP
.B \-\-inhibit\-cache
Do not use
.IR /etc/ld.so.cache .
.TP
.BI \-\-library\-path " path"
Use
.I path
instead of
.B LD_LIBRARY_PATH
environment variable setting (see below).
The names
.IR ORIGIN ,
.IR LIB ,
and
.IR PLATFORM
are interpreted as for the
.BR LD_LIBRARY_PATH
environment variable.
.TP
.BI \-\-inhibit\-rpath " list"
Ignore RPATH and RUNPATH information in object names in
.IR list .
This option is ignored when running in secure-execution mode (see below).
The objects in
.I list
are delimited by colons or spaces.
.TP
.B \-\-list
List all dependencies and how they are resolved.
.TP
.BR \-\-preload " \fIlist\fP (since glibc 2.30)"
Preload the objects specified in
.IR list .
The objects in
.I list
are delimited by colons or spaces.
The objects are preloaded as explained in the description of the
.BR LD_PRELOAD
environment variable below.
.IP
By contrast with
.BR LD_PRELOAD ,
the
.BR \-\-preload
option provides a way to perform preloading for a single executable
without affecting preloading performed in any child process that executes
a new program.
.TP
.B \-\-verify
Verify that program is dynamically linked and this dynamic linker can handle
it.
.SH ENVIRONMENT
Various environment variables influence the operation of the dynamic linker.
.\"
.SS 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:
.BR GCONV_PATH ,
.BR GETCONF_DIR ,
.BR HOSTALIASES ,
.BR LOCALDOMAIN ,
.BR LOCPATH ,
.BR MALLOC_TRACE ,
.BR NIS_PATH ,
.BR NLSPATH ,
.BR RESOLV_HOST_CONF ,
.BR RES_OPTIONS ,
.BR TMPDIR ,
and
.BR TZDIR .
.PP
A binary is executed in secure-execution mode if the
.B AT_SECURE
entry in the auxiliary vector (see
.BR getauxval (3))
has a nonzero value.
This entry may have a nonzero value for various reasons, including:
.IP * 3
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.
.IP *
A process with a non-root user ID executed a binary that
conferred capabilities to the process.
.IP *
A nonzero value may have been set by a Linux Security Module.
.\"
.SS Environment variables
Among the more important environment variables are the following:
.TP
.BR 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
.IR "readelf\ \-n"
as a section labeled
.BR 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.
.IP
.BR 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
.IR myprog :
.IP
.in +4n
.EX
$ \fBLD_ASSUME_KERNEL=2.2.5 ./myprog\fP
.EE
.in
.IP
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,
.BR LD_ASSUME_KERNEL
can be used to select the version of the object that is used
(dependent on the directory search order).
.IP
Historically, the most common use of the
.BR 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
.BR pthreads (7).
.TP
.BR 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.
.TP
.B 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.
.IP
This variable is ignored in secure-execution mode.
.IP
Within the pathnames specified in
.BR LD_LIBRARY_PATH ,
the dynamic linker expands the tokens
.IR $ORIGIN ,
.IR $LIB ,
and
.IR $PLATFORM
(or the versions using curly braces around the names)
as described above in
.IR "Rpath token expansion" .
Thus, for example,
the following would cause a library to be searched for in either the
.I lib
or
.I lib64
subdirectory below the directory containing the program to be executed:
.IP
.in +4n
.EX
$ \fBLD_LIBRARY_PATH='$ORIGIN/$LIB' prog\fP
.EE
.in
.IP
(Note the use of single quotes, which prevent expansion of
.I $ORIGIN
and
.I $LIB
as shell variables!)
.TP
.B 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.
.IP
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.
.IP
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).
.IP
Within the names specified in the
.BR LD_PRELOAD
list, the dynamic linker understands the tokens
.IR $ORIGIN ,
.IR $LIB ,
and
.IR $PLATFORM
(or the versions using curly braces around the names)
as described above in
.IR "Rpath token expansion" .
(See also the discussion of quoting under the description of
.BR LD_LIBRARY_PATH .)
.\" Tested with the following:
.\"
.\"     LD_PRELOAD='$LIB/libmod.so' LD_LIBRARY_PATH=. ./prog
.\"
.\" which will preload the libmod.so in 'lib' or 'lib64', using it
.\" in preference to the version in '.'.
.IP
There are various methods of specifying libraries to be preloaded,
and these are handled in the following order:
.RS
.IP (1) 4
The
.BR LD_PRELOAD
environment variable.
.IP (2)
The
.B \-\-preload
command-line option when invoking the dynamic linker directly.
.IP (3)
The
.I /etc/ld.so.preload
file (described below).
.RE
.TP
.BR LD_TRACE_LOADED_OBJECTS
If set (to any value), causes the program to list its dynamic
dependencies, as if run by
.BR ldd (1),
instead of running normally.
.PP
Then there are lots of more or less obscure variables,
many obsolete or only for internal use.
.TP
.BR 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)
and there is no support for escaping the separator.
These objects can be used to audit the operation of the dynamic linker.
.IP
.B LD_AUDIT
is ignored in secure-execution mode.
.IP
The dynamic linker will notify the audit
shared objects at so-called auditing checkpoints\(emfor example,
loading a new shared object, resolving a symbol,
or calling a symbol from another shared object\(emby
calling an appropriate function within the audit shared object.
For details, see
.BR rtld-audit (7).
The auditing interface is largely compatible with that provided on Solaris,
as described in its
.IR "Linker and Libraries Guide" ,
in the chapter
.IR "Runtime Linker Auditing Interface" .
.IP
Within the names specified in the
.BR LD_AUDIT
list, the dynamic linker understands the tokens
.IR $ORIGIN ,
.IR $LIB ,
and
.IR $PLATFORM
(or the versions using curly braces around the names)
as described above in
.IR "Rpath token expansion" .
(See also the discussion of quoting under the description of
.BR LD_LIBRARY_PATH .)
.IP
Since glibc 2.13,
.\" commit 8e9f92e9d5d7737afdacf79b76d98c4c42980508
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.
.TP
.BR 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
.BR LD_DEBUG
(with the categories
.IR bindings
and
.IR symbols ),
one can observe all run-time function bindings.
.TP
.BR 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:
.RS
.TP 12
.I help
Specifying
.IR 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.
.TP
.I all
Print all debugging information (except
.IR statistics
and
.IR unused ;
see below).
.TP
.I bindings
Display information about which definition each symbol is bound to.
.TP
.I files
Display progress for input file.
.TP
.I libs
Display library search paths.
.TP
.I reloc
Display relocation processing.
.TP
.I scopes
Display scope information.
.TP
.I statistics
Display relocation statistics.
.TP
.I symbols
Display search paths for each symbol look-up.
.TP
.I unused
Determine unused DSOs.
.TP
.I versions
Display version dependencies.
.RE
.IP
Since glibc 2.3.4,
.B LD_DEBUG
is ignored in secure-execution mode, unless the file
.IR /etc/suid\-debug
exists (the content of the file is irrelevant).
.TP
.BR LD_DEBUG_OUTPUT " (since glibc 2.1)"
By default,
.B LD_DEBUG
output is written to standard error.
If
.B 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.
.IP
.B LD_DEBUG_OUTPUT
is ignored in secure-execution mode.
.TP
.BR 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.
.IP
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.)
.IP
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,
.\" More precisely 2.1.92
.\" See weak handling
.\"     https://www.sourceware.org/ml/libc-hacker/2000-06/msg00029.html
.\"     To: GNU libc hacker <libc-hacker at sourceware dot cygnus dot com>
.\"     Subject: weak handling
.\"     From: Ulrich Drepper <drepper at redhat dot com>
.\"     Date: 07 Jun 2000 20:08:12 -0700
.\"     Reply-To: drepper at cygnus dot com (Ulrich Drepper)
the dynamic linker was modified to provide the current behavior
(which was the behavior that was provided by most other implementations
at that time).
.IP
Defining the
.B 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.)
.IP
Since glibc 2.3.4,
.B LD_DYNAMIC_WEAK
is ignored in secure-execution mode.
.TP
.BR LD_HWCAP_MASK " (since glibc 2.1)"
Mask for hardware capabilities.
.TP
.BR LD_ORIGIN_PATH " (since glibc 2.1)"
Path where the binary is found.
.\" Used only if $ORIGIN can't be determined by normal means
.\" (from the origin path saved at load time, or from /proc/self/exe)?
.IP
Since glibc 2.4,
.B LD_ORIGIN_PATH
is ignored in secure-execution mode.
.TP
.BR LD_POINTER_GUARD " (glibc from 2.4 to 2.22)"
Set to 0 to disable pointer guarding.
Any other value enables pointer guarding, which is also the default.
Pointer guarding is a security mechanism whereby some pointers to code
stored in writable program memory (return addresses saved by
.BR setjmp (3)
or function pointers used by various glibc internals) are mangled
semi-randomly to make it more difficult for an attacker to hijack
the pointers for use in the event of a buffer overrun or
stack-smashing attack.
Since glibc 2.23,
.\" commit a014cecd82b71b70a6a843e250e06b541ad524f7
.B LD_POINTER_GUARD
can no longer be used to disable pointer guarding,
which is now always enabled.
.TP
.BR LD_PROFILE " (since glibc 2.1)"
The name of a (single) shared object to be profiled,
specified either as a pathname or a soname.
Profiling output is appended to the file whose name is:
"\fI$LD_PROFILE_OUTPUT\fP/\fI$LD_PROFILE\fP.profile".
.IP
Since glibc 2.2.5,
.BR LD_PROFILE
is ignored in secure-execution mode.
.TP
.BR LD_PROFILE_OUTPUT " (since glibc 2.1)"
Directory where
.B LD_PROFILE
output should be written.
If this variable is not defined, or is defined as an empty string,
then the default is
.IR /var/tmp .
.IP
.B LD_PROFILE_OUTPUT
is ignored in secure-execution mode; instead
.IR /var/profile
is always used.
(This detail is relevant only before glibc 2.2.5,
since in later glibc versions,
.B LD_PROFILE
is also ignored in secure-execution mode.)
.TP
.BR 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
.BR getauxval (3)).
.IP
Since glibc 2.3.4,
.B LD_SHOW_AUXV
is ignored in secure-execution mode.
.TP
.BR 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
.BR ldd (1)
to get a list of the objects that might be traced.)
If the object name is not recognized,
.\" (This is what seems to happen, from experimenting)
then all prelinking activity is traced.
.TP
.BR LD_USE_LOAD_BIAS " (since glibc 2.3.3)"
.\" http://sources.redhat.com/ml/libc-hacker/2003-11/msg00127.html
.\" Subject: [PATCH] Support LD_USE_LOAD_BIAS
.\" Jakub Jelinek
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
.B LD_USE_LOAD_BIAS
is defined with the value 1, both executables and PIEs
will honor the base addresses.
If
.B LD_USE_LOAD_BIAS
is defined with the value 0,
neither executables nor PIEs will honor the base addresses.
.IP
Since glibc 2.3.3, this variable is ignored in secure-execution mode.
.TP
.BR LD_VERBOSE " (since glibc 2.1)"
If set to a nonempty string,
output symbol versioning information about the
program if the
.B LD_TRACE_LOADED_OBJECTS
environment variable has been set.
.TP
.BR LD_WARN " (since glibc 2.1.3)
If set to a nonempty string, warn about unresolved symbols.
.TP
.BR 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
.BR mmap (2)
.BR 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.
.IP
Because
.B MAP_32BIT
reduces the address range available for address space layout
randomization (ASLR),
.B LD_PREFER_MAP_32BIT_EXEC
is always disabled in secure-execution mode.
.SH FILES
.PD 0
.TP
.I /lib/ld.so
a.out dynamic linker/loader
.TP
.IR /lib/ld\-linux.so. { 1 , 2 }
ELF dynamic linker/loader
.TP
.I /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
.BR ldconfig (8).
.TP
.I /etc/ld.so.preload
File containing a whitespace-separated list of ELF shared objects to
be loaded before the program.
See the discussion of
.BR LD_PRELOAD
above.
If both
.BR LD_PRELOAD
and
.I /etc/ld.so.preload
are employed, the libraries specified by
.BR LD_PRELOAD
are preloaded first.
.I /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.)
.TP
.I lib*.so*
shared objects
.PD
.SH NOTES
.SS 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
.IR /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:
.TP
.B Alpha
ev4, ev5, ev56, ev6, ev67
.TP
.B MIPS
loongson2e, loongson2f, octeon, octeon2
.TP
.B 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
.TP
.B SPARC
flush, muldiv, stbar, swap, ultra3, v9, v9v, v9v2
.TP
.B s390
dfp, eimm, esan3, etf3enh, g5, highgprs, hpage, ldisp, msa, stfle,
z900, z990, z9-109, z10, zarch
.TP
.B 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
.SH SEE ALSO
.BR ld (1),
.BR ldd (1),
.BR pldd (1),
.BR sprof (1),
.BR dlopen (3),
.BR getauxval (3),
.BR elf (5),
.BR capabilities (7),
.BR rtld-audit (7),
.BR ldconfig (8),
.BR sln (8)
.\" .SH AUTHORS
.\" ld.so: David Engel, Eric Youngdale, Peter MacDonald, Hongjiu Lu, Linus
.\"  Torvalds, Lars Wirzenius and Mitch D'Souza
.\" ld\-linux.so: Roland McGrath, Ulrich Drepper and others.
.\"
.\" In the above, (libc5) stands for David Engel's ld.so/ld\-linux.so.