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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ARGUMENTS | EXAMPLES | FILES | SEE ALSO | BUGS | AUTHORS | COLOPHON |
prelink(8) System Manager's Manual prelink(8)
prelink - prelink ELF shared libraries and binaries to speed up
startup time
prelink [OPTION...] [FILES]
prelink is a program that modifies ELF shared libraries and ELF
dynamically linked binaries in such a way that the time needed for
the dynamic linker to perform relocations at startup significantly
decreases. Due to fewer relocations, the run-time memory consumption
decreases as well (especially the number of unshareable pages). The
prelinking information is only used at startup time if none of the
dependent libraries have changed since prelinking; otherwise programs
are relocated normally.
prelink first collects ELF binaries to be prelinked and all the ELF
shared libraries they depend on. Then it assigns a unique virtual
address space slot to each library and relinks the shared library to
that base address. When the dynamic linker attempts to load such a
library, unless that virtual address space slot is already occupied,
it maps the library into the given slot. After this is done,
prelink, with the help of dynamic linker, resolves all relocations in
the binary or library against its dependent libraries and stores the
relocations into the ELF object. It also stores a list of all
dependent libraries together with their checksums into the binary or
library. For binaries, it also computes a list of conflicts
(relocations that resolve differently in the binary's symbol search
scope than in the smaller search scope in which the dependent library
was resolved) and stores it into a special ELF section.
At runtime, the dynamic linker first checks whether all dependent
libraries were successfully mapped into their designated address
space slots, and whether they have not changed since the prelinking
was done. If all checks are successful, the dynamic linker just
replays the list of conflicts (which is usually significantly shorter
than total number of relocations) instead of relocating each library.
-v --verbose
Verbose mode. Print the virtual address slots assigned to
libraries and what binary or library is currently being
prelinked.
-n --dry-run
Don't actually prelink anything; just collect the
binaries/libraries, assign them addresses, and with -v print
what would be prelinked.
-a --all
Prelink all binaries and dependent libraries found in
directory hierarchies specified in /etc/prelink.conf.
Normally, only binaries specified on the command line and
their dependent libraries are prelinked.
-m --conserve-memory
When assigning addresses to libraries, allow overlap of
address space slots provided that the two libraries are not
present together in any of the binaries or libraries. This
results in a smaller virtual address space range used for
libraries. On the other hand, if prelink sees a binary during
incremental prelinking which puts together two libraries which
were not present together in any other binary and were given
the same virtual address space slots, then the binary cannot
be prelinked. Without this option, each library is assigned a
unique virtual address space slot.
-R --random
When assigning addresses to libraries, start with a random
address within the architecture-dependent virtual address
space range. This can make some buffer overflow attacks
slightly harder to exploit, because libraries are not present
on the same addresses across different machines. Normally,
assigning virtual addresses starts at the bottom of the
architecture-dependent range.
-r --reloc-only=ADDRESS
Instead of prelinking, just relink given shared libraries to
the specified base address.
-N --no-update-cache
Don't save the cache file after prelinking. Normally, the
list of libraries (and with -m binaries also) is stored into
the /etc/prelink.cache file together with their given address
space slots and dependencies, so the cache can be used during
incremental prelinking (prelinking without -a option).
-c --config-file=CONFIG
Specify an alternate config file instead of default
/etc/prelink.conf.
-C --cache-file=CACHE
Specify an alternate cache file instead of default
/etc/prelink.cache.
-f --force
Force re-prelinking even for already prelinked objects whose
dependencies are unchanged. This option causes new virtual
address space slots to be assigned to all libraries.
Normally, only binaries or libraries which are either not
prelinked yet, or whose dependencies have changed, are
prelinked.
-q --quick
Run prelink in quick mode. This mode checks just mtime and
ctime timestamps of libraries and binaries stored in the cache
file. If they are unchanged from the last prelink run, it is
assumed that the library in question did not change, without
parsing or verifying its ELF headers.
-p --print-cache
Print the contents of the cache file (normally
/etc/prelink.cache) and exit.
--dynamic-linker=LDSO
Specify an alternate dynamic linker instead of the default.
--ld-library-path=PATH
Specify a special LD_LIBRARY_PATH to be used when prelink
queries the dynamic linker about symbol resolution details.
--libs-only
Only prelink ELF shared libraries, don't prelink any binaries.
-h --dereference
When processing command line directory arguments, follow
symbolic links when walking directory hierarchies.
-l --one-file-system
When processing command line directory arguments, limit
directory tree walk to a single file system.
-u --undo
Revert binaries and libraries to their original content before
they were prelinked. Without the -a option, this causes only
the binaries and libraries specified on the command line to be
reverted to their original state (and e.g. not their
dependencies). If used together with the -a option, all
binaries and libraries from command line, all their
dependencies, all binaries found in directories specified on
command line and in the config file, and all their
dependencies are undone.
-y --verify
Verifies a prelinked binary or library. This option can be
used only on a single binary or library. It first applies an
--undo operation on the file, then prelinks just that file
again and compares this with the original file. If both are
identical, it prints the file after --undo operation on
standard output and exits with zero status. Otherwise it exits
with error status. Thus if --verify operation returns zero
exit status and its standard output is equal to the content of
the binary or library before prelinking, you can be sure that
nobody modified the binaries or libraries after prelinking.
Similarly with message digests and checksums (unless you
trigger the improbable case of modified file and original file
having the same digest or checksum).
--md5 This is similar to --verify option, except instead of
outputting the content of the binary or library before
prelinking to standard output, MD5 digest is printed. See
md5sum(1).
--sha This is similar to --verify option, except instead of
outputting the content of the binary or library before
prelinking to standard output, SHA1 digest is printed. See
sha1sum(1).
--exec-shield --no-exec-shield
On IA-32, if the kernel supports Exec-Shield, prelink attempts
to lay libraries out similarly to how the kernel places them
(i.e. if possible below the binary, most widely used into the
ASCII armor zone). These switches allow overriding prelink
detection of whether Exec-Shield is supported or not.
-b --black-list=PATH
This option allows blacklisting certain paths, libraries or
binaries. Prelink will not touch them during prelinking.
-o --undo-output=FILE
When performing an --undo operation, don't overwrite the
prelinked binary or library with its original content (before
it was prelinked), but save that into the specified file.
-V --version
Print version and exit.
-? --help
Print short help and exit.
Command-line arguments should be either directory hierarchies (in
which case -l and -h options apply), or particular ELF binaries or
shared libraries. Specifying a shared library explicitly on the
command line causes it to be prelinked even if no binary is linked
against it. Otherwise, binaries are collected together and only the
libraries they depend on are prelinked with them.
# /usr/sbin/prelink -avmR
prelinks all binaries found in directories specified in
/etc/prelink.conf and all their dependent libraries, assigning
libraries unique virtual address space slots only if they ever appear
together, and starts assigning libraries at a random address.
# /usr/sbin/prelink -vm ~/bin/progx
prelinks ~/bin/progx program and all its dependent libraries (unless
they were prelinked already e.g. during prelink -a invocation).
# /usr/sbin/prelink -au
reverts all binaries and libraries to their original content.
# /usr/sbin/prelink -y /bin/prelinked_prog >
/tmp/original_prog; echo $? verifies whether
/bin/prelinked_prog is unchanged.
/etc/prelink.cache Binary file containing a list of prelinked
libraries and/or binaries together with their
assigned virtual address space slots and
dependencies. You can run /usr/sbin/prelink -p
to see what is stored in there.
/etc/prelink.conf Configuration file containing a list of directory
hierarchies that contain ELF shared libraries or
binaries which should be prelinked. This
configuration file is used in -a mode to find
binaries which should be prelinked and also, no
matter whether -a is given or not, to limit which
dependent shared libraries should be prelinked.
If prelink finds a dependent library of some
binary or other library which is not present in
any of the directories specified either in
/etc/prelink.conf or on the command line, then it
cannot be prelinked. Each line of the config
file should be either a comment starting with #,
or a directory name, or a blacklist
specification. Directory names can be prefixed
by the -l switch, meaning the tree walk of the
given directory is only limited to one file
system; or the -h switch, meaning the tree walk
of the given directory follows symbolic links. A
blacklist specification should be prefixed by -b
and optionally also -l or -h if needed. A
blacklist entry can be either an absolute
directory name (in that case all files in that
directory hierarchy are ignored by the
prelinker); an absolute filename (then that
particular library or binary is skipped); or a
glob pattern without a / character in it (then
all files matching that glob in any directory are
ignored).
ldd(1), ld.so(8).
prelink Some architectures, including IA-64 and HPPA, are not yet
supported.
Jakub Jelinek <jakub@redhat.com>.
This page is part of the prelink (prelink ELF shared libraries and
binaries) project. Information about the project can be found at
⟨http://people.redhat.com/jakub/prelink/⟩. It is not known how to
report bugs for this man page; if you know, please send a mail to
man-pages@man7.org. This page was obtained from the tarball pre‐
link-20130503.tar.bz2 fetched from
⟨http://people.redhat.com/jakub/prelink/⟩ on 2017-07-05. If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
01 March 2007 prelink(8)