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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ARGUMENTS | EXAMPLES | FILES | SEE ALSO | BUGS | AUTHORS | COLOPHON |
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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
prelink-20130503.tar.bz2 fetched from
⟨http://people.redhat.com/jakub/prelink/⟩ on 2025-02-02. If you
discover 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)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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