NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ARGUMENTS | EXAMPLES | FILES | SEE ALSO | BUGS | AUTHORS | COLOPHON

prelink(8)                 System Manager's Manual                prelink(8)

NAME         top

       prelink  -  prelink  ELF  shared  libraries  and binaries to speed up
       startup time

SYNOPSIS         top

       prelink [OPTION...] [FILES]

DESCRIPTION         top

       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.

OPTIONS         top

       -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.

ARGUMENTS         top

       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.

EXAMPLES         top

              # /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.

FILES         top

       /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).

SEE ALSO         top

       ldd(1), ld.so(8).

BUGS         top

       prelink Some architectures, including IA-64 and HPPA, are not yet
       supported.

AUTHORS         top

       Jakub Jelinek <jakub@redhat.com>.

COLOPHON         top

       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)