prelink



prelink(8)                                                          prelink(8)




NAME

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


SYNOPSIS

       prelink [OPTION...] [FILES]


DESCRIPTION

       prelink is a program which modifies ELF shared libraries and ELF dynam-
       ically linked binaries, so that the time which dynamic linker needs for
       their relocation at startup significantly decreases  and  also  due  to
       fewer  relocations the run-time memory consumption decreases too (espe-
       cially number of unshareable pages).  Such  prelinking  information  is
       only  used  if  all its dependant libraries have not changed since pre-
       linking, otherwise programs are relocated normally.

       prelink first collects ELF binaries which should be prelinked  and  all
       the  ELF shared libraries they depend on. Then it assigns a unique vir-
       tual address space slot for 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
       will  map it 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 dependant libraries and stores the relocations into
       the ELF object.  It also stores  a  list  of  all  dependant  libraries
       together  with  their  checksums into the binary or library.  For bina-
       ries, it also computes a list of conflicts (relocations  which  resolve
       differently  in  the  binary’s  symbol search scope than in the smaller
       search scope in which the dependant library was resolved) and stores it
       into a special ELF section.

       At  runtime,  the  dynamic  linker  first  checks whether all dependant
       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

       -v --verbose
              Verbose mode.  Print the  virtual  address  slot  assignment  to
              libraries  and  print  what binary or library is currently being
              prelinked.

       -n --dry-run
              Don’t  actually  prelink  anything,  just  collect   the   bina-
              ries/libraries,  assign  them  addresses  and with -v print what
              would be prelinked.

       -a --all
              Prelink all binaries and dependant libraries found in  directory
              hierarchies specified in /etc/prelink.conf.  Normally only bina-
              ries specified from command line and their  dependant  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
              smaller virtual address space range used for libraries,  on  the
              other  side  if  during  incremental  prelinking  prelink sees a
              binary 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.   Nor-
              mally  each  library  is assigned a unique virtual address space
              slot.

       -R --random
              When assigning addresses to libraries, start with random address
              within architecture dependant virtual address space range.  This
              can  make  some  buffer  overflow  attacks  slightly  harder  to
              exploit, because libraries are not present on the same addresses
              accross  different  machines.    Normally,   assigning   virtual
              addresses  starts at the bottom of architecture dependant 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  cache  file  after  prelinking.  Normally,  list of
              libraries (and with -m binaries also) is stored  into  /etc/pre-
              link.cache  file  together  with their given address space slots
              and dependencies, so it can be used during incremental  prelink-
              ing (prelinking without -a option).

       -c --config-file=CONFIG
              Specify  alternate  config  file  instead  of  default /etc/pre-
              link.conf.

       -C --cache-file=CACHE
              Specify  alternate  cache  file  instead  of  default  /etc/pre-
              link.cache.

       -f --force
              Force re-prelinking even for already prelinked objects for which
              no dependencies changed. 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
              some of their dependencies 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 and no pars-
              ing of its ELF headers and verifying it is done.

       -p --print-cache
              Print   the  content  of  the  cache  file  (normally  /etc/pre-
              link.cache) and exit.

       --dynamic-linker=LDSO
              Specify alternate dynamic linker instead of the default.

       --ld-library-path=PATH
              Specify special LD_LIBRARY_PATH to be used when prelink  queries
              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 sym-
              bolic links when walking directory hierarchies.

       -l --one-file-system
              When processing command line directory arguments,  limit  direc-
              tory tree walk to a single filesystem.

       -u --undo
              Revert  binaries  and libraries to their original content before
              they were prelinked.  Without -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  dependen-
              cies).  If  used  together  with  -a  option  all  binaries  and
              libraries from command line, all their dependencies,  all  bina-
              ries  found in directories specified on command line and in con-
              fig file and all their dependencies will be 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
              exit  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 unprobable case of
              modified file and original file having the same digest or check-
              sum).

       --md5  This is similar to --verify option, except instead of  outputing
              the  content of the binary or library before prelinking to stan-
              dard output MD5 digest is printed.  See md5sum(1).

       --sha  This is similar to --verify option, except instead of  outputing
              the  content of the binary or library before prelinking to stan-
              dard output SHA1 digest is printed.  See sha1sum(1).

       --exec-shield --no-exec-shield
              On IA-32, if kernel supports Exec-Shield,  prelink  attempts  to
              lay  libraries  out similarly to how kernel places them (i.e. if
              possible below the binary, most widely used into the ASCII armor
              zone).   These  switches  allow to override prelink detection of
              whether Exec-Shield is supported or not.

       -b --black-list=PATH
              This option allows to  blacklist  certain  paths,  libraries  or
              binaries.  Prelink will not touch them during prelinking.

       -o --undo-output=FILE
              When  doing  --undo  operation,  don’t  overwrite  the prelinked
              binary or library with its original content (before it was  pre-
              linked), but save that into the specified file.

       -V --version
              Print version and exit.

       -? --help
              Print short help and exit.


ARGUMENTS

       Command line arguments should be either directory hierarchies (in which
       case -l and -h options apply), or particular  ELF  binaries  or  shared
       libraries.   Unlike  when  walking  directory hierarchies, specifying a
       shared library explicitely on the command line causes  it  to  be  pre-
       linked  even if no binary is linked against it. Normally, only binaries
       are collected together with all libraries they depend on.


EXAMPLES

              # /usr/sbin/prelink -avmR
       will  prelink  all  binaries  found   in   directories   specified   in
       /etc/prelink.conf   and   all   their  dependant  libraries,  assigning
       libraries unique virtual address space slots only if they  ever  appear
       together and will start assigning at random address.
              # /usr/sbin/prelink -vm ~/bin/progx
       will  prelink  ~/bin/progx  program  and  all  its  dependant libraries
       (unless they were prelinked already e.g. during prelink -a invocation).
              # /usr/sbin/prelink -au
       will revert all binaries and libraries to their original content.
              # /usr/sbin/prelink -y /bin/prelinked_prog > /tmp/original_prog;
              echo $?  will verify  whether  /bin/prelinked_prog  hasn’t  been
              changed.


FILES

       /etc/prelink.cache  Binary  file containing list of prelinked libraries
                           and/or binaries together with their  assigned  vir-
                           tual 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 which can contain ELF shared  libraries
                           or  binaries  which should be prelinked.  This con-
                           figuration 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 dependant
                           shared  libraries  should  be prelinked. If prelink
                           finds a dependant library of some binary  or  other
                           library which is not present in any of the directo-
                           ries specified in /etc/prelink.conf and neither  in
                           any  of  the  directories  specified on the command
                           line, then it cannot be prelinked.   Each  line  of
                           the  config  file should be either comment starting
                           with #, or a directory name, or a blacklist  speci-
                           fication.   Directory  names  can be prefixed by -l
                           switch, meaning tree walk of  the  given  directory
                           will  be  only  limited  to  one  filesystem, or -h
                           switch, meaning tree walk of  the  given  directory
                           will  follow  symbolic links.  Blacklist specifica-
                           tion should be prefixed by -b and  optionally  also
                           -l  or  -h if needed.  It should be either absolute
                           directory name (in that  case  all  files  in  that
                           directory  hierarchy will be blacklisted), absolute
                           filename (in that case that particular  library  or
                           binary  will  not  be touched by prelink) or a glob
                           pattern without / character in it (then  all  files
                           matching  that glob in any directory will be black-
                           listed).

SEE ALSO

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

BUGS

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

AUTHORS

       Jakub Jelinek <jakub@redhat.com>.



                               23 November 2004                     prelink(8)

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