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prelink(8)									       prelink(8)

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

       prelink [OPTION...] [FILES]

       prelink	is a program which modifies ELF shared libraries and ELF dynamically linked bina-
       ries, so that the time which dynamic linker needs for their relocation at startup signifi-
       cantly  decreases  and  also  due  to  fewer  relocations  the run-time memory consumption
       decreases too (especially number of unshareable pages).	Such  prelinking  information  is
       only used if all its dependant libraries have not changed since prelinking, otherwise pro-
       grams 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 virtual 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 occu-
       pied, 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  success-
       fully  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 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 binaries/libraries, assign them
	      addresses and with -v print what would be prelinked.

       -a --all
	      Prelink all binaries and dependant libraries found in directory hierarchies  speci-
	      fied  in /etc/prelink.conf.  Normally only binaries specified from command line and
	      their dependant libraries are prelinked.

       -m --conserve-memory
	      When assigning addresses to libraries, allow overlap of address  space  slots  pro-
	      vided  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.
	      Normally each library is assigned a unique virtual address space slot.

       -R --random
	      When assigning addresses to libraries, start with random address	within	architec-
	      ture  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

       -N --no-update-cache
	      Don't  save  cache  file after prelinking. Normally, list of libraries (and with -m
	      binaries also) is stored into /etc/prelink.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/prelink.conf.

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

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

	      Specify alternate dynamic linker instead of the default.

	      Specify  special	LD_LIBRARY_PATH  to  be  used when prelink queries dynamic linker
	      about symbol resolution details.

	      Only prelink ELF shared libraries, don't prelink any binaries.

       -h --dereference
	      When processing command line directory arguments, follow symbolic links when  walk-
	      ing directory hierarchies.

       -l --one-file-system
	      When  processing	command  line directory arguments, limit directory 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 com-
	      mand line to be reverted to their original state (and e.g. not their dependencies).
	      If  used	together with -a option all binaries and libraries from command line, all
	      their dependencies, all binaries found in directories specified on command line and
	      in config 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 identi-
	      cal, 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 bina-
	      ries 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 checksum).

       -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.  Unlike when walking
       directory hierarchies, specifying a shared library explicitely on the command line  causes
       it  to  be  prelinked  even if no binary is linked against it. Normally, only binaries are
       collected together with all libraries they depend on.

	      # /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  pre-
       linked 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 ver-
	      ify whether /bin/prelinked_prog hasn't been changed.

       /etc/prelink.cache  Binary file containing list of  prelinked  libraries  and/or  binaries
			   together with their assigned virtual address space slots and dependen-
			   cies.  You can run /usr/sbin/prelink -p  to	see  what  is  stored  in
       /etc/prelink.conf   Configuration  file	containing  a list of directory hierarchies which
			   can contain ELF shared libraries or	binaries  which  should  be  pre-
			   linked.   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 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 directories specified in /etc/pre-
			   link.conf and neither in any of the directories specified on the  com-
			   mand  line, then it cannot be prelinked.  Each line of the config file
			   should be either comment starting with #, or directory name. It 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.
       ldd(1), ld.so(8).
       prelink Some architectures, including IA-64, HPPA and MIPS are not yet supported.
       Jakub Jelinek <jakub@redhat.com>.

					   14 July 2002 			       prelink(8)
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