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

NAME

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

SYNOPSIS

       prelink [OPTION...] [FILES]

DESCRIPTION

       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 reloca-
       tions 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

       -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  pre-
	      linked.

       -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 verify-
	      ing 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.

       -T --timestamp-output
	      Prefix output with timestamps.

       -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 direc-
	      tories 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 --ver-
	      ify 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).

       -y --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).

       -y --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  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.  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

	      # /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

       /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

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

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

       Jakub Jelinek <jakub@redhat.com>.

								   01 March 2007							prelink(8)