prelink(8) prelink(8) NAME prelink - prelink ELF shared libraries and binaries to speed up startup time(1,2,n) SYNOPSIS prelink [OPTION...] [FILES] DESCRIPTION prelink is a program which modifies ELF shared libraries and ELF dynam- ically linked binaries, so that the time(1,2,n) 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(3,n) 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(5,8) address space slot for each library and relinks the shared library to that base address. When the dynamic linker attempts to load(7,n) such a library, unless that virtual(5,8) 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(1,8) 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(1,8) the binary's symbol search scope than in(1,8) the smaller search scope in(1,8) 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(5,8) 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(1,8) directory hierarchies specified in(1,8) /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(1,8) any of the binaries or libraries. This results in(1,8) smaller virtual(5,8) address space range used for libraries, on the other side if(3,n) during incremental prelinking prelink sees a binary which puts(3,n) together two libraries which were not present together in(1,8) any other binary and were given the same virtual(5,8) address space slots, then the binary cannot be prelinked. Nor- mally each library is assigned a unique virtual(5,8) address space slot. -R --random When assigning addresses to libraries, start with random(3,4,6) address within architecture dependant virtual(5,8) 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(5,8) 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(1,n) after prelinking. Normally, list of libraries (and with -m binaries also) is stored into /etc/pre- link.cache file(1,n) 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(1,5) file(1,n) instead of default /etc/pre- link.conf. -C --cache-file=CACHE Specify alternate cache file(1,n) 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(5,8) 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(1,8) quick mode. This mode checks just mtime and ctime timestamps of libraries and binaries stored in(1,8) the cache file. If they are unchanged from the last prelink run, it is assumed that the library in(1,8) 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(1,n) (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(1,8) directories specified on command line and in(1,8) con- fig(1,5) file(1,n) 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(1,n), then prelinks just that file(1,n) again and compares this with the original file. If both are identical, it prints the file(1,n) after --undo operation on standard output and exit(3,n,1 builtins) with zero status. Otherwise it exits with error(8,n) status. Thus if(3,n) --verify operation returns zero exit(3,n,1 builtins) 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(1,n) and original file(1,n) 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). -V --version Print version(1,3,5) and exit. -? --help Print short help and exit. ARGUMENTS Command line arguments should be either directory hierarchies (in(1,8) 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(3,n) 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(1,8) directories specified in(1,8) /etc/pre- link.conf and all their dependant libraries, assigning libraries unique virtual(5,8) address space slots only if(3,n) they ever appear together and will start assigning at random(3,4,6) 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(1,3x,1 builtins) $? will verify(1,8) whether /bin/prelinked_prog hasn't been changed. FILES /etc/prelink.cache Binary file(1,n) containing list of prelinked libraries and/or binaries together with their assigned vir- tual(5,8) address space slots and dependencies. You can run /usr/sbin/prelink -p to see what is stored in(1,8) there. /etc/prelink.conf Configuration file(1,n) containing a list of directory hierarchies which can contain ELF shared libraries or binaries which should be prelinked. This con- figuration file(1,n) is used in(1,8) -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(1,8) any of the directo- ries specified in(1,8) /etc/prelink.conf and neither in(1,8) any of the directories specified on the command line, then it cannot be prelinked. Each line of the config(1,5) file(1,n) should be either comment starting with #, or directory name. It can be prefixed by -l switch(1,n), meaning tree walk of the given directory will be only limited to one filesystem, or -h switch(1,n), meaning tree walk of the given directory will follow symbolic links. 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>. 5 August 2003 prelink(8)