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 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 (especially number of unshareable pages). Such prelinking information is only used if all its dependant libraries have not changed since prelinking, 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 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 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 binaries, 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.

-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 specified 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 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. Normally 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/prelink.cache file together with their given address space slots and dependencies, so it can be used during incremental prelinking (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.

-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 parsing of its ELF headers and verifying it is done.

-p --print-cache

Print the content of the cache file (normally /etc/prelink.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 symbolic links when walking 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 command 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 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 checksum).

-y --md5

This is similar to --verify option, except instead of outputing 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 outputing 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 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 prelinked), but save that into the specified file.

-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 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.

/etc/prelink.cache

Binary file containing 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 which can 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 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/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 specification. 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 specification 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 blacklisted).

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

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

Jakub Jelinek <jakub@redhat.com>.