Provided by: prelink_0.0.20131005-1_amd64 
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 relocations 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 prelinked.
-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 verifying 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.
--layout-page-size=SIZE
Layout start of libraries at given boundary.
--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 directories 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 --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 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 prelinked), but save that into the specified file.
-V --version
Print version and exit.
-? --help
Print short help and exit.
--usage
Print short usage message.
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>.
19 July 2013 prelink(8)