Provided by: prelink_0.0.20131005-1.1_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)