SYNOPSIS
prelink [OPTION...] [FILES]
DESCRIPTION
prelink is a program which modifies ELF shared libraries and ELF dynam-
ically 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 (espe-
cially number of unshareable pages). Such prelinking information is
only used if 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 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 bina-
ries, 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.
OPTIONS
-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 bina-
ries/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 bina-
ries specified from command line and their dependant libraries
are prelinked.
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/pre-
link.cache file 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 file instead of default /etc/pre-
link.conf.
-C --cache-file=CACHE
Specify alternate cache file 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 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.
-p --print-cache
Print the content of the cache file (normally /etc/prelink.conf)
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-
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 check-
sum).
-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. Unlike when walking directory hierarchies, specifying a
shared library explicitely on the command line causes it to be pre-
linked even if 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 directories specified in /etc/pre-
link.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.
FILES
/etc/prelink.cache Binary file containing list of prelinked libraries
and/or binaries together with their assigned vir-
tual 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 con-
figuration file is used in -a mode to find binaries
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>.
14 July 2002 prelink(8)
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