prelink(8)prelink(8)NAMEprelink - prelink ELF shared libraries and binaries to speed up startup
time
SYNOPSISprelink [OPTION...] [FILES]
DESCRIPTIONprelink is a program that modifies ELF shared libraries and ELF dynami‐
cally 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 bina‐
ries, 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 pre‐
linked.
-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 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 prelink‐
ing 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. With‐
out 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 vir‐
tual addresses starts at the bottom of the architecture-depen‐
dent 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 incre‐
mental prelinking (prelinking without -a option).
-c --config-file=CONFIG
Specify an alternate config file instead of default /etc/pre‐
link.conf.
-C --cache-file=CACHE
Specify an alternate cache file instead of default /etc/pre‐
link.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/pre‐
link.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.
--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 file system.
-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 dependen‐
cies). If used together with the -a option, all binaries and
libraries from command line, all their dependencies, all bina‐
ries 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 check‐
sum).
--md5 This is similar to --verify option, except instead of outputting
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 outputting
the content of the binary or library before prelinking to stan‐
dard 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 pre‐
linked 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.
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. Oth‐
erwise, 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/pre‐
link.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 dependen‐
cies. 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 configu‐
ration 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 depen‐
dent shared libraries should be prelinked. If pre‐
link 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 pre‐
linked. 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 black‐
list 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 ALSOldd(1), ld.so(8).
BUGSprelink Some architectures, including IA-64 and HPPA, are not yet sup‐
ported.
AUTHORS
Jakub Jelinek <jakub@redhat.com>.
01 March 2007 prelink(8)
