1prelink(8) System Manager's Manual prelink(8)
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6 prelink - prelink ELF shared libraries and binaries to speed up startup
7 time
8
10 prelink [OPTION...] [FILES]
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13 prelink is a program which modifies ELF shared libraries and ELF dynam‐
14 ically linked binaries, so that the time which dynamic linker needs for
15 their relocation at startup significantly decreases and also due to
16 fewer relocations the run-time memory consumption decreases too (espe‐
17 cially number of unshareable pages). Such prelinking information is
18 only used if all its dependant libraries have not changed since pre‐
19 linking, otherwise programs are relocated normally.
20 prelink first collects ELF binaries which should be prelinked and all
22 the ELF shared libraries they depend on. Then it assigns a unique vir‐
23 tual address space slot for each library and relinks the shared library
24 to that base address. When the dynamic linker attempts to load such a
25 library, unless that virtual address space slot is already occupied, it
26 will map it into the given slot. After this is done, prelink with the
27 help of dynamic linker resolves all relocations in the binary or
28 library against its dependant libraries and stores the relocations into
29 the ELF object. It also stores a list of all dependant libraries
30 together with their checksums into the binary or library. For bina‐
31 ries, it also computes a list of conflicts (relocations which resolve
32 differently in the binary's symbol search scope than in the smaller
33 search scope in which the dependant library was resolved) and stores it
34 into a special ELF section.
35 At runtime, the dynamic linker first checks whether all dependant
37 libraries were successfully mapped into their designated address space
38 slots and whether they have not changed since the prelinking was done.
39 If all checks are successful, the dynamic linker just replays the list
40 of conflicts (which is usually significantly shorter than total number
41 of relocations) instead of relocating each library.
42
44 -v --verbose
45 Verbose mode. Print the virtual address slot assignment to
46 libraries and print what binary or library is currently being
47 prelinked.
48 -n --dry-run
50 Don't actually prelink anything, just collect the bina‐
51 ries/libraries, assign them addresses and with -v print what
52 would be prelinked.
53 -a --all
55 Prelink all binaries and dependant libraries found in directory
56 hierarchies specified in /etc/prelink.conf. Normally only bina‐
57 ries specified from command line and their dependant libraries
58 are prelinked.
59 -m --conserve-memory
61 When assigning addresses to libraries, allow overlap of address
62 space slots provided that the two libraries are not present
63 together in any of the binaries or libraries. This results in
64 smaller virtual address space range used for libraries, on the
65 other side if during incremental prelinking prelink sees a
66 binary which puts together two libraries which were not present
67 together in any other binary and were given the same virtual
68 address space slots, then the binary cannot be prelinked. Nor‐
69 mally each library is assigned a unique virtual address space
70 slot.
71 -R --random
73 When assigning addresses to libraries, start with random address
74 within architecture dependant virtual address space range. This
75 can make some buffer overflow attacks slightly harder to
76 exploit, because libraries are not present on the same addresses
77 accross different machines. Normally, assigning virtual
78 addresses starts at the bottom of architecture dependant range.
79 -r --reloc-only=ADDRESS
81 Instead of prelinking, just relink given shared libraries to the
82 specified base address.
83 -N --no-update-cache
85 Don't save cache file after prelinking. Normally, list of
86 libraries (and with -m binaries also) is stored into /etc/pre‐
87 link.cache file together with their given address space slots
88 and dependencies, so it can be used during incremental prelink‐
89 ing (prelinking without -a option).
90 -c --config-file=CONFIG
92 Specify alternate config file instead of default /etc/pre‐
93 link.conf.
94 -C --cache-file=CACHE
96 Specify alternate cache file instead of default /etc/pre‐
97 link.cache.
98 -f --force
100 Force re-prelinking even for already prelinked objects for which
101 no dependencies changed. This option causes new virtual address
102 space slots to be assigned to all libraries. Normally, only
103 binaries or libraries which are either not prelinked yet, or
104 some of their dependencies changed, are prelinked.
105 -q --quick
107 Run prelink in quick mode. This mode checks just mtime and
108 ctime timestamps of libraries and binaries stored in the cache
109 file. If they are unchanged from the last prelink run, it is
110 assumed that the library in question did not change and no pars‐
111 ing of its ELF headers and verifying it is done.
112 -p --print-cache
114 Print the content of the cache file (normally /etc/pre‐
115 link.cache) and exit.
116 --dynamic-linker=LDSO
118 Specify alternate dynamic linker instead of the default.
119 --ld-library-path=PATH
121 Specify special LD_LIBRARY_PATH to be used when prelink queries
122 dynamic linker about symbol resolution details.
123 --libs-only
125 Only prelink ELF shared libraries, don't prelink any binaries.
126 -h --dereference
128 When processing command line directory arguments, follow sym‐
129 bolic links when walking directory hierarchies.
130 -l --one-file-system
132 When processing command line directory arguments, limit direc‐
133 tory tree walk to a single filesystem.
134 -u --undo
136 Revert binaries and libraries to their original content before
137 they were prelinked. Without -a option this causes only the
138 binaries and libraries specified on the command line to be
139 reverted to their original state (and e.g. not their dependen‐
140 cies). If used together with -a option all binaries and
141 libraries from command line, all their dependencies, all bina‐
142 ries found in directories specified on command line and in con‐
143 fig file and all their dependencies will be undone.
144 -y --verify
146 Verifies a prelinked binary or library. This option can be used
147 only on a single binary or library. It first applies an --undo
148 operation on the file, then prelinks just that file again and
149 compares this with the original file. If both are identical, it
150 prints the file after --undo operation on standard output and
151 exit with zero status. Otherwise it exits with error status.
152 Thus if --verify operation returns zero exit status and its
153 standard output is equal to the content of the binary or library
154 before prelinking, you can be sure that nobody modified the
155 binaries or libraries after prelinking. Similarly with message
156 digests and checksums (unless you trigger the unprobable case of
157 modified file and original file having the same digest or check‐
158 sum).
159 --md5 This is similar to --verify option, except instead of outputing
161 the content of the binary or library before prelinking to stan‐
162 dard output MD5 digest is printed. See md5sum(1).
163 --sha This is similar to --verify option, except instead of outputing
165 the content of the binary or library before prelinking to stan‐
166 dard output SHA1 digest is printed. See sha1sum(1).
167 --exec-shield --no-exec-shield
169 On IA-32, if kernel supports Exec-Shield, prelink attempts to
170 lay libraries out similarly to how kernel places them (i.e. if
171 possible below the binary, most widely used into the ASCII armor
172 zone). These switches allow to override prelink detection of
173 whether Exec-Shield is supported or not.
174 -b --black-list=PATH
176 This option allows to blacklist certain paths, libraries or
177 binaries. Prelink will not touch them during prelinking.
178 -o --undo-output=FILE
180 When doing --undo operation, don't overwrite the prelinked
181 binary or library with its original content (before it was pre‐
182 linked), but save that into the specified file.
183 -V --version
185 Print version and exit.
186 -? --help
188 Print short help and exit.
189
191 Command line arguments should be either directory hierarchies (in which
192 case -l and -h options apply), or particular ELF binaries or shared
193 libraries. Unlike when walking directory hierarchies, specifying a
194 shared library explicitely on the command line causes it to be pre‐
195 linked even if no binary is linked against it. Normally, only binaries
196 are collected together with all libraries they depend on.
197
199 # /usr/sbin/prelink -avmR
200 will prelink all binaries found in directories specified in /etc/pre‐
201 link.conf and all their dependant libraries, assigning libraries unique
202 virtual address space slots only if they ever appear together and will
203 start assigning at random address.
204 # /usr/sbin/prelink -vm ~/bin/progx
205 will prelink ~/bin/progx program and all its dependant libraries
206 (unless they were prelinked already e.g. during prelink -a invocation).
207 # /usr/sbin/prelink -au
208 will revert all binaries and libraries to their original content.
209 # /usr/sbin/prelink -y /bin/prelinked_prog > /tmp/original_prog;
210 echo $? will verify whether /bin/prelinked_prog hasn't been
211 changed.
212
214 /etc/prelink.cache Binary file containing list of prelinked libraries
215 and/or binaries together with their assigned vir‐
216 tual address space slots and dependencies. You can
217 run /usr/sbin/prelink -p to see what is stored in
218 there.
219 /etc/prelink.conf Configuration file containing a list of directory
220 hierarchies which can contain ELF shared libraries
221 or binaries which should be prelinked. This con‐
222 figuration file is used in -a mode to find binaries
223 which should be prelinked and also no matter
224 whether -a is given or not to limit which dependant
225 shared libraries should be prelinked. If prelink
226 finds a dependant library of some binary or other
227 library which is not present in any of the directo‐
228 ries specified in /etc/prelink.conf and neither in
229 any of the directories specified on the command
230 line, then it cannot be prelinked. Each line of
231 the config file should be either comment starting
232 with #, or a directory name, or a blacklist speci‐
233 fication. Directory names can be prefixed by -l
234 switch, meaning tree walk of the given directory
235 will be only limited to one filesystem, or -h
236 switch, meaning tree walk of the given directory
237 will follow symbolic links. Blacklist specifica‐
238 tion should be prefixed by -b and optionally also
239 -l or -h if needed. It should be either absolute
240 directory name (in that case all files in that
241 directory hierarchy will be blacklisted), absolute
242 filename (in that case that particular library or
243 binary will not be touched by prelink) or a glob
244 pattern without / character in it (then all files
245 matching that glob in any directory will be black‐
246 listed).
248 ldd(1), ld.so(8).
250 prelink Some architectures, including IA-64, HPPA and MIPS, are not yet
251 supported.
253 Jakub Jelinek <jakub@redhat.com>.
254 23 November 2004 prelink(8)