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 that modifies ELF shared libraries and ELF dynami‐
14 cally linked binaries in such a way that the time needed for the
15 dynamic linker to perform relocations at startup significantly
16 decreases. Due to fewer relocations, the run-time memory consumption
17 decreases as well (especially the number of unshareable pages). The
18 prelinking information is only used at startup time if none of the
19 dependent libraries have changed since prelinking; otherwise programs
20 are relocated normally.
21 prelink first collects ELF binaries to be prelinked and all the ELF
23 shared libraries they depend on. Then it assigns a unique virtual
24 address space slot to each library and relinks the shared library to
25 that base address. When the dynamic linker attempts to load such a
26 library, unless that virtual address space slot is already occupied, it
27 maps the library into the given slot. After this is done, prelink,
28 with the help of dynamic linker, resolves all relocations in the binary
29 or library against its dependent libraries and stores the relocations
30 into the ELF object. It also stores a list of all dependent libraries
31 together with their checksums into the binary or library. For bina‐
32 ries, it also computes a list of conflicts (relocations that resolve
33 differently in the binary's symbol search scope than in the smaller
34 search scope in which the dependent library was resolved) and stores it
35 into a special ELF section.
36 At runtime, the dynamic linker first checks whether all dependent
38 libraries were successfully mapped into their designated address space
39 slots, and whether they have not changed since the prelinking was done.
40 If all checks are successful, the dynamic linker just replays the list
41 of conflicts (which is usually significantly shorter than total number
42 of relocations) instead of relocating each library.
43
45 -v --verbose
46 Verbose mode. Print the virtual address slots assigned to
47 libraries and what binary or library is currently being pre‐
48 linked.
49 -n --dry-run
51 Don't actually prelink anything; just collect the bina‐
52 ries/libraries, assign them addresses, and with -v print what
53 would be prelinked.
54 -a --all
56 Prelink all binaries and dependent libraries found in directory
57 hierarchies specified in /etc/prelink.conf. Normally, only
58 binaries specified on the command line and their dependent
59 libraries are prelinked.
60 -m --conserve-memory
62 When assigning addresses to libraries, allow overlap of address
63 space slots provided that the two libraries are not present
64 together in any of the binaries or libraries. This results in a
65 smaller virtual address space range used for libraries. On the
66 other hand, if prelink sees a binary during incremental prelink‐
67 ing which puts together two libraries which were not present
68 together in any other binary and were given the same virtual
69 address space slots, then the binary cannot be prelinked. With‐
70 out this option, each library is assigned a unique virtual
71 address space slot.
72 -R --random
74 When assigning addresses to libraries, start with a random
75 address within the architecture-dependent virtual address space
76 range. This can make some buffer overflow attacks slightly
77 harder to exploit, because libraries are not present on the same
78 addresses across different machines. Normally, assigning vir‐
79 tual addresses starts at the bottom of the architecture-depen‐
80 dent range.
81 -r --reloc-only=ADDRESS
83 Instead of prelinking, just relink given shared libraries to the
84 specified base address.
85 -N --no-update-cache
87 Don't save the cache file after prelinking. Normally, the list
88 of libraries (and with -m binaries also) is stored into the
89 /etc/prelink.cache file together with their given address space
90 slots and dependencies, so the cache can be used during incre‐
91 mental prelinking (prelinking without -a option).
92 -c --config-file=CONFIG
94 Specify an alternate config file instead of default /etc/pre‐
95 link.conf.
96 -C --cache-file=CACHE
98 Specify an alternate cache file instead of default /etc/pre‐
99 link.cache.
100 -f --force
102 Force re-prelinking even for already prelinked objects whose
103 dependencies are unchanged. This option causes new virtual
104 address space slots to be assigned to all libraries. Normally,
105 only binaries or libraries which are either not prelinked yet,
106 or whose dependencies have changed, are prelinked.
107 -q --quick
109 Run prelink in quick mode. This mode checks just mtime and
110 ctime timestamps of libraries and binaries stored in the cache
111 file. If they are unchanged from the last prelink run, it is
112 assumed that the library in question did not change, without
113 parsing or verifying its ELF headers.
114 -p --print-cache
116 Print the contents of the cache file (normally /etc/pre‐
117 link.cache) and exit.
118 --dynamic-linker=LDSO
120 Specify an alternate dynamic linker instead of the default.
121 --ld-library-path=PATH
123 Specify a special LD_LIBRARY_PATH to be used when prelink
124 queries the dynamic linker about symbol resolution details.
125 --libs-only
127 Only prelink ELF shared libraries, don't prelink any binaries.
128 -h --dereference
130 When processing command line directory arguments, follow sym‐
131 bolic links when walking directory hierarchies.
132 -l --one-file-system
134 When processing command line directory arguments, limit direc‐
135 tory tree walk to a single file system.
136 -u --undo
138 Revert binaries and libraries to their original content before
139 they were prelinked. Without the -a option, this causes only
140 the binaries and libraries specified on the command line to be
141 reverted to their original state (and e.g. not their dependen‐
142 cies). If used together with the -a option, all binaries and
143 libraries from command line, all their dependencies, all bina‐
144 ries found in directories specified on command line and in the
145 config file, and all their dependencies are undone.
146 -y --verify
148 Verifies a prelinked binary or library. This option can be used
149 only on a single binary or library. It first applies an --undo
150 operation on the file, then prelinks just that file again and
151 compares this with the original file. If both are identical, it
152 prints the file after --undo operation on standard output and
153 exits with zero status. Otherwise it exits with error status.
154 Thus if --verify operation returns zero exit status and its
155 standard output is equal to the content of the binary or library
156 before prelinking, you can be sure that nobody modified the
157 binaries or libraries after prelinking. Similarly with message
158 digests and checksums (unless you trigger the improbable case of
159 modified file and original file having the same digest or check‐
160 sum).
161 --md5 This is similar to --verify option, except instead of outputting
163 the content of the binary or library before prelinking to stan‐
164 dard output, MD5 digest is printed. See md5sum(1).
165 --sha This is similar to --verify option, except instead of outputting
167 the content of the binary or library before prelinking to stan‐
168 dard output, SHA1 digest is printed. See sha1sum(1).
169 --exec-shield --no-exec-shield
171 On IA-32, if the kernel supports Exec-Shield, prelink attempts
172 to lay libraries out similarly to how the kernel places them
173 (i.e. if possible below the binary, most widely used into the
174 ASCII armor zone). These switches allow overriding prelink
175 detection of whether Exec-Shield is supported or not.
176 -b --black-list=PATH
178 This option allows blacklisting certain paths, libraries or
179 binaries. Prelink will not touch them during prelinking.
180 -o --undo-output=FILE
182 When performing an --undo operation, don't overwrite the pre‐
183 linked binary or library with its original content (before it
184 was prelinked), but save that into the specified file.
185 -V --version
187 Print version and exit.
188 -? --help
190 Print short help and exit.
191
193 Command-line arguments should be either directory hierarchies (in which
194 case -l and -h options apply), or particular ELF binaries or shared
195 libraries. Specifying a shared library explicitly on the command line
196 causes it to be prelinked even if no binary is linked against it. Oth‐
197 erwise, binaries are collected together and only the libraries they
198 depend on are prelinked with them.
199
201 # /usr/sbin/prelink -avmR
202 prelinks all binaries found in directories specified in /etc/pre‐
203 link.conf and all their dependent libraries, assigning libraries unique
204 virtual address space slots only if they ever appear together, and
205 starts assigning libraries at a random address.
206 # /usr/sbin/prelink -vm ~/bin/progx
207 prelinks ~/bin/progx program and all its dependent libraries (unless
208 they were prelinked already e.g. during prelink -a invocation).
209 # /usr/sbin/prelink -au
210 reverts all binaries and libraries to their original content.
211 # /usr/sbin/prelink -y /bin/prelinked_prog > /tmp/original_prog;
212 echo $? verifies whether /bin/prelinked_prog is unchanged.
213
215 /etc/prelink.cache Binary file containing a list of prelinked
216 libraries and/or binaries together with their
217 assigned virtual address space slots and dependen‐
218 cies. You can run /usr/sbin/prelink -p to see what
219 is stored in there.
220 /etc/prelink.conf Configuration file containing a list of directory
221 hierarchies that contain ELF shared libraries or
222 binaries which should be prelinked. This configu‐
223 ration file is used in -a mode to find binaries
224 which should be prelinked and also, no matter
225 whether -a is given or not, to limit which depen‐
226 dent shared libraries should be prelinked. If pre‐
227 link finds a dependent library of some binary or
228 other library which is not present in any of the
229 directories specified either in /etc/prelink.conf
230 or on the command line, then it cannot be pre‐
231 linked. Each line of the config file should be
232 either a comment starting with #, or a directory
233 name, or a blacklist specification. Directory
234 names can be prefixed by the -l switch, meaning the
235 tree walk of the given directory is only limited to
236 one file system; or the -h switch, meaning the tree
237 walk of the given directory follows symbolic links.
238 A blacklist specification should be prefixed by -b
239 and optionally also -l or -h if needed. A black‐
240 list entry can be either an absolute directory name
241 (in that case all files in that directory hierarchy
242 are ignored by the prelinker); an absolute filename
243 (then that particular library or binary is
244 skipped); or a glob pattern without a / character
245 in it (then all files matching that glob in any
246 directory are ignored).
248 ldd(1), ld.so(8).
250 prelink Some architectures, including IA-64 and HPPA, are not yet sup‐
251 ported.
253 Jakub Jelinek <jakub@redhat.com>.
254 01 March 2007 prelink(8)