1prelink(8) System Manager's Manual prelink(8)
2
6 prelink - prelink ELF shared libraries and binaries to speed up startup
7 time
8
10 prelink [OPTION...] [FILES]
11
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 --layout-page-size=SIZE
127 Layout start of libraries at given boundary.
128 --libs-only
130 Only prelink ELF shared libraries, don't prelink any binaries.
131 -h --dereference
133 When processing command line directory arguments, follow sym‐
134 bolic links when walking directory hierarchies.
135 -l --one-file-system
137 When processing command line directory arguments, limit direc‐
138 tory tree walk to a single file system.
139 -u --undo
141 Revert binaries and libraries to their original content before
142 they were prelinked. Without the -a option, this causes only
143 the binaries and libraries specified on the command line to be
144 reverted to their original state (and e.g. not their dependen‐
145 cies). If used together with the -a option, all binaries and
146 libraries from command line, all their dependencies, all bina‐
147 ries found in directories specified on command line and in the
148 config file, and all their dependencies are undone.
149 -y --verify
151 Verifies a prelinked binary or library. This option can be used
152 only on a single binary or library. It first applies an --undo
153 operation on the file, then prelinks just that file again and
154 compares this with the original file. If both are identical, it
155 prints the file after --undo operation on standard output and
156 exits with zero status. Otherwise it exits with error status.
157 Thus if --verify operation returns zero exit status and its
158 standard output is equal to the content of the binary or library
159 before prelinking, you can be sure that nobody modified the
160 binaries or libraries after prelinking. Similarly with message
161 digests and checksums (unless you trigger the improbable case of
162 modified file and original file having the same digest or check‐
163 sum).
164 --md5 This is similar to --verify option, except instead of outputting
166 the content of the binary or library before prelinking to stan‐
167 dard output, MD5 digest is printed. See md5sum(1).
168 --sha This is similar to --verify option, except instead of outputting
170 the content of the binary or library before prelinking to stan‐
171 dard output, SHA1 digest is printed. See sha1sum(1).
172 --exec-shield --no-exec-shield
174 On IA-32, if the kernel supports Exec-Shield, prelink attempts
175 to lay libraries out similarly to how the kernel places them
176 (i.e. if possible below the binary, most widely used into the
177 ASCII armor zone). These switches allow overriding prelink
178 detection of whether Exec-Shield is supported or not.
179 -b --black-list=PATH
181 This option allows blacklisting certain paths, libraries or
182 binaries. Prelink will not touch them during prelinking.
183 -o --undo-output=FILE
185 When performing an --undo operation, don't overwrite the pre‐
186 linked binary or library with its original content (before it
187 was prelinked), but save that into the specified file.
188 -V --version
190 Print version and exit.
191 -? --help
193 Print short help and exit.
194 --usage
196 Print short usage message.
197
199 Command-line arguments should be either directory hierarchies (in which
200 case -l and -h options apply), or particular ELF binaries or shared
201 libraries. Specifying a shared library explicitly on the command line
202 causes it to be prelinked even if no binary is linked against it. Oth‐
203 erwise, binaries are collected together and only the libraries they
204 depend on are prelinked with them.
205
207 # /usr/sbin/prelink -avmR
208 prelinks all binaries found in directories specified in /etc/pre‐
209 link.conf and all their dependent libraries, assigning libraries unique
210 virtual address space slots only if they ever appear together, and
211 starts assigning libraries at a random address.
212 # /usr/sbin/prelink -vm ~/bin/progx
213 prelinks ~/bin/progx program and all its dependent libraries (unless
214 they were prelinked already e.g. during prelink -a invocation).
215 # /usr/sbin/prelink -au
216 reverts all binaries and libraries to their original content.
217 # /usr/sbin/prelink -y /bin/prelinked_prog > /tmp/original_prog;
218 echo $? verifies whether /bin/prelinked_prog is unchanged.
219
221 /etc/prelink.cache Binary file containing a list of prelinked
222 libraries and/or binaries together with their
223 assigned virtual address space slots and dependen‐
224 cies. You can run /usr/sbin/prelink -p to see what
225 is stored in there.
226 /etc/prelink.conf Configuration file containing a list of directory
227 hierarchies that contain ELF shared libraries or
228 binaries which should be prelinked. This configu‐
229 ration file is used in -a mode to find binaries
230 which should be prelinked and also, no matter
231 whether -a is given or not, to limit which depen‐
232 dent shared libraries should be prelinked. If pre‐
233 link finds a dependent library of some binary or
234 other library which is not present in any of the
235 directories specified either in /etc/prelink.conf
236 or on the command line, then it cannot be pre‐
237 linked. Each line of the config file should be
238 either a comment starting with #, or a directory
239 name, or a blacklist specification. Directory
240 names can be prefixed by the -l switch, meaning the
241 tree walk of the given directory is only limited to
242 one file system; or the -h switch, meaning the tree
243 walk of the given directory follows symbolic links.
244 A blacklist specification should be prefixed by -b
245 and optionally also -l or -h if needed. A black‐
246 list entry can be either an absolute directory name
247 (in that case all files in that directory hierarchy
248 are ignored by the prelinker); an absolute filename
249 (then that particular library or binary is
250 skipped); or a glob pattern without a / character
251 in it (then all files matching that glob in any
252 directory are ignored).
254 ldd(1), ld.so(8).
256 prelink Some architectures, including IA-64 and HPPA, are not yet sup‐
257 ported.
259 Jakub Jelinek <jakub@redhat.com>.
260 19 July 2013 prelink(8)