rtld: Implement ELF filters (DT_FILTER and DT_AUXILIARY)
[dragonfly.git] / libexec / rtld-elf / rtld.1
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1.\" Copyright (c) 1995 Paul Kranenburg
2.\" All rights reserved.
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
6.\" are met:
7.\" 1. Redistributions of source code must retain the above copyright
8.\" notice, this list of conditions and the following disclaimer.
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11.\" documentation and/or other materials provided with the distribution.
12.\" 3. All advertising materials mentioning features or use of this software
13.\" must display the following acknowledgment:
14.\" This product includes software developed by Paul Kranenburg.
15.\" 3. The name of the author may not be used to endorse or promote products
16.\" derived from this software without specific prior written permission
17.\"
18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
19.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
20.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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23.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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27.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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f4f4bfd5 29.\" $FreeBSD$
fcf53d9b 30.\"
35b2b265 31.Dd January 20, 2012
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32.Dt RTLD 1
33.Os
34.Sh NAME
a1eee96a 35.Nm ld-elf.so.2 ,
fcf53d9b 36.Nm ld-elf.so.1 ,
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37.Nm rtld ,
38.Nm _rtld_functrace
984263bc 39.Nd run-time link-editor
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40.Sh SYNOPSIS
41.Ft int
42.Fn _rtld_functrace "const char *callerso" "const char *calleeso" "const char *calleefun" "void *stack"
984263bc 43.Sh DESCRIPTION
fcf53d9b 44The
984263bc 45.Nm
fcf53d9b 46utility is a self-contained shared object providing run-time
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47support for loading and link-editing shared objects into a process'
48address space.
49It is also commonly known as the dynamic linker.
50It uses the data structures
51contained within dynamically linked programs to determine which shared
52libraries are needed and loads them using the
53.Xr mmap 2
54system call.
55.Pp
56After all shared libraries have been successfully loaded,
57.Nm
58proceeds to resolve external references from both the main program and
59all objects loaded.
60A mechanism is provided for initialization routines
61to be called on a per-object basis, giving a shared object an opportunity
62to perform any extra set-up before execution of the program proper begins.
63This is useful for C++ libraries that contain static constructors.
64.Pp
fcf53d9b 65When resolving dependencies for the loaded objects,
984263bc 66.Nm
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67may be allowed to translate dynamic token strings in rpath and soname
68by setting
69.Fl "z origin"
70option of the static linker
71.Xr ld 1 .
72The following strings are recognized now:
73.Bl -tag -width ".Pa $PLATFORM"
74.It Pa $ORIGIN
75Translated to the full path of the loaded object.
76.It Pa $OSNAME
77Translated to the name of the operating system implementation.
78.It Pa $OSREL
79Translated to the release level of the operating system.
80.It Pa $PLATFORM
81Translated to the machine hardware platform.
82.El
83.Pp
84The
85.Nm
86utility itself is loaded by the kernel together with any dynamically-linked
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87program that is to be executed.
88The kernel transfers control to the
89dynamic linker.
90After the dynamic linker has finished loading,
91relocating, and initializing the program and its required shared
92objects, it transfers control to the entry point of the program.
93.Pp
fcf53d9b 94To locate the required shared objects in the file system,
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95.Nm
96may use a
97.Dq hints
98file prepared by the
99.Xr ldconfig 8
100utility.
101.Pp
fcf53d9b 102The
984263bc 103.Nm
fcf53d9b 104utility
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105recognizes a number of environment variables that can be used to modify
106its behaviour as follows:
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107.Bl -tag -width ".Ev LD_LIBMAP_DISABLE"
108.It Ev LD_DUMP_REL_POST
109If set,
110.Nm
111will print a table containing all relocations after symbol
112binding and relocation.
113.It Ev LD_DUMP_REL_PRE
114If set,
115.Nm
116will print a table containing all relocations before symbol
117binding and relocation.
118.It Ev LD_LIBMAP
119A library replacement list in the same format as
120.Xr libmap.conf 5 .
121For convenience, the characters
122.Ql =
123and
124.Ql \&,
125can be used instead of a space and a newline.
126This variable is parsed after
127.Xr libmap.conf 5 ,
128and will override its entries.
129This variable is unset for set-user-ID and set-group-ID programs.
130.It Ev LD_LIBMAP_DISABLE
131If set, disables the use of
132.Xr libmap.conf 5
133and
134.Ev LD_LIBMAP .
135This variable is unset for set-user-ID and set-group-ID programs.
136.It Ev LD_ELF_HINTS_PATH
137This variable will override the default location of
138.Dq hints
139file.
140This variable is unset for set-user-ID and set-group-ID programs.
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141.It Ev LD_LIBRARY_PATH
142A colon separated list of directories, overriding the default search path
143for shared libraries.
fcf53d9b 144This variable is unset for set-user-ID and set-group-ID programs.
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145.It Ev LD_PRELOAD
146A list of shared libraries, separated by colons and/or white space,
147to be linked in before any
148other shared libraries.
149If the directory is not specified then
150the directories specified by
151.Ev LD_LIBRARY_PATH
152will be searched first
153followed by the set of built-in standard directories.
fcf53d9b 154This variable is unset for set-user-ID and set-group-ID programs.
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155.It Ev LD_BIND_NOW
156When set to a nonempty string, causes
157.Nm
158to relocate all external function calls before starting execution of the
159program.
160Normally, function calls are bound lazily, at the first call
161of each function.
162.Ev LD_BIND_NOW
163increases the start-up time of a program, but it avoids run-time
164surprises caused by unexpectedly undefined functions.
165.It Ev LD_TRACE_LOADED_OBJECTS
166When set to a nonempty string, causes
167.Nm
168to exit after loading the shared objects and printing a summary which includes
169the absolute pathnames of all objects, to standard output.
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170.It Ev LD_TRACE_LOADED_OBJECTS_ALL
171When set to a nonempty string, causes
172.Nm
173to expand the summary to indicate which objects caused each object to
174be loaded.
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175.It Ev LD_TRACE_LOADED_OBJECTS_FMT1
176.It Ev LD_TRACE_LOADED_OBJECTS_FMT2
177When set, these variables are interpreted as format strings a la
178.Xr printf 3
179to customize the trace output and are used by
180.Xr ldd 1 Ns 's
181.Fl f
182option and allows
183.Xr ldd 1
184to be operated as a filter more conveniently.
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185If the dependency name starts with string
186.Pa lib ,
187.Ev LD_TRACE_LOADED_OBJECTS_FMT1
188is used, otherwise
189.Ev LD_TRACE_LOADED_OBJECTS_FMT2
190is used.
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191The following conversions can be used:
192.Bl -tag -width 4n
193.It Li %a
194The main program's name
195(also known as
196.Dq __progname ) .
197.It Li \&%A
198The value of the environment variable
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199.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME .
200Typically used to print both the names of programs and shared libraries
201being inspected using
202.Xr ldd 1 .
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203.It Li %o
204The library name.
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205.It Li %p
206The full pathname as determined by
207.Nm rtld Ns 's
208library search rules.
209.It Li %x
210The library's load address.
211.El
212.Pp
213Additionally,
214.Ql \en
215and
216.Ql \et
217are recognized and have their usual meaning.
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218.It Ev LD_UTRACE
219If set,
220.Nm
221will log events such as the loading and unloading of shared objects via
222.Xr utrace 2 .
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223.Pp
224.It Ev LD_LOADFLTR
225If set,
226.Nm
227will process the filtee dependencies of the loaded objects immediately,
228instead of postponing it until required.
229Normally, the filtees are opened at the time of the first symbol resolution
230from the filter object.
984263bc 231.El
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232.Pp
233If a shared object preloaded by the
234.Ev LD_PRELOAD
235mechanism contains a public symbol
236.Dq _rtld_functrace ,
237.Nm
238will transfer control to this function each time
239it needs to resolve an unbound function symbol.
240By returning a non-zero value,
241.Fn _rtld_functrace
242can advise the linker to keep tracing the specified
243combination of caller shared object and called function;
244returning 0 will prevent
245.Fn _rtld_functrace
246to be called for this combination again.
247.Pp
248When implementing a custom
249.Fn _rtld_functrace
250function, be aware of the possibility that
251.Fn _rtld_functrace
252might be called for functions called on its behalf,
253or that multiple threads could enter
254.Fn _rtld_functrace
255at the same time.
a1eee96a 256.Sh DIFFERENCES BETWEEN .1 and .2
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257ABI changes have been made to support TLS allocation and initialization
258and to give threading libraries a chance to complete initialization of the
259TCB prior to the calling of the _init() functions for the dynamically loaded
260libraries.
984263bc 261.Sh FILES
fcf53d9b 262.Bl -tag -width ".Pa /var/run/ld-elf.so.hints" -compact
984263bc 263.It Pa /var/run/ld-elf.so.hints
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264Hints file.
265.It Pa /etc/libmap.conf
266The libmap configuration file.
984263bc 267.El
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268.Sh EXAMPLES
269To set up an
270.Fn _rtld_functrace
271for printing out the functions as they are called, this code can be used:
272.Bd -literal -offset indent
273#include <string.h>
274
4ee0d876 275static int nl = 10;
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276
277int
278_rtld_functrace(const char *callerso, const char *calleeso,
279 const char *calleefun, void *stack)
280{
281 write(2, "calling ", 8);
282 write(2, calleefun, strlen(calleefun));
283 write(2, &nl, 1);
284 return 1;
285}
286.Ed
287.Pp
288If put in a file named
289.Pa ft.c
290and compiled with
291.Bd -literal -offset indent
292$ cc -shared ft.c -o ft.so
293.Ed
294.Pp
295setting
296.Ev LD_PRELOAD
297to the path of
298.Pa ft.so
299will activate it.
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300.Sh SEE ALSO
301.Xr ld 1 ,
302.Xr ldd 1 ,
303.Xr elf 5 ,
fcf53d9b 304.Xr libmap.conf 5 ,
984263bc 305.Xr ldconfig 8