Annotation of src/libexec/ld.elf_so/ld.elf_so.1, Revision 1.11.2.1
1.11.2.1! wrstuden 1: .\" $NetBSD: ld.elf_so.1,v 1.11 2008/04/30 13:10:52 martin Exp $
1.1 skrll 2: .\"
3: .\" Copyright (c) 2001 The NetBSD Foundation, Inc.
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6: .\" This code is derived from software contributed to The NetBSD Foundation
1.2 wiz 7: .\" by Nick Hudson.
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1.9 christos 30: .Dd May 18, 2007
1.1 skrll 31: .Dt LD.ELF_SO 1
32: .Os
33: .Sh NAME
34: .Nm ld.elf_so
35: .Nd run-time link-editor (linker)
36: .Sh DESCRIPTION
37: .Nm
1.7 wiz 38: is a self-contained, position independent program
39: image providing run-time support for loading and
40: link-editing shared objects into a process' address space.
41: It uses information stored in data structures within the binary (see
1.2 wiz 42: .Xr elf 5 )
1.1 skrll 43: and environment variables to determine which shared objects are needed.
44: These shared objects are loaded at a convenient virtual address using the
45: .Xr mmap 2
1.7 wiz 46: system call.
47: After all shared objects have been successfully loaded,
1.1 skrll 48: .Nm
1.7 wiz 49: proceeds to resolve external references from both
50: the main program and all objects loaded.
51: Once all required references are resolved control is
1.1 skrll 52: passed to the program via its entry point.
53: .Ss Startup
54: On the execution of a dynamically linked binary the kernel will load
55: the program and its run-time linker as specified in the PT_INTERP
1.7 wiz 56: section in the program header.
57: At this point, instead of passing control directly to the program,
58: the kernel passes control to the specified linker.
59: An auxiliary vector of information is passed that includes
1.1 skrll 60: the address of the program header, the size of each entry in the header,
1.7 wiz 61: and the number of entries.
62: The entry point of the program and the base address of where
1.1 skrll 63: .Nm
64: is loaded is also supplied.
65: .Pp
66: Careful use of code allows
67: .Nm
1.7 wiz 68: to relocate itself before proceeding.
69: Specifically the use of global variables and
70: large switch statements is not allowed.
71: The later can cause the output of a jump table that
72: can use the equivalent of a global variable.
1.1 skrll 73: .Ss Finding objects
74: Each
75: .Xr elf 5
76: object file may contain information in its dynamic (PT_DYNAMIC) section
77: about which shared objects it requires (often referred to as dependencies).
78: These dependencies are specified in the optional DT_NEEDED entry within
1.7 wiz 79: the dynamic section.
80: Each DT_NEEDED entry refers to a filename string of
1.1 skrll 81: the shared object that is to be searched for.
82: .Pp
83: The linker will search for libraries in three lists of paths:
84: .Bl -enum
85: .It
86: A user defined list of paths as specified in LD_LIBRARY_PATH and
1.2 wiz 87: .Xr ld.so.conf 5 .
1.1 skrll 88: .Pp
89: The use of ld.so.conf should be avoided as the setting of a global search
90: path can present a security risk.
91: .It
92: A list of paths specified within a shared object using a DT_RPATH entry in
1.7 wiz 93: the dynamic section.
94: This is defined at shared object link time.
1.1 skrll 95: .It
96: The list of default paths which is set to
97: .Pa /usr/lib .
98: .El
99: .Pp
1.9 christos 100: .Nm
101: will expand the following variables if present in the paths:
102: .Bl -tag -width $PLATFORM
103: .It $HWCAP
104: Processor hardware capabilities, for example FPU, MMX, SSE.
105: Currently unimplemented.
106: .It $ISALIST
107: List of instructions sets this processor can execute.
108: Currently unimplemented.
109: .It $ORIGIN
110: The directory of the main object.
111: .It $OSNAME
112: The value of the
1.11.2.1! wrstuden 113: .Dv kern.ostype
1.9 christos 114: .Xr sysctl 3 .
115: .It $OSREL
116: The value of the
117: .Dv kern.osrelease
118: .Xr sysctl 3 .
119: .It $PLATFORM
120: The value of the
121: .Dv hw.machine_arch
122: .Xr sysctl 3 .
123: .El
124: .Pp
125: Both
126: .Dv ${VARIABLE}
127: and
128: .Dv $VARIABLE
129: are recognized.
130: .Pp
1.1 skrll 131: The filename string can be considered free form, however, it will almost
1.7 wiz 132: always take the form lib\*[Lt]name\*[Gt].so.\*[Lt]number\*[Gt],
133: where name specifies the
1.1 skrll 134: .Sq library
135: name and number is conceptually the library's major version number.
136: .Pp
1.7 wiz 137: This name and another of the form lib\*[Lt]name\*[Gt].so are normally
138: symbolic links to the real shared object which has a filename of the form
1.4 ross 139: lib\*[Lt]name\*[Gt].so.\*[Lt]major\*[Gt].\*[Lt]minor\*[Gt][.\*[Lt]teeny\*[Gt]].
1.1 skrll 140: This naming convention allows a versioning scheme similar to
1.2 wiz 141: .Xr a.out 5 .
1.1 skrll 142: .Ss Relocation
143: .Nm
144: will perform all necessary relocations immediately except for relocations
1.7 wiz 145: relating to the Procedure Linkage Table (PLT).
146: The PLT is used as a indirection method for procedure
147: calls to globally defined functions.
148: It allows, through the use of intermediate code, the delayed binding of
149: a call to a globally defined function to be performed at procedure call time.
150: This
1.1 skrll 151: .Sq lazy
152: method is the default (see LD_BIND_NOW).
153: .Ss Initialization
154: A mechanism is provided for initialization and termination routines
155: to be called, on a per-object basis before execution of the program proper
1.7 wiz 156: begins or after the program has completed.
157: This gives a shared object an opportunity to perform
158: any extra set-up or completion work.
1.1 skrll 159: .Pp
160: The DT_INIT and DT_FINI entries in the dynamic section specify the addresses
161: of the initialization and termination functions, respectively, for
162: the shared object.
163: .Nm
164: arranges for each initialization function to be called before control is passed
165: to the program and for the termination functions to be called by using
166: .Xr atexit 3 .
167: .Pp
168: This mechanism is exploited by the system-supplied constructor
169: initialization and destructor code located in
170: .Pa /usr/lib/crtbeginS.o
171: and
172: .Pa /usr/lib/crtendS.o .
173: These files are automatically included by
174: .Xr cc 1
175: and
176: .Xr c++ 1
1.7 wiz 177: in the list of object-code files passed to
1.1 skrll 178: .Xr ld 1
179: when building a shared C or C++ object.
180: .Sh ENVIRONMENT
181: If the following environment variables exist they will be used by
1.8 wiz 182: .Nm .
1.1 skrll 183: .Bl -tag -width "LD_LIBRARY_PATH"
184: .It Ev LD_LIBRARY_PATH
185: A colon separated list of directories, overriding the default search path
186: for shared libraries.
187: .It Ev LD_PRELOAD
1.6 bjh21 188: A colon or space separated list of shared object filenames to be loaded
1.1 skrll 189: .Em after
190: the main program but
191: .Em before
192: its shared object dependencies.
1.7 wiz 193: Space is allowed as a separator for backwards compatibility only.
194: Support may be removed in a future release and should not be relied upon.
1.1 skrll 195: .It Ev LD_BIND_NOW
196: If defined immediate binding of Procedure Link Table (PLT) entries is
197: performed instead of the default lazy method.
198: .It Ev LD_DEBUG
199: If defined a variety of debug information will be written to the standard
200: error of an dynamically linked executed when it is run.
1.5 wiz 201: This variable is only recognized if
202: .Nm
203: was compiled with debugging support
204: .Sy ( -DDEBUG ) .
1.1 skrll 205: .El
206: .Sh FILES
207: .Bl -tag -width /etc/ld.so.conf -compact
208: .It Pa /etc/ld.so.conf
209: library location hints supplied by the system administrator.
210: .El
211: .Sh SEE ALSO
212: .Xr ld 1 ,
213: .Xr ld.aout_so 1 ,
1.2 wiz 214: .Xr dlfcn 3 ,
215: .Xr elf 5
1.1 skrll 216: .Sh HISTORY
217: The ELF shared library model employed first appeared in Sys V R4.
1.9 christos 218: .Pp
219: The path expansion variables first appeared in Solaris 10, and
220: in
1.10 tnn 221: .Nx 5.0 .
1.1 skrll 222: .Sh SECURITY CONSIDERATIONS
223: The environment variables
224: .Ev LD_LIBRARY_PATH
225: and
226: .Ev LD_PRELOAD
227: are not honored when executing in a set-user-ID or set-group-ID environment.
228: This action is taken to prevent malicious substitution of shared object
229: dependencies or interposition of symbols.
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