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1 .\" $NetBSD: elf.3,v 1.2 2014/03/09 16:58:04 christos Exp $
2 .\"
3 .\" Copyright (c) 2006-2008,2011 Joseph Koshy. All rights reserved.
4 .\"
5 .\" Redistribution and use in source and binary forms, with or without
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14 .\" This software is provided by Joseph Koshy ``as is'' and
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16 .\" implied warranties of merchantability and fitness for a particular purpose
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26 .\" Id: elf.3 2885 2013-01-11 02:11:28Z jkoshy
27 .\"
28 .Dd August 14, 2011
29 .Os
30 .Dt ELF 3
31 .Sh NAME
32 .Nm elf
33 .Nd API for manipulating ELF objects
34 .Sh LIBRARY
35 .Lb libelf
36 .Sh SYNOPSIS
37 .In libelf.h
38 .Sh DESCRIPTION
39 The
40 .Lb libelf
41 provides functions that allow an application to read and manipulate
42 ELF object files, and to read
43 .Xr ar 1
44 archives.
45 The library allows the manipulation of ELF objects in a byte ordering
46 and word-size independent way, allowing an application to read and
47 create ELF objects for 32 and 64 bit architectures and for little-
48 and big-endian machines.
49 The library is capable of processing ELF objects that use extended
50 section numbering.
51 .Pp
52 This manual page serves to provide an overview of the functionality in
53 the ELF library.
54 Further information may found in the manual pages for individual
55 .Xr ELF 3
56 functions that comprise the library.
57 .Ss ELF Concepts
58 As described in
59 .Xr elf 5 ,
60 ELF files contain several data structures that are laid out in a
61 specific way.
62 ELF files begin with an
63 .Dq Executable Header ,
64 and may contain an optional
65 .Dq Program Header Table ,
66 and optional data in the form of ELF
67 .Dq sections .
68 A
69 .Dq Section Header Table
70 describes the content of the data in these sections.
71 .Pp
72 ELF objects have an associated
73 .Dq "ELF class"
74 which denotes the natural machine word size for the architecture
75 the object is associated with.
76 Objects for 32 bit architectures have an ELF class of
77 .Dv ELFCLASS32 .
78 Objects for 64 bit architectures have an ELF class of
79 .Dv ELFCLASS64 .
80 .Pp
81 ELF objects also have an associated
82 .Dq endianness
83 which denotes the endianness of the machine architecture associated
84 with the object.
85 This may be
86 .Dv ELFDATA2LSB
87 for little-endian architectures and
88 .Dv ELFDATA2MSB
89 for big-endian architectures.
90 .Pp
91 ELF objects are also associated with an API version number.
92 This version number determines the layout of the individual components
93 of an ELF file and the semantics associated with these.
94 .Ss Data Representation And Translation
95 The
96 .Xr ELF 3
97 library distinguishes between
98 .Dq native
99 representations of ELF data structures and their
100 .Dq file
101 representations.
102 .Pp
103 An application would work with ELF data in its
104 .Dq native
105 representation, i.e., using the native byteorder and alignment mandated
106 by the processor the application is running on.
107 The
108 .Dq file
109 representation of the same data could use a different byte ordering
110 and follow different constraints on object alignment than these native
111 constraints.
112 .Pp
113 Accordingly, the
114 .Xr ELF 3
115 library offers translation facilities
116 .Xr ( elf32_xlatetof 3 ,
117 .Xr elf32_xlatetom 3 ,
118 .Xr elf64_xlatetof 3
119 and
120 .Xr elf64_xlatetom 3 )
121 to and from these
122 representations and also provides higher-level APIs that retrieve and store
123 data from the ELF object in a transparent manner.
124 .Ss Library Working Version
125 Conceptually, there are three version numbers associated with an
126 application using the ELF library to manipulate ELF objects:
127 .Bl -bullet -compact -offset indent
128 .It
129 The ELF version that the application was compiled against.
130 This version determines the ABI expected by the application.
131 .It
132 The ELF version of the ELF object being manipulated by the
133 application through the ELF library.
134 .It
135 The ELF version (or set of versions) supported by the ELF library itself.
136 .El
137 .Pp
138 In order to facilitate working with ELF objects of differing versions,
139 the ELF library requires the application to call the
140 .Fn elf_version
141 function before invoking many of its operations, in order to inform
142 the library of the application's desired working version.
143 .Pp
144 In the current implementation, all three versions have to be
145 .Dv EV_CURRENT .
146 .Ss Namespace use
147 The ELF library uses the following prefixes:
148 .Bl -tag -width "ELF_F_*"
149 .It Dv elf_
150 Used for class-independent functions.
151 .It Dv elf32_
152 Used for functions working with 32 bit ELF objects.
153 .It Dv elf64_
154 Used for functions working with 64 bit ELF objects.
155 .It Dv Elf_
156 Used for class-independent data types.
157 .It Dv ELF_C_
158 Used for command values used in a few functions.
159 These symbols are defined as members of the
160 .Vt Dv Elf_Cmd
161 enumeration.
162 .It Dv ELF_E_
163 Used for error numbers.
164 .It Dv ELF_F_
165 Used for flags.
166 .It Dv ELF_K_
167 These constants define the kind of file associated with an ELF
168 descriptor.
169 See
170 .Xr elf_kind 3 .
171 The symbols are defined by the
172 .Vt Elf_Kind
173 enumeration.
174 .It Dv ELF_T_
175 These values are defined by the
176 .Vt Elf_Type
177 enumeration, and denote the types of ELF data structures
178 that can be present in an ELF object.
179 .El
180 .Pp
181 In addition, the library uses symbols with prefixes
182 .Dv _ELF
183 and
184 .Dv _libelf
185 for its internal use.
186 .Ss Descriptors
187 Applications communicate with the library using descriptors.
188 These are:
189 .Bl -tag -width ".Vt Elf_Data"
190 .It Vt Elf
191 An
192 .Vt Elf
193 descriptor represents an ELF object or an
194 .Xr ar 1
195 archive.
196 It is allocated using one of the
197 .Fn elf_begin
198 or
199 .Fn elf_memory
200 functions.
201 An
202 .Vt Elf
203 descriptor can be used to read and write data to an ELF file.
204 An
205 .Vt Elf
206 descriptor can be associated with zero or more
207 .Vt Elf_Scn
208 section descriptors.
209 .Pp
210 Given an ELF descriptor, the application may retrieve the ELF
211 object's class-dependent
212 .Dq "Executable Header"
213 structures using the
214 .Fn elf32_getehdr
215 or
216 .Fn elf64_getehdr
217 functions.
218 A new Ehdr structure may be allocated using the
219 .Fn elf64_newehdr
220 or
221 .Fn elf64_newehdr
222 functions.
223 .Pp
224 The
225 .Dq "Program Header Table"
226 associated with an ELF descriptor may be allocated using the
227 .Fn elf32_getphdr
228 or
229 .Fn elf64_getphdr
230 functions.
231 A new program header table may be allocated or an existing table
232 resized using the
233 .Fn elf32_newphdr
234 or
235 .Fn elf64_newphdr
236 functions.
237 .Pp
238 The
239 .Vt Elf
240 structure is opaque and has no members visible to the
241 application.
242 .\" TODO describe the Elf_Arhdr and Elf_Arsym structures.
243 .It Vt Elf_Data
244 An
245 .Vt Elf_Data
246 data structure describes an individual chunk of a ELF file as
247 represented in memory.
248 It has the following application-visible members:
249 .Bl -tag -width ".Vt unsigned int d_version" -compact
250 .It Vt "uint64_t d_align"
251 The in-file alignment of the data buffer within its containing ELF section.
252 This value must be non-zero and a power of two.
253 .It Vt "void *d_buf"
254 A pointer to data in memory.
255 .It Vt "uint64_t d_off"
256 The offset within the containing section where this descriptor's data
257 would be placed.
258 This field will be computed by the library unless the application
259 requests full control of the ELF object's layout.
260 .It Vt "uint64_t d_size"
261 The number of bytes of data in this descriptor.
262 .It Vt "Elf_Type d_type"
263 The ELF type (see below) of the data in this descriptor.
264 .It Vt "unsigned int d_version"
265 The operating version for the data in this buffer.
266 .El
267 .Pp
268 .Vt Elf_Data
269 descriptors are usually associated with
270 .Vt Elf_Scn
271 descriptors.
272 Existing data descriptors associated with an ELF section may be
273 structures are retrieved using the
274 .Fn elf_getdata
275 and
276 .Fn elf_rawdata
277 functions.
278 The
279 .Fn elf_newdata
280 function may be used to attach new data descriptors to an ELF section.
281 .It Vt Elf_Scn
282 .Vt Elf_Scn
283 descriptors represent a section in an ELF object.
284 .Pp
285 They are retrieved using the
286 .Fn elf_getscn
287 function.
288 An application may iterate through the existing sections of an ELF
289 object using the
290 .Fn elf_nextscn
291 function.
292 New sections may be allocated using the
293 .Fn elf_newscn
294 function.
295 .Pp
296 The
297 .Vt Elf_Scn
298 descriptor is opaque and contains no application modifiable fields.
299 .El
300 .Ss Supported Elf Types
301 The following ELF datatypes are supported by the library.
302 .Pp
303 .Bl -tag -width ".Dv ELF_T_SYMINFO" -compact
304 .It Dv ELF_T_ADDR
305 Machine addresses.
306 .It Dv ELF_T_BYTE
307 Byte data.
308 The library will not attempt to translate byte data.
309 .It Dv ELF_T_CAP
310 Software and hardware capability records.
311 .It Dv ELF_T_DYN
312 Records used in a section of type
313 .Dv SHT_DYNAMIC .
314 .It Dv ELF_T_EHDR
315 ELF executable header.
316 .It Dv ELF_T_GNUHASH
317 GNU-style hash tables.
318 .It Dv ELF_T_HALF
319 16-bit unsigned words.
320 .It Dv ELF_T_LWORD
321 64 bit unsigned words.
322 .It Dv ELF_T_MOVE
323 ELF Move records.
324 .\".It Dv ELF_T_MOVEP
325 .\" As yet unsupported.
326 .It Dv ELF_T_NOTE
327 ELF Note structures.
328 .It Dv ELF_T_OFF
329 File offsets.
330 .It Dv ELF_T_PHDR
331 ELF program header table entries.
332 .It Dv ELF_T_REL
333 ELF relocation entries.
334 .It Dv ELF_T_RELA
335 ELF relocation entries with addends.
336 .It Dv ELF_T_SHDR
337 ELF section header entries.
338 .It Dv ELF_T_SWORD
339 Signed 32-bit words.
340 .It Dv ELF_T_SXWORD
341 Signed 64-bit words.
342 .It Dv ELF_T_SYMINFO
343 ELF symbol information.
344 .It Dv ELF_T_SYM
345 ELF symbol table entries.
346 .It Dv ELF_T_VDEF
347 Symbol version definition records.
348 .It Dv ELF_T_VNEED
349 Symbol version requirement records.
350 .It Dv ELF_T_WORD
351 Unsigned 32-bit words.
352 .It Dv ELF_T_XWORD
353 Unsigned 64-bit words.
354 .El
355 .Pp
356 The symbol
357 .Dv ELF_T_NUM
358 denotes the number of Elf types known to the library.
359 .Pp
360 The following table shows the mapping between ELF section types
361 defined in
362 .Xr elf 5
363 and the types supported by the library.
364 .Bl -column ".Dv SHT_PREINIT_ARRAY" ".Dv ELF_T_SYMINFO"
365 .It Em Section Type Ta Em "Library Type" Ta Em Description
366 .It Dv SHT_DYNAMIC Ta Dv ELF_T_DYN Ta Xo
367 .Sq .dynamic
368 section entries.
369 .Xc
370 .It Dv SHT_DYNSYM Ta Dv ELF_T_SYM Ta Symbols for dynamic linking.
371 .It Dv SHT_FINI_ARRAY Ta Dv ELF_T_ADDR Ta Termination function pointers.
372 .It Dv SHT_GROUP Ta Dv ELF_T_WORD Ta Section group marker.
373 .It Dv SHT_HASH Ta Dv ELF_T_HASH Ta Symbol hashes.
374 .It Dv SHT_INIT_ARRAY Ta Dv ELF_T_ADDR Ta Initialization function pointers.
375 .It Dv SHT_NOBITS Ta Dv ELF_T_BYTE Ta Xo
376 Empty sections.
377 See
378 .Xr elf 5 .
379 .Xc
380 .It Dv SHT_NOTE Ta Dv ELF_T_NOTE Ta ELF note records.
381 .It Dv SHT_PREINIT_ARRAY Ta Dv ELF_T_ADDR Ta Pre-initialization function pointers.
382 .It Dv SHT_PROGBITS Ta Dv ELF_T_BYTE Ta Machine code.
383 .It Dv SHT_REL Ta Dv ELF_T_REL Ta ELF relocation records.
384 .It Dv SHT_RELA Ta Dv ELF_T_RELA Ta Relocation records with addends.
385 .It Dv SHT_STRTAB Ta Dv ELF_T_BYTE Ta String tables.
386 .It Dv SHT_SYMTAB Ta Dv ELF_T_SYM Ta Symbol tables.
387 .It Dv SHT_SYMTAB_SHNDX Ta Dv ELF_T_WORD Ta Used with extended section numbering.
388 .It Dv SHT_GNU_verdef Ta Dv ELF_T_VDEF Ta Symbol version definitions.
389 .It Dv SHT_GNU_verneed Ta Dv ELF_T_VNEED Ta Symbol versioning requirements.
390 .It Dv SHT_GNU_versym Ta Dv ELF_T_HALF Ta Version symbols.
391 .It Dv SHT_SUNW_move Ta Dv ELF_T_MOVE Ta ELF move records.
392 .It Dv SHT_SUNW_syminfo Ta Dv ELF_T_SYMINFO Ta Additional symbol flags.
393 .El
394 .Ss Functional Grouping
395 This section contains a brief overview of the available functionality
396 in the ELF library.
397 Each function listed here is described further in its own manual page.
398 .Bl -tag -width indent
399 .It "Archive Access"
400 .Bl -tag -compact
401 .It Fn elf_getarsym
402 Retrieve the archive symbol table.
403 .It Fn elf_getarhdr
404 Retrieve the archive header for an object.
405 .It Fn elf_getbase
406 Retrieve the offset of a member inside an archive.
407 .It Fn elf_next
408 Iterate through an
409 .Xr ar 1
410 archive.
411 .It Fn elf_rand
412 Random access inside an
413 .Xr ar 1
414 archive.
415 .El
416 .It "Data Structures"
417 .Bl -tag -compact
418 .It Fn elf_getdata
419 Retrieve translated data for an ELF section.
420 .It Fn elf_getscn
421 Retrieve the section descriptor for a named section.
422 .It Fn elf_ndxscn
423 Retrieve the index for a section.
424 .It Fn elf_newdata
425 Add a new
426 .Vt Elf_Data
427 descriptor to an ELF section.
428 .It Fn elf_newscn
429 Add a new section descriptor to an ELF descriptor.
430 .It Fn elf_nextscn
431 Iterate through the sections in an ELF object.
432 .It Fn elf_rawdata
433 Retrieve untranslated data for an ELF section.
434 .It Fn elf_rawfile
435 Return a pointer to the untranslated file contents for an ELF object.
436 .It Fn elf32_getehdr , Fn elf64_getehdr
437 Retrieve the Executable Header in an ELF object.
438 .It Fn elf32_getphdr , Fn elf64_getphdr
439 Retrieve the Program Header Table in an ELF object.
440 .It Fn elf32_getshdr , Fn elf64_getshdr
441 Retrieve the ELF section header associated with an
442 .Vt Elf_Scn
443 descriptor.
444 .It Fn elf32_newehdr , Fn elf64_newehdr
445 Allocate an Executable Header in an ELF object.
446 .It Fn elf32_newphdr , Fn elf64_newphdr
447 Allocate or resize the Program Header Table in an ELF object.
448 .El
449 .It "Data Translation"
450 .Bl -tag -compact
451 .It Fn elf32_xlatetof , Fn elf64_xlatetof
452 Translate an ELF data structure from its native representation to its
453 file representation.
454 .It Fn elf32_xlatetom , Fn elf64_xlatetom
455 Translate an ELF data structure from its file representation to a
456 native representation.
457 .El
458 .It "Error Reporting"
459 .Bl -tag -compact
460 .It Fn elf_errno
461 Retrieve the current error.
462 .It Fn elf_errmsg
463 Retrieve a human readable description of the current error.
464 .El
465 .It "Initialization"
466 .Bl -tag -compact
467 .It Fn elf_begin
468 Opens an
469 .Xr ar 1
470 archive or ELF object given a file descriptor.
471 .It Fn elf_end
472 Close an ELF descriptor and release all its resources.
473 .It Fn elf_memory
474 Opens an
475 .Xr ar 1
476 archive or ELF object present in a memory arena.
477 .It Fn elf_version
478 Sets the operating version.
479 .El
480 .It "IO Control"
481 .Bl -tag -width ".Fn elf_setshstrndx" -compact
482 .It Fn elf_cntl
483 Manage the association between and ELF descriptor and its underlying file.
484 .It Fn elf_flagdata
485 Mark an
486 .Vt Elf_Data
487 descriptor as dirty.
488 .It Fn elf_flagehdr
489 Mark the ELF Executable Header in an ELF descriptor as dirty.
490 .It Fn elf_flagphdr
491 Mark the ELF Program Header Table in an ELF descriptor as dirty.
492 .It Fn elf_flagscn
493 Mark an
494 .Vt Elf_Scn
495 descriptor as dirty.
496 .It Fn elf_flagshdr
497 Mark an ELF Section Header as dirty.
498 .It Fn elf_setshstrndx
499 Set the index of the section name string table for the ELF object.
500 .It Fn elf_update
501 Recompute ELF object layout and optionally write the modified object
502 back to the underlying file.
503 .El
504 .It "Queries"
505 .Bl -tag -width ".Fn elf_getshstrndx" -compact
506 .It Fn elf32_checksum , Fn elf64_checkum
507 Compute checksum of an ELF object.
508 .It Fn elf_getident
509 Retrieve the identification bytes for an ELF object.
510 .It Fn elf_getshnum
511 Retrieve the number of sections in an ELF object.
512 .It Fn elf_getshstrndx
513 Retrieve the section index of the section name string table in
514 an ELF object.
515 .It Fn elf_hash
516 Compute the ELF hash value of a string.
517 .It Fn elf_kind
518 Query the kind of object associated with an ELF descriptor.
519 .It Fn elf32_fsize , Fn elf64_fsize
520 Return the size of the file representation of an ELF type.
521 .El
522 .El
523 .Ss Controlling ELF Object Layout
524 In the usual mode of operation, library will compute section
525 offsets and alignments based on the contents of an ELF descriptor's
526 sections without need for further intervention by the
527 application.
528 .Pp
529 However, if the application wishes to take complete charge of the
530 layout of the ELF file, it may set the
531 .Dv ELF_F_LAYOUT
532 flag on an ELF descriptor using
533 .Xr elf_flagelf 3 ,
534 following which the library will use the data offsets and alignments
535 specified by the application when laying out the file.
536 Application control of file layout is described further in the
537 .Xr elf_update 3
538 manual page.
539 .Pp
540 Gaps in between sections will be filled with the fill character
541 set by function
542 .Fn elf_fill .
543 .Ss Error Handling
544 In case an error is encountered, these library functions set an
545 internal error number and signal the presence of the error by
546 returning an special return value.
547 The application can check the
548 current error number by calling
549 .Xr elf_errno 3 .
550 A human readable description of the recorded error is available by
551 calling
552 .Xr elf_errmsg 3 .
553 .Ss Memory Management Rules
554 The library keeps track of all
555 .Vt Elf_Scn
556 and
557 .Vt Elf_Data
558 descriptors associated with an ELF descriptor and recovers them
559 when the descriptor is closed using
560 .Xr elf_end 3 .
561 Thus the application must not call
562 .Xr free 3
563 on data structures allocated by the ELF library.
564 .Pp
565 Conversely the library will not
566 free data that it has not allocated.
567 As an example, an application may call
568 .Xr elf_newdata 3
569 to allocate a new
570 .Vt Elf_Data
571 descriptor and can set the
572 .Va d_off
573 member of the descriptor to point to a region of memory allocated
574 using
575 .Xr malloc 3 .
576 It is the applications responsibility to free this arena, though the
577 library will reclaim the space used by the
578 .Vt Elf_Data
579 descriptor itself.
580 .Sh SEE ALSO
581 .Xr gelf 3 ,
582 .Xr elf 5
583 .Sh HISTORY
584 The original ELF(3) API was developed for Unix System V.
585 This implementation first appeared in
586 .Fx 7.0
587 and
588 .Nx 6.0 .
589 .Sh AUTHORS
590 The ELF library was written by
591 .An "Joseph Koshy"
592 .Aq jkoshy@FreeBSD.org .