1.\" Copyright (c) 2006-2008,2011 Joseph Koshy. All rights reserved. 2.\" 3.\" Redistribution and use in source and binary forms, with or without 4.\" modification, are permitted provided that the following conditions 5.\" are met: 6.\" 1. Redistributions of source code must retain the above copyright 7.\" notice, this list of conditions and the following disclaimer. 8.\" 2. Redistributions in binary form must reproduce the above copyright 9.\" notice, this list of conditions and the following disclaimer in the 10.\" documentation and/or other materials provided with the distribution. 11.\" 12.\" This software is provided by Joseph Koshy ``as is'' and 13.\" any express or implied warranties, including, but not limited to, the 14.\" implied warranties of merchantability and fitness for a particular purpose 15.\" are disclaimed. in no event shall Joseph Koshy be liable 16.\" for any direct, indirect, incidental, special, exemplary, or consequential 17.\" damages (including, but not limited to, procurement of substitute goods 18.\" or services; loss of use, data, or profits; or business interruption) 19.\" however caused and on any theory of liability, whether in contract, strict 20.\" liability, or tort (including negligence or otherwise) arising in any way 21.\" out of the use of this software, even if advised of the possibility of 22.\" such damage. 23.\" 24.\" $Id: elf.3 3195 2015-05-12 17:22:19Z emaste $ 25.\" 26.Dd July 28, 2014 27.Os 28.Dt ELF 3 29.Sh NAME 30.Nm elf 31.Nd API for manipulating ELF objects 32.Sh LIBRARY 33.Lb libelf 34.Sh SYNOPSIS 35.In libelf.h 36.Sh DESCRIPTION 37The 38.Lb libelf 39provides functions that allow an application to read and manipulate 40ELF object files, and to read 41.Xr ar 1 42archives. 43The library allows the manipulation of ELF objects in a byte ordering 44and word-size independent way, allowing an application to read and 45create ELF objects for 32 and 64 bit architectures and for little- 46and big-endian machines. 47The library is capable of processing ELF objects that use extended 48section numbering. 49.Pp 50This manual page serves to provide an overview of the functionality in 51the ELF library. 52Further information may found in the manual pages for individual 53.Xr ELF 3 54functions that comprise the library. 55.Ss ELF Concepts 56As described in 57.Xr elf 5 , 58ELF files contain several data structures that are laid out in a 59specific way. 60ELF files begin with an 61.Dq Executable Header , 62and may contain an optional 63.Dq Program Header Table , 64and optional data in the form of ELF 65.Dq sections . 66A 67.Dq Section Header Table 68describes the content of the data in these sections. 69.Pp 70ELF objects have an associated 71.Dq "ELF class" 72which denotes the natural machine word size for the architecture 73the object is associated with. 74Objects for 32 bit architectures have an ELF class of 75.Dv ELFCLASS32 . 76Objects for 64 bit architectures have an ELF class of 77.Dv ELFCLASS64 . 78.Pp 79ELF objects also have an associated 80.Dq endianness 81which denotes the endianness of the machine architecture associated 82with the object. 83This may be 84.Dv ELFDATA2LSB 85for little-endian architectures and 86.Dv ELFDATA2MSB 87for big-endian architectures. 88.Pp 89ELF objects are also associated with an API version number. 90This version number determines the layout of the individual components 91of an ELF file and the semantics associated with these. 92.Ss Data Representation And Translation 93The 94.Xr ELF 3 95library distinguishes between 96.Dq native 97representations of ELF data structures and their 98.Dq file 99representations. 100.Pp 101An application would work with ELF data in its 102.Dq native 103representation, i.e., using the native byteorder and alignment mandated 104by the processor the application is running on. 105The 106.Dq file 107representation of the same data could use a different byte ordering 108and follow different constraints on object alignment than these native 109constraints. 110.Pp 111Accordingly, the 112.Xr ELF 3 113library offers translation facilities 114.Xr ( elf32_xlatetof 3 , 115.Xr elf32_xlatetom 3 , 116.Xr elf64_xlatetof 3 117and 118.Xr elf64_xlatetom 3 ) 119to and from these 120representations and also provides higher-level APIs that retrieve and store 121data from the ELF object in a transparent manner. 122.Ss Library Working Version 123Conceptually, there are three version numbers associated with an 124application using the ELF library to manipulate ELF objects: 125.Bl -bullet -compact -offset indent 126.It 127The ELF version that the application was compiled against. 128This version determines the ABI expected by the application. 129.It 130The ELF version of the ELF object being manipulated by the 131application through the ELF library. 132.It 133The ELF version (or set of versions) supported by the ELF library itself. 134.El 135.Pp 136In order to facilitate working with ELF objects of differing versions, 137the ELF library requires the application to call the 138.Fn elf_version 139function before invoking many of its operations, in order to inform 140the library of the application's desired working version. 141.Pp 142In the current implementation, all three versions have to be 143.Dv EV_CURRENT . 144.Ss Namespace use 145The ELF library uses the following prefixes: 146.Bl -tag -width "ELF_F_*" 147.It Dv elf_ 148Used for class-independent functions. 149.It Dv elf32_ 150Used for functions working with 32 bit ELF objects. 151.It Dv elf64_ 152Used for functions working with 64 bit ELF objects. 153.It Dv Elf_ 154Used for class-independent data types. 155.It Dv ELF_C_ 156Used for command values used in a few functions. 157These symbols are defined as members of the 158.Vt Dv Elf_Cmd 159enumeration. 160.It Dv ELF_E_ 161Used for error numbers. 162.It Dv ELF_F_ 163Used for flags. 164.It Dv ELF_K_ 165These constants define the kind of file associated with an ELF 166descriptor. 167See 168.Xr elf_kind 3 . 169The symbols are defined by the 170.Vt Elf_Kind 171enumeration. 172.It Dv ELF_T_ 173These values are defined by the 174.Vt Elf_Type 175enumeration, and denote the types of ELF data structures 176that can be present in an ELF object. 177.El 178.Pp 179In addition, the library uses symbols with prefixes 180.Dv _ELF 181and 182.Dv _libelf 183for its internal use. 184.Ss Descriptors 185Applications communicate with the library using descriptors. 186These are: 187.Bl -tag -width ".Vt Elf_Data" 188.It Vt Elf 189An 190.Vt Elf 191descriptor represents an ELF object or an 192.Xr ar 1 193archive. 194It is allocated using one of the 195.Fn elf_begin 196or 197.Fn elf_memory 198functions. 199An 200.Vt Elf 201descriptor can be used to read and write data to an ELF file. 202An 203.Vt Elf 204descriptor can be associated with zero or more 205.Vt Elf_Scn 206section descriptors. 207.Pp 208Given an ELF descriptor, the application may retrieve the ELF 209object's class-dependent 210.Dq "Executable Header" 211structures using the 212.Fn elf32_getehdr 213or 214.Fn elf64_getehdr 215functions. 216A new Ehdr structure may be allocated using the 217.Fn elf64_newehdr 218or 219.Fn elf64_newehdr 220functions. 221.Pp 222The 223.Dq "Program Header Table" 224associated with an ELF descriptor may be allocated using the 225.Fn elf32_getphdr 226or 227.Fn elf64_getphdr 228functions. 229A new program header table may be allocated or an existing table 230resized using the 231.Fn elf32_newphdr 232or 233.Fn elf64_newphdr 234functions. 235.Pp 236The 237.Vt Elf 238structure is opaque and has no members visible to the 239application. 240.\" TODO describe the Elf_Arhdr and Elf_Arsym structures. 241.It Vt Elf_Data 242An 243.Vt Elf_Data 244data structure describes an individual chunk of a ELF file as 245represented in memory. 246It has the following application-visible members: 247.Bl -tag -width ".Vt unsigned int d_version" -compact 248.It Vt "uint64_t d_align" 249The in-file alignment of the data buffer within its containing ELF section. 250This value must be non-zero and a power of two. 251.It Vt "void *d_buf" 252A pointer to data in memory. 253.It Vt "uint64_t d_off" 254The offset within the containing section where this descriptor's data 255would be placed. 256This field will be computed by the library unless the application 257requests full control of the ELF object's layout. 258.It Vt "uint64_t d_size" 259The number of bytes of data in this descriptor. 260.It Vt "Elf_Type d_type" 261The ELF type (see below) of the data in this descriptor. 262.It Vt "unsigned int d_version" 263The operating version for the data in this buffer. 264.El 265.Pp 266.Vt Elf_Data 267descriptors are usually associated with 268.Vt Elf_Scn 269descriptors. 270Existing data descriptors associated with an ELF section may be 271structures are retrieved using the 272.Fn elf_getdata 273and 274.Fn elf_rawdata 275functions. 276The 277.Fn elf_newdata 278function may be used to attach new data descriptors to an ELF section. 279.It Vt Elf_Scn 280.Vt Elf_Scn 281descriptors represent a section in an ELF object. 282.Pp 283They are retrieved using the 284.Fn elf_getscn 285function. 286An application may iterate through the existing sections of an ELF 287object using the 288.Fn elf_nextscn 289function. 290New sections may be allocated using the 291.Fn elf_newscn 292function. 293.Pp 294The 295.Vt Elf_Scn 296descriptor is opaque and contains no application modifiable fields. 297.El 298.Ss Supported Elf Types 299The following ELF datatypes are supported by the library. 300.Pp 301.Bl -tag -width ".Dv ELF_T_SYMINFO" -compact 302.It Dv ELF_T_ADDR 303Machine addresses. 304.It Dv ELF_T_BYTE 305Byte data. 306The library will not attempt to translate byte data. 307.It Dv ELF_T_CAP 308Software and hardware capability records. 309.It Dv ELF_T_DYN 310Records used in a section of type 311.Dv SHT_DYNAMIC . 312.It Dv ELF_T_EHDR 313ELF executable header. 314.It Dv ELF_T_GNUHASH 315GNU-style hash tables. 316.It Dv ELF_T_HALF 31716-bit unsigned words. 318.It Dv ELF_T_LWORD 31964 bit unsigned words. 320.It Dv ELF_T_MOVE 321ELF Move records. 322.\".It Dv ELF_T_MOVEP 323.\" As yet unsupported. 324.It Dv ELF_T_NOTE 325ELF Note structures. 326.It Dv ELF_T_OFF 327File offsets. 328.It Dv ELF_T_PHDR 329ELF program header table entries. 330.It Dv ELF_T_REL 331ELF relocation entries. 332.It Dv ELF_T_RELA 333ELF relocation entries with addends. 334.It Dv ELF_T_SHDR 335ELF section header entries. 336.It Dv ELF_T_SWORD 337Signed 32-bit words. 338.It Dv ELF_T_SXWORD 339Signed 64-bit words. 340.It Dv ELF_T_SYMINFO 341ELF symbol information. 342.It Dv ELF_T_SYM 343ELF symbol table entries. 344.It Dv ELF_T_VDEF 345Symbol version definition records. 346.It Dv ELF_T_VNEED 347Symbol version requirement records. 348.It Dv ELF_T_WORD 349Unsigned 32-bit words. 350.It Dv ELF_T_XWORD 351Unsigned 64-bit words. 352.El 353.Pp 354The symbol 355.Dv ELF_T_NUM 356denotes the number of Elf types known to the library. 357.Pp 358The following table shows the mapping between ELF section types 359defined in 360.Xr elf 5 361and the types supported by the library. 362.Bl -column ".Dv SHT_PREINIT_ARRAY" ".Dv ELF_T_SYMINFO" 363.It Em Section Type Ta Em "Library Type" Ta Em Description 364.It Dv SHT_DYNAMIC Ta Dv ELF_T_DYN Ta Xo 365.Sq .dynamic 366section entries. 367.Xc 368.It Dv SHT_DYNSYM Ta Dv ELF_T_SYM Ta Symbols for dynamic linking. 369.It Dv SHT_FINI_ARRAY Ta Dv ELF_T_ADDR Ta Termination function pointers. 370.It Dv SHT_GNU_HASH Ta Dv ELF_T_GNUHASH Ta GNU hash sections. 371.It Dv SHT_GNU_LIBLIST Ta Dv ELF_T_WORD Ta List of libraries to be pre-linked. 372.It Dv SHT_GNU_verdef Ta Dv ELF_T_VDEF Ta Symbol version definitions. 373.It Dv SHT_GNU_verneed Ta Dv ELF_T_VNEED Ta Symbol versioning requirements. 374.It Dv SHT_GNU_versym Ta Dv ELF_T_HALF Ta Version symbols. 375.It Dv SHT_GROUP Ta Dv ELF_T_WORD Ta Section group marker. 376.It Dv SHT_HASH Ta Dv ELF_T_HASH Ta Symbol hashes. 377.It Dv SHT_INIT_ARRAY Ta Dv ELF_T_ADDR Ta Initialization function pointers. 378.It Dv SHT_NOBITS Ta Dv ELF_T_BYTE Ta Xo 379Empty sections. 380See 381.Xr elf 5 . 382.Xc 383.It Dv SHT_NOTE Ta Dv ELF_T_NOTE Ta ELF note records. 384.It Dv SHT_PREINIT_ARRAY Ta Dv ELF_T_ADDR Ta Pre-initialization function pointers. 385.It Dv SHT_PROGBITS Ta Dv ELF_T_BYTE Ta Machine code. 386.It Dv SHT_REL Ta Dv ELF_T_REL Ta ELF relocation records. 387.It Dv SHT_RELA Ta Dv ELF_T_RELA Ta Relocation records with addends. 388.It Dv SHT_STRTAB Ta Dv ELF_T_BYTE Ta String tables. 389.It Dv SHT_SYMTAB Ta Dv ELF_T_SYM Ta Symbol tables. 390.It Dv SHT_SYMTAB_SHNDX Ta Dv ELF_T_WORD Ta Used with extended section numbering. 391.It Dv SHT_SUNW_dof Ta Dv ELF_T_BYTE Ta Xo 392Used by 393.Xr dtrace 1 . 394.Xc 395.It Dv SHT_SUNW_move Ta Dv ELF_T_MOVE Ta ELF move records. 396.It Dv SHT_SUNW_syminfo Ta Dv ELF_T_SYMINFO Ta Additional symbol flags. 397.It Dv SHT_SUNW_verdef Ta Dv ELF_T_VDEF Ta Xo 398Same as 399.Dv SHT_GNU_verdef . 400.Xc 401.It Dv SHT_SUNW_verneed Ta Dv ELF_T_VNEED Ta Xo 402Same as 403.Dv SHT_GNU_verneed . 404.Xc 405.It Dv SHT_SUNW_versym Ta Dv ELF_T_HALF Ta Xo 406Same as 407.Dv SHT_GNU_versym . 408.Xc 409.El 410.Pp 411Section types in the range 412.Ns [ Dv SHT_LOOS , 413.Dv SHT_HIUSER ] 414are otherwise considered to be of type 415.Dv ELF_T_BYTE . 416.Ss Functional Grouping 417This section contains a brief overview of the available functionality 418in the ELF library. 419Each function listed here is described further in its own manual page. 420.Bl -tag -width indent 421.It "Archive Access" 422.Bl -tag -compact 423.It Fn elf_getarsym 424Retrieve the archive symbol table. 425.It Fn elf_getarhdr 426Retrieve the archive header for an object. 427.It Fn elf_getbase 428Retrieve the offset of a member inside an archive. 429.It Fn elf_next 430Iterate through an 431.Xr ar 1 432archive. 433.It Fn elf_rand 434Random access inside an 435.Xr ar 1 436archive. 437.El 438.It "Data Structures" 439.Bl -tag -compact 440.It Fn elf_getdata 441Retrieve translated data for an ELF section. 442.It Fn elf_getscn 443Retrieve the section descriptor for a named section. 444.It Fn elf_ndxscn 445Retrieve the index for a section. 446.It Fn elf_newdata 447Add a new 448.Vt Elf_Data 449descriptor to an ELF section. 450.It Fn elf_newscn 451Add a new section descriptor to an ELF descriptor. 452.It Fn elf_nextscn 453Iterate through the sections in an ELF object. 454.It Fn elf_rawdata 455Retrieve untranslated data for an ELF section. 456.It Fn elf_rawfile 457Return a pointer to the untranslated file contents for an ELF object. 458.It Fn elf32_getehdr , Fn elf64_getehdr 459Retrieve the Executable Header in an ELF object. 460.It Fn elf32_getphdr , Fn elf64_getphdr 461Retrieve the Program Header Table in an ELF object. 462.It Fn elf32_getshdr , Fn elf64_getshdr 463Retrieve the ELF section header associated with an 464.Vt Elf_Scn 465descriptor. 466.It Fn elf32_newehdr , Fn elf64_newehdr 467Allocate an Executable Header in an ELF object. 468.It Fn elf32_newphdr , Fn elf64_newphdr 469Allocate or resize the Program Header Table in an ELF object. 470.El 471.It "Data Translation" 472.Bl -tag -compact 473.It Fn elf32_xlatetof , Fn elf64_xlatetof 474Translate an ELF data structure from its native representation to its 475file representation. 476.It Fn elf32_xlatetom , Fn elf64_xlatetom 477Translate an ELF data structure from its file representation to a 478native representation. 479.El 480.It "Error Reporting" 481.Bl -tag -compact 482.It Fn elf_errno 483Retrieve the current error. 484.It Fn elf_errmsg 485Retrieve a human readable description of the current error. 486.El 487.It "Initialization" 488.Bl -tag -compact 489.It Fn elf_begin 490Opens an 491.Xr ar 1 492archive or ELF object given a file descriptor. 493.It Fn elf_end 494Close an ELF descriptor and release all its resources. 495.It Fn elf_memory 496Opens an 497.Xr ar 1 498archive or ELF object present in a memory arena. 499.It Fn elf_version 500Sets the operating version. 501.El 502.It "IO Control" 503.Bl -tag -width ".Fn elf_setshstrndx" -compact 504.It Fn elf_cntl 505Manage the association between and ELF descriptor and its underlying file. 506.It Fn elf_flagdata 507Mark an 508.Vt Elf_Data 509descriptor as dirty. 510.It Fn elf_flagehdr 511Mark the ELF Executable Header in an ELF descriptor as dirty. 512.It Fn elf_flagphdr 513Mark the ELF Program Header Table in an ELF descriptor as dirty. 514.It Fn elf_flagscn 515Mark an 516.Vt Elf_Scn 517descriptor as dirty. 518.It Fn elf_flagshdr 519Mark an ELF Section Header as dirty. 520.It Fn elf_setshstrndx 521Set the index of the section name string table for the ELF object. 522.It Fn elf_update 523Recompute ELF object layout and optionally write the modified object 524back to the underlying file. 525.El 526.It "Queries" 527.Bl -tag -width ".Fn elf_getshstrndx" -compact 528.It Fn elf32_checksum , Fn elf64_checkum 529Compute checksum of an ELF object. 530.It Fn elf_getident 531Retrieve the identification bytes for an ELF object. 532.It Fn elf_getshnum 533Retrieve the number of sections in an ELF object. 534.It Fn elf_getshstrndx 535Retrieve the section index of the section name string table in 536an ELF object. 537.It Fn elf_hash 538Compute the ELF hash value of a string. 539.It Fn elf_kind 540Query the kind of object associated with an ELF descriptor. 541.It Fn elf32_fsize , Fn elf64_fsize 542Return the size of the file representation of an ELF type. 543.El 544.El 545.Ss Controlling ELF Object Layout 546In the usual mode of operation, library will compute section 547offsets and alignments based on the contents of an ELF descriptor's 548sections without need for further intervention by the 549application. 550.Pp 551However, if the application wishes to take complete charge of the 552layout of the ELF file, it may set the 553.Dv ELF_F_LAYOUT 554flag on an ELF descriptor using 555.Xr elf_flagelf 3 , 556following which the library will use the data offsets and alignments 557specified by the application when laying out the file. 558Application control of file layout is described further in the 559.Xr elf_update 3 560manual page. 561.Pp 562Gaps in between sections will be filled with the fill character 563set by function 564.Fn elf_fill . 565.Ss Error Handling 566In case an error is encountered, these library functions set an 567internal error number and signal the presence of the error by 568returning an special return value. 569The application can check the 570current error number by calling 571.Xr elf_errno 3 . 572A human readable description of the recorded error is available by 573calling 574.Xr elf_errmsg 3 . 575.Ss Memory Management Rules 576The library keeps track of all 577.Vt Elf_Scn 578and 579.Vt Elf_Data 580descriptors associated with an ELF descriptor and recovers them 581when the descriptor is closed using 582.Xr elf_end 3 . 583Thus the application must not call 584.Xr free 3 585on data structures allocated by the ELF library. 586.Pp 587Conversely the library will not 588free data that it has not allocated. 589As an example, an application may call 590.Xr elf_newdata 3 591to allocate a new 592.Vt Elf_Data 593descriptor and can set the 594.Va d_off 595member of the descriptor to point to a region of memory allocated 596using 597.Xr malloc 3 . 598It is the applications responsibility to free this arena, though the 599library will reclaim the space used by the 600.Vt Elf_Data 601descriptor itself. 602.Sh SEE ALSO 603.Xr gelf 3 , 604.Xr elf 5 605.Sh HISTORY 606The original ELF(3) API was developed for Unix System V. 607The current implementation of the ELF(3) API appeared in 608.Fx 7.0 . 609.Sh AUTHORS 610The ELF library was written by 611.An Joseph Koshy Aq Mt jkoshy@FreeBSD.org . 612