1.\" Copyright (c) 1999 Jeroen Ruigrok van der Werven 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. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd July 25, 2022 28.Dt ELF 5 29.Os 30.Sh NAME 31.Nm elf 32.Nd format of ELF executable binary files 33.Sh SYNOPSIS 34.In elf.h 35.Sh DESCRIPTION 36The header file 37.In elf.h 38defines the format of ELF executable binary files. 39Amongst these files are 40normal executable files, relocatable object files, core files and shared 41libraries. 42.Pp 43An executable file using the ELF file format consists of an ELF header, 44followed by a program header table or a section header table, or both. 45The ELF header is always at offset zero of the file. 46The program header 47table and the section header table's offset in the file are defined in the 48ELF header. 49The two tables describe the rest of the particularities of 50the file. 51.Pp 52Applications which wish to process ELF binary files for their native 53architecture only should include 54.In elf.h 55in their source code. 56These applications should need to refer to 57all the types and structures by their generic names 58.Dq Elf_xxx 59and to the macros by 60.Dq ELF_xxx . 61Applications written this way can be compiled on any architecture, 62regardless whether the host is 32-bit or 64-bit. 63.Pp 64Should an application need to process ELF files of an unknown 65architecture then the application needs to include both 66.In sys/elf32.h 67and 68.In sys/elf64.h 69instead of 70.In elf.h . 71Furthermore, all types and structures need to be identified by either 72.Dq Elf32_xxx 73or 74.Dq Elf64_xxx . 75The macros need to be identified by 76.Dq ELF32_xxx 77or 78.Dq ELF64_xxx . 79.Pp 80Whatever the system's architecture is, it will always include 81.In sys/elf_common.h 82as well as 83.In sys/elf_generic.h . 84.Pp 85These header files describe the above mentioned headers as C structures 86and also include structures for dynamic sections, relocation sections and 87symbol tables. 88.Pp 89The following types are being used for 32-bit architectures: 90.Bd -literal -offset indent 91Elf32_Addr Unsigned 32-bit program address 92Elf32_Half Unsigned 16-bit field 93Elf32_Lword Unsigned 64-bit field 94Elf32_Off Unsigned 32-bit file offset 95Elf32_Sword Signed 32-bit field or integer 96Elf32_Word Unsigned 32-bit field or integer 97.Ed 98.Pp 99For 64-bit architectures we have the following types: 100.Bd -literal -offset indent 101Elf64_Addr Unsigned 64-bit program address 102Elf64_Half Unsigned 16-bit field 103Elf64_Lword Unsigned 64-bit field 104Elf64_Off Unsigned 64-bit file offset 105Elf64_Sword Signed 32-bit field 106Elf64_Sxword Signed 64-bit field or integer 107Elf64_Word Unsigned 32-bit field 108Elf64_Xword Unsigned 64-bit field or integer 109.Ed 110.Pp 111All data structures that the file format defines follow the 112.Dq natural 113size and alignment guidelines for the relevant class. 114If necessary, 115data structures contain explicit padding to ensure 4-byte alignment 116for 4-byte objects, to force structure sizes to a multiple of 4, etc. 117.Pp 118The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 119.Bd -literal -offset indent 120typedef struct { 121 unsigned char e_ident[EI_NIDENT]; 122 Elf32_Half e_type; 123 Elf32_Half e_machine; 124 Elf32_Word e_version; 125 Elf32_Addr e_entry; 126 Elf32_Off e_phoff; 127 Elf32_Off e_shoff; 128 Elf32_Word e_flags; 129 Elf32_Half e_ehsize; 130 Elf32_Half e_phentsize; 131 Elf32_Half e_phnum; 132 Elf32_Half e_shentsize; 133 Elf32_Half e_shnum; 134 Elf32_Half e_shstrndx; 135} Elf32_Ehdr; 136.Ed 137.Bd -literal -offset indent 138typedef struct { 139 unsigned char e_ident[EI_NIDENT]; 140 Elf64_Half e_type; 141 Elf64_Half e_machine; 142 Elf64_Word e_version; 143 Elf64_Addr e_entry; 144 Elf64_Off e_phoff; 145 Elf64_Off e_shoff; 146 Elf64_Word e_flags; 147 Elf64_Half e_ehsize; 148 Elf64_Half e_phentsize; 149 Elf64_Half e_phnum; 150 Elf64_Half e_shentsize; 151 Elf64_Half e_shnum; 152 Elf64_Half e_shstrndx; 153} Elf64_Ehdr; 154.Ed 155.Pp 156The fields have the following meanings: 157.Pp 158.Bl -tag -width "e_phentsize" -compact -offset indent 159.It Dv e_ident 160This array of bytes specifies to interpret the file, 161independent of the processor or the file's remaining contents. 162Within this array everything is named by macros, which start with 163the prefix 164.Sy EI_ 165and may contain values which start with the prefix 166.Sy ELF . 167The following macros are defined: 168.Pp 169.Bl -tag -width "EI_ABIVERSION" -compact 170.It Dv EI_MAG0 171The first byte of the magic number. 172It must be filled with 173.Sy ELFMAG0 . 174.It Dv EI_MAG1 175The second byte of the magic number. 176It must be filled with 177.Sy ELFMAG1 . 178.It Dv EI_MAG2 179The third byte of the magic number. 180It must be filled with 181.Sy ELFMAG2 . 182.It Dv EI_MAG3 183The fourth byte of the magic number. 184It must be filled with 185.Sy ELFMAG3 . 186.It Dv EI_CLASS 187The fifth byte identifies the architecture for this binary: 188.Pp 189.Bl -tag -width "ELFCLASSNONE" -compact 190.It Dv ELFCLASSNONE 191This class is invalid. 192.It Dv ELFCLASS32 193This defines the 32-bit architecture. 194It supports machines with files 195and virtual address spaces up to 4 Gigabytes. 196.It Dv ELFCLASS64 197This defines the 64-bit architecture. 198.El 199.It Dv EI_DATA 200The sixth byte specifies the data encoding of the processor-specific 201data in the file. 202Currently these encodings are supported: 203.Pp 204.Bl -tag -width "ELFDATA2LSB" -compact 205.It Dv ELFDATANONE 206Unknown data format. 207.It Dv ELFDATA2LSB 208Two's complement, little-endian. 209.It Dv ELFDATA2MSB 210Two's complement, big-endian. 211.El 212.It Dv EI_VERSION 213The version number of the ELF specification: 214.Pp 215.Bl -tag -width "EV_CURRENT" -compact 216.It Dv EV_NONE 217Invalid version. 218.It Dv EV_CURRENT 219Current version. 220.El 221.It Dv EI_OSABI 222This byte identifies the operating system 223and ABI to which the object is targeted. 224Some fields in other ELF structures have flags 225and values that have platform specific meanings; 226the interpretation of those fields is determined by the value of this byte. 227The following values are currently defined: 228.Pp 229.Bl -tag -width "ELFOSABI_STANDALONE" -compact 230.It Dv ELFOSABI_SYSV 231UNIX System V ABI. 232.It Dv ELFOSABI_HPUX 233HP-UX operating system ABI. 234.It Dv ELFOSABI_NETBSD 235.Nx 236operating system ABI. 237.It Dv ELFOSABI_LINUX 238GNU/Linux operating system ABI. 239.It Dv ELFOSABI_HURD 240GNU/Hurd operating system ABI. 241.It Dv ELFOSABI_86OPEN 24286Open Common IA32 ABI. 243.It Dv ELFOSABI_SOLARIS 244Solaris operating system ABI. 245.It Dv ELFOSABI_MONTEREY 246Monterey project ABI. 247.It Dv ELFOSABI_IRIX 248IRIX operating system ABI. 249.It Dv ELFOSABI_FREEBSD 250.Fx 251operating system ABI. 252.It Dv ELFOSABI_TRU64 253TRU64 UNIX operating system ABI. 254.It Dv ELFOSABI_ARM 255ARM architecture ABI. 256.It Dv ELFOSABI_STANDALONE 257Standalone (embedded) ABI. 258.El 259.It Dv EI_ABIVERSION 260This byte identifies the version of the ABI 261to which the object is targeted. 262This field is used to distinguish among incompatible versions of an ABI. 263The interpretation of this version number 264is dependent on the ABI identified by the EI_OSABI field. 265Applications conforming to this specification use the value 0. 266.It Dv EI_PAD 267Start of padding. 268These bytes are reserved and set to zero. 269Programs 270which read them should ignore them. 271The value for EI_PAD will change in 272the future if currently unused bytes are given meanings. 273.It Dv EI_BRAND 274Start of architecture identification. 275.It Dv EI_NIDENT 276The size of the e_ident array. 277.El 278.Pp 279.It Dv e_type 280This member of the structure identifies the object file type: 281.Pp 282.Bl -tag -width "ET_NONE" -compact 283.It Dv ET_NONE 284An unknown type. 285.It Dv ET_REL 286A relocatable file. 287.It Dv ET_EXEC 288An executable file. 289.It Dv ET_DYN 290A shared object. 291.It Dv ET_CORE 292A core file. 293.El 294.Pp 295.It Dv e_machine 296This member specifies the required architecture for an individual file: 297.Pp 298.Bl -tag -width "EM_MIPS_RS4_BE" -compact 299.It Dv EM_NONE 300An unknown machine. 301.It Dv EM_M32 302AT&T WE 32100. 303.It Dv EM_SPARC 304Sun Microsystems SPARC. 305.It Dv EM_386 306Intel 80386. 307.It Dv EM_68K 308Motorola 68000. 309.It Dv EM_88K 310Motorola 88000. 311.It Dv EM_486 312Intel 80486. 313.It Dv EM_860 314Intel 80860. 315.It Dv EM_MIPS 316MIPS RS3000 (big-endian only). 317.It Dv EM_MIPS_RS4_BE 318MIPS RS4000 (big-endian only). 319.It Dv EM_SPARC64 320SPARC v9 64-bit unofficial. 321.It Dv EM_PARISC 322HPPA. 323.It Dv EM_PPC 324PowerPC. 325.It Dv EM_ALPHA 326Compaq [DEC] Alpha. 327.El 328.Pp 329.It Dv e_version 330This member identifies the file version: 331.Pp 332.Bl -tag -width "EV_CURRENT" -compact 333.It Dv EV_NONE 334Invalid version 335.It Dv EV_CURRENT 336Current version 337.El 338.It Dv e_entry 339This member gives the virtual address to which the system first transfers 340control, thus starting the process. 341If the file has no associated entry 342point, this member holds zero. 343.It Dv e_phoff 344This member holds the program header table's file offset in bytes. 345If 346the file has no program header table, this member holds zero. 347.It Dv e_shoff 348This member holds the section header table's file offset in bytes. 349If the 350file has no section header table this member holds zero. 351.It Dv e_flags 352This member holds processor-specific flags associated with the file. 353Flag 354names take the form EF_`machine_flag'. 355Currently no flags have been defined. 356.It Dv e_ehsize 357This member holds the ELF header's size in bytes. 358.It Dv e_phentsize 359This member holds the size in bytes of one entry in the file's program header 360table; all entries are the same size. 361.It Dv e_phnum 362This member holds the number of entries in the program header 363table. 364If the file is using extended program header numbering, then the 365.Sy e_phnum 366member will contain the value 367.Dv PN_XNUM 368and the actual number of program header table entries will be stored 369in the 370.Sy sh_info 371member of the section header at index 372.Dv SHN_UNDEF . 373The product of 374.Sy e_phentsize 375and the number of program header table entries gives the program 376header table's size in bytes. 377If a file has no program header, 378.Sy e_phnum 379holds the value zero. 380.It Dv e_shentsize 381This member holds a sections header's size in bytes. 382A section header is one 383entry in the section header table; all entries are the same size. 384.It Dv e_shnum 385This member holds the number of entries in the section header table. 386If the file is using extended section numbering, then the 387.Sy e_shnum 388member will be zero and the actual section number will be stored in the 389.Sy sh_size 390member of the section header at index 391.Dv SHN_UNDEF . 392If a file has no section header table, both the 393.Sy e_shnum 394and the 395.Sy e_shoff 396fields of the ELF header will be zero. 397The product of 398.Sy e_shentsize 399and the number of sections in the file gives the section header 400table's size in bytes. 401.It Dv e_shstrndx 402This member holds the section header table index of the entry associated 403with the section name string table. 404If extended section numbering is being used, this field will hold the 405value 406.Sy SHN_XINDEX , 407and the actual section header table index will be present in the 408.Sy sh_link 409field of the section header entry at index 410.Dv SHN_UNDEF . 411If the file has no section name string 412table, this member holds the value 413.Sy SHN_UNDEF . 414.El 415.Pp 416An executable or shared object file's program header table is an array of 417structures, each describing a segment or other information the system needs 418to prepare the program for execution. 419An object file 420.Em segment 421contains one or more 422.Em sections . 423Program headers are meaningful only for executable and shared object files. 424A file specifies its own program header size with the ELF header's 425.Sy e_phentsize 426and 427.Sy e_phnum 428members. 429As with the Elf executable header, the program header 430also has different versions depending on the architecture: 431.Bd -literal -offset indent 432typedef struct { 433 Elf32_Word p_type; 434 Elf32_Off p_offset; 435 Elf32_Addr p_vaddr; 436 Elf32_Addr p_paddr; 437 Elf32_Word p_filesz; 438 Elf32_Word p_memsz; 439 Elf32_Word p_flags; 440 Elf32_Word p_align; 441} Elf32_Phdr; 442.Ed 443.Bd -literal -offset indent 444typedef struct { 445 Elf64_Word p_type; 446 Elf64_Word p_flags; 447 Elf64_Off p_offset; 448 Elf64_Addr p_vaddr; 449 Elf64_Addr p_paddr; 450 Elf64_Xword p_filesz; 451 Elf64_Xword p_memsz; 452 Elf64_Xword p_align; 453} Elf64_Phdr; 454.Ed 455.Pp 456The main difference between the 32-bit and the 64-bit program header lies 457only in the location of a 458.Sy p_flags 459member in the total struct. 460.Pp 461.Bl -tag -width "p_offset" -compact -offset indent 462.It Dv p_type 463This member of the Phdr struct tells what kind of segment this array 464element describes or how to interpret the array element's information. 465.Pp 466.Bl -tag -width "PT_DYNAMIC" -compact 467.It Dv PT_NULL 468The array element is unused and the other members' values are undefined. 469This lets the program header have ignored entries. 470.It Dv PT_LOAD 471The array element specifies a loadable segment, described by 472.Sy p_filesz 473and 474.Sy p_memsz . 475The bytes from the file are mapped to the beginning of the memory 476segment. 477If the segment's memory size 478.Pq Sy p_memsz 479is larger than the file size 480.Pq Sy p_filesz , 481the 482.Dq extra 483bytes are defined to hold the value 0 and to follow the segment's 484initialized area. 485The file size may not be larger than the memory size. 486Loadable segment entries in the program header table appear in ascending 487order, sorted on the 488.Sy p_vaddr 489member. 490.It Dv PT_DYNAMIC 491The array element specifies dynamic linking information. 492.It Dv PT_INTERP 493The array element specifies the location and size of a null-terminated 494path name to invoke as an interpreter. 495This segment type is meaningful 496only for executable files (though it may occur for shared objects). 497However 498it may not occur more than once in a file. 499If it is present it must precede 500any loadable segment entry. 501.It Dv PT_NOTE 502The array element specifies the location and size for auxiliary information. 503.It Dv PT_SHLIB 504This segment type is reserved but has unspecified semantics. 505Programs that 506contain an array element of this type do not conform to the ABI. 507.It Dv PT_PHDR 508The array element, if present, specifies the location and size of the program 509header table itself, both in the file and in the memory image of the program. 510This segment type may not occur more than once in a file. 511Moreover, it may 512only occur if the program header table is part of the memory image of the 513program. 514If it is present it must precede any loadable segment entry. 515.It Dv PT_LOPROC 516This value up to and including 517.Sy PT_HIPROC 518are reserved for processor-specific semantics. 519.It Dv PT_HIPROC 520This value down to and including 521.Sy PT_LOPROC 522are reserved for processor-specific semantics. 523.El 524.Pp 525.It Dv p_offset 526This member holds the offset from the beginning of the file at which 527the first byte of the segment resides. 528.It Dv p_vaddr 529This member holds the virtual address at which the first byte of the 530segment resides in memory. 531.It Dv p_paddr 532On systems for which physical addressing is relevant, this member is 533reserved for the segment's physical address. 534Under 535.Bx 536this member is 537not used and must be zero. 538.It Dv p_filesz 539This member holds the number of bytes in the file image of the segment. 540It may be zero. 541.It Dv p_memsz 542This member holds the number of bytes in the memory image of the segment. 543It may be zero. 544.It Dv p_flags 545This member holds flags relevant to the segment: 546.Pp 547.Bl -tag -width "PF_X" -compact 548.It Dv PF_X 549An executable segment. 550.It Dv PF_W 551A writable segment. 552.It Dv PF_R 553A readable segment. 554.El 555.Pp 556A text segment commonly has the flags 557.Sy PF_X 558and 559.Sy PF_R . 560A data segment commonly has 561.Sy PF_X , 562.Sy PF_W 563and 564.Sy PF_R . 565.It Dv p_align 566This member holds the value to which the segments are aligned in memory 567and in the file. 568Loadable process segments must have congruent values for 569.Sy p_vaddr 570and 571.Sy p_offset , 572modulo the page size. 573Values of zero and one mean no alignment is required. 574Otherwise, 575.Sy p_align 576should be a positive, integral power of two, and 577.Sy p_vaddr 578should equal 579.Sy p_offset , 580modulo 581.Sy p_align . 582.El 583.Pp 584An file's section header table lets one locate all the file's sections. 585The 586section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 587The 588ELF header's 589.Sy e_shoff 590member gives the byte offset from the beginning of the file to the section 591header table. 592.Sy e_shnum 593holds the number of entries the section header table contains. 594.Sy e_shentsize 595holds the size in bytes of each entry. 596.Pp 597A section header table index is a subscript into this array. 598Some section 599header table indices are reserved. 600An object file does not have sections for 601these special indices: 602.Pp 603.Bl -tag -width "SHN_LORESERVE" -compact 604.It Dv SHN_UNDEF 605This value marks an undefined, missing, irrelevant, or otherwise meaningless 606section reference. 607For example, a symbol 608.Dq defined 609relative to section number 610.Sy SHN_UNDEF 611is an undefined symbol. 612.It Dv SHN_LORESERVE 613This value specifies the lower bound of the range of reserved indices. 614.It Dv SHN_LOPROC 615This value up to and including 616.Sy SHN_HIPROC 617are reserved for processor-specific semantics. 618.It Dv SHN_HIPROC 619This value down to and including 620.Sy SHN_LOPROC 621are reserved for processor-specific semantics. 622.It Dv SHN_ABS 623This value specifies absolute values for the corresponding reference. 624For 625example, symbols defined relative to section number 626.Sy SHN_ABS 627have absolute values and are not affected by relocation. 628.It Dv SHN_COMMON 629Symbols defined relative to this section are common symbols, such as FORTRAN 630COMMON or unallocated C external variables. 631.It Dv SHN_HIRESERVE 632This value specifies the upper bound of the range of reserved indices. 633The 634system reserves indices between 635.Sy SHN_LORESERVE 636and 637.Sy SHN_HIRESERVE , 638inclusive. 639The section header table does not contain entries for the 640reserved indices. 641.El 642.Pp 643The section header has the following structure: 644.Bd -literal -offset indent 645typedef struct { 646 Elf32_Word sh_name; 647 Elf32_Word sh_type; 648 Elf32_Word sh_flags; 649 Elf32_Addr sh_addr; 650 Elf32_Off sh_offset; 651 Elf32_Word sh_size; 652 Elf32_Word sh_link; 653 Elf32_Word sh_info; 654 Elf32_Word sh_addralign; 655 Elf32_Word sh_entsize; 656} Elf32_Shdr; 657.Ed 658.Bd -literal -offset indent 659typedef struct { 660 Elf64_Word sh_name; 661 Elf64_Word sh_type; 662 Elf64_Xword sh_flags; 663 Elf64_Addr sh_addr; 664 Elf64_Off sh_offset; 665 Elf64_Xword sh_size; 666 Elf64_Word sh_link; 667 Elf64_Word sh_info; 668 Elf64_Xword sh_addralign; 669 Elf64_Xword sh_entsize; 670} Elf64_Shdr; 671.Ed 672.Pp 673.Bl -tag -width "sh_addralign" -compact 674.It Dv sh_name 675This member specifies the name of the section. 676Its value is an index 677into the section header string table section, giving the location of 678a null-terminated string. 679.It Dv sh_type 680This member categorizes the section's contents and semantics. 681.Pp 682.Bl -tag -width "SHT_PROGBITS" -compact 683.It Dv SHT_NULL 684This value marks the section header as inactive. 685It does not 686have an associated section. 687Other members of the section header 688have undefined values. 689.It Dv SHT_PROGBITS 690The section holds information defined by the program, whose 691format and meaning are determined solely by the program. 692.It Dv SHT_SYMTAB 693This section holds a symbol table. 694Typically, 695.Sy SHT_SYMTAB 696provides symbols for link editing, though it may also be used 697for dynamic linking. 698As a complete symbol table, it may contain 699many symbols unnecessary for dynamic linking. 700An object file can 701also contain a 702.Sy SHN_DYNSYM 703section. 704.It Dv SHT_STRTAB 705This section holds a string table. 706An object file may have multiple 707string table sections. 708.It Dv SHT_RELA 709This section holds relocation entries with explicit addends, such 710as type 711.Sy Elf32_Rela 712for the 32-bit class of object files. 713An object may have multiple 714relocation sections. 715.It Dv SHT_HASH 716This section holds a symbol hash table. 717All object participating in 718dynamic linking must contain a symbol hash table. 719An object file may 720have only one hash table. 721.It Dv SHT_DYNAMIC 722This section holds information for dynamic linking. 723An object file may 724have only one dynamic section. 725.It Dv SHT_NOTE 726This section holds information that marks the file in some way. 727.It Dv SHT_NOBITS 728A section of this type occupies no space in the file but otherwise 729resembles 730.Sy SHN_PROGBITS . 731Although this section contains no bytes, the 732.Sy sh_offset 733member contains the conceptual file offset. 734.It Dv SHT_REL 735This section holds relocation offsets without explicit addends, such 736as type 737.Sy Elf32_Rel 738for the 32-bit class of object files. 739An object file may have multiple 740relocation sections. 741.It Dv SHT_SHLIB 742This section is reserved but has unspecified semantics. 743.It Dv SHT_DYNSYM 744This section holds a minimal set of dynamic linking symbols. 745An 746object file can also contain a 747.Sy SHN_SYMTAB 748section. 749.It Dv SHT_LOPROC 750This value up to and including 751.Sy SHT_HIPROC 752are reserved for processor-specific semantics. 753.It Dv SHT_HIPROC 754This value down to and including 755.Sy SHT_LOPROC 756are reserved for processor-specific semantics. 757.It Dv SHT_LOUSER 758This value specifies the lower bound of the range of indices reserved for 759application programs. 760.It Dv SHT_HIUSER 761This value specifies the upper bound of the range of indices reserved for 762application programs. 763Section types between 764.Sy SHT_LOUSER 765and 766.Sy SHT_HIUSER 767may be used by the application, without conflicting with current or future 768system-defined section types. 769.El 770.Pp 771.It Dv sh_flags 772Sections support one-bit flags that describe miscellaneous attributes. 773If a flag bit is set in 774.Sy sh_flags , 775the attribute is 776.Dq on 777for the section. 778Otherwise, the attribute is 779.Dq off 780or does not apply. 781Undefined attributes are set to zero. 782.Pp 783.Bl -tag -width "SHF_EXECINSTR" -compact 784.It Dv SHF_WRITE 785This section contains data that should be writable during process 786execution. 787.It Dv SHF_ALLOC 788The section occupies memory during process execution. 789Some control 790sections do not reside in the memory image of an object file. 791This 792attribute is off for those sections. 793.It Dv SHF_EXECINSTR 794The section contains executable machine instructions. 795.It Dv SHF_MASKPROC 796All bits included in this mask are reserved for processor-specific 797semantics. 798.It Dv SHF_COMPRESSED 799The section data is compressed. 800.El 801.Pp 802.It Dv sh_addr 803If the section will appear in the memory image of a process, this member 804holds the address at which the section's first byte should reside. 805Otherwise, the member contains zero. 806.It Dv sh_offset 807This member's value holds the byte offset from the beginning of the file 808to the first byte in the section. 809One section type, 810.Sy SHT_NOBITS , 811occupies no space in the file, and its 812.Sy sh_offset 813member locates the conceptual placement in the file. 814.It Dv sh_size 815This member holds the section's size in bytes. 816Unless the section type 817is 818.Sy SHT_NOBITS , 819the section occupies 820.Sy sh_size 821bytes in the file. 822A section of type 823.Sy SHT_NOBITS 824may have a non-zero size, but it occupies no space in the file. 825.It Dv sh_link 826This member holds a section header table index link, whose interpretation 827depends on the section type. 828.It Dv sh_info 829This member holds extra information, whose interpretation depends on the 830section type. 831.It Dv sh_addralign 832Some sections have address alignment constraints. 833If a section holds a 834doubleword, the system must ensure doubleword alignment for the entire 835section. 836That is, the value of 837.Sy sh_addr 838must be congruent to zero, modulo the value of 839.Sy sh_addralign . 840Only zero and positive integral powers of two are allowed. 841Values of zero 842or one mean the section has no alignment constraints. 843.It Dv sh_entsize 844Some sections hold a table of fixed-sized entries, such as a symbol table. 845For such a section, this member gives the size in bytes for each entry. 846This member contains zero if the section does not hold a table of 847fixed-size entries. 848.El 849.Pp 850Various sections hold program and control information: 851.Bl -tag -width ".shstrtab" -compact 852.It .bss 853(Block Started by Symbol) 854This section holds uninitialized data that contributes to the program's 855memory image. 856By definition, the system initializes the data with zeros 857when the program begins to run. 858This section is of type 859.Sy SHT_NOBITS . 860The attributes types are 861.Sy SHF_ALLOC 862and 863.Sy SHF_WRITE . 864.It .comment 865This section holds version control information. 866This section is of type 867.Sy SHT_PROGBITS . 868No attribute types are used. 869.It .data 870This section holds initialized data that contribute to the program's 871memory image. 872This section is of type 873.Sy SHT_PROGBITS . 874The attribute types are 875.Sy SHF_ALLOC 876and 877.Sy SHF_WRITE . 878.It .data1 879This section holds initialized data that contribute to the program's 880memory image. 881This section is of type 882.Sy SHT_PROGBITS . 883The attribute types are 884.Sy SHF_ALLOC 885and 886.Sy SHF_WRITE . 887.It .debug 888This section holds information for symbolic debugging. 889The contents 890are unspecified. 891This section is of type 892.Sy SHT_PROGBITS . 893No attribute types are used. 894.It .dynamic 895This section holds dynamic linking information. 896The section's attributes 897will include the 898.Sy SHF_ALLOC 899bit. 900Whether the 901.Sy SHF_WRITE 902bit is set is processor-specific. 903This section is of type 904.Sy SHT_DYNAMIC . 905See the attributes above. 906.It .dynstr 907This section holds strings needed for dynamic linking, most commonly 908the strings that represent the names associated with symbol table entries. 909This section is of type 910.Sy SHT_STRTAB . 911The attribute type used is 912.Sy SHF_ALLOC . 913.It .dynsym 914This section holds the dynamic linking symbol table. 915This section is of type 916.Sy SHT_DYNSYM . 917The attribute used is 918.Sy SHF_ALLOC . 919.It .fini 920This section holds executable instructions that contribute to the process 921termination code. 922When a program exits normally the system arranges to 923execute the code in this section. 924This section is of type 925.Sy SHT_PROGBITS . 926The attributes used are 927.Sy SHF_ALLOC 928and 929.Sy SHF_EXECINSTR . 930.It .got 931This section holds the global offset table. 932This section is of type 933.Sy SHT_PROGBITS . 934The attributes are processor-specific. 935.It .hash 936This section holds a symbol hash table. 937This section is of type 938.Sy SHT_HASH . 939The attribute used is 940.Sy SHF_ALLOC . 941.It .init 942This section holds executable instructions that contribute to the process 943initialization code. 944When a program starts to run the system arranges to 945execute the code in this section before calling the main program entry point. 946This section is of type 947.Sy SHT_PROGBITS . 948The attributes used are 949.Sy SHF_ALLOC 950and 951.Sy SHF_EXECINSTR . 952.It .interp 953This section holds the pathname of a program interpreter. 954If the file has 955a loadable segment that includes the section, the section's attributes will 956include the 957.Sy SHF_ALLOC 958bit. 959Otherwise, that bit will be off. 960This section is of type 961.Sy SHT_PROGBITS . 962.It .line 963This section holds line number information for symbolic debugging, which 964describes the correspondence between the program source and the machine code. 965The contents are unspecified. 966This section is of type 967.Sy SHT_PROGBITS . 968No attribute types are used. 969.It .note 970This section holds information in the 971.Dq Note Section 972format described below. 973This section is of type 974.Sy SHT_NOTE . 975No attribute types are used. 976.It .plt 977This section holds the procedure linkage table. 978This section is of type 979.Sy SHT_PROGBITS . 980The attributes are processor-specific. 981.It .relNAME 982This section holds relocation information as described below. 983If the file 984has a loadable segment that includes relocation, the section's attributes 985will include the 986.Sy SHF_ALLOC 987bit. 988Otherwise the bit will be off. 989By convention, 990.Dq NAME 991is supplied by the section to which the relocations apply. 992Thus a relocation 993section for 994.Sy .text 995normally would have the name 996.Sy .rel.text . 997This section is of type 998.Sy SHT_REL . 999.It .relaNAME 1000This section holds relocation information as described below. 1001If the file 1002has a loadable segment that includes relocation, the section's attributes 1003will include the 1004.Sy SHF_ALLOC 1005bit. 1006Otherwise the bit will be off. 1007By convention, 1008.Dq NAME 1009is supplied by the section to which the relocations apply. 1010Thus a relocation 1011section for 1012.Sy .text 1013normally would have the name 1014.Sy .rela.text . 1015This section is of type 1016.Sy SHT_RELA . 1017.It .rodata 1018This section holds read-only data that typically contributes to a 1019non-writable segment in the process image. 1020This section is of type 1021.Sy SHT_PROGBITS . 1022The attribute used is 1023.Sy SHF_ALLOC . 1024.It .rodata1 1025This section hold read-only data that typically contributes to a 1026non-writable segment in the process image. 1027This section is of type 1028.Sy SHT_PROGBITS . 1029The attribute used is 1030.Sy SHF_ALLOC . 1031.It .shstrtab 1032This section holds section names. 1033This section is of type 1034.Sy SHT_STRTAB . 1035No attribute types are used. 1036.It .strtab 1037This section holds strings, most commonly the strings that represent the 1038names associated with symbol table entries. 1039If the file has a loadable 1040segment that includes the symbol string table, the section's attributes 1041will include the 1042.Sy SHF_ALLOC 1043bit. 1044Otherwise the bit will be off. 1045This section is of type 1046.Sy SHT_STRTAB . 1047.It .symtab 1048This section holds a symbol table. 1049If the file has a loadable segment 1050that includes the symbol table, the section's attributes will include 1051the 1052.Sy SHF_ALLOC 1053bit. 1054Otherwise the bit will be off. 1055This section is of type 1056.Sy SHT_SYMTAB . 1057.It .text 1058This section holds the 1059.Dq text , 1060or executable instructions, of a program. 1061This section is of type 1062.Sy SHT_PROGBITS . 1063The attributes used are 1064.Sy SHF_ALLOC 1065and 1066.Sy SHF_EXECINSTR . 1067.It .jcr 1068This section holds information about Java classes that must 1069be registered. 1070.It .eh_frame 1071This section holds information used for C++ exception-handling. 1072.El 1073.Pp 1074A section with the 1075.Dv SHF_COMPRESSED 1076flag set contains a compressed copy of the section data. 1077Compressed section data begins with an 1078.Vt Elf64_Chdr 1079or 1080.Vt Elf32_Chdr structure 1081which encodes the compression algorithm and some characteristics of the 1082uncompressed data. 1083.Bd -literal -offset indent 1084typedef struct { 1085 Elf32_Word ch_type; 1086 Elf32_Word ch_size; 1087 Elf32_Word ch_addralign; 1088} Elf32_Chdr; 1089.Ed 1090.Bd -literal -offset indent 1091typedef struct { 1092 Elf64_Word ch_type; 1093 Elf64_Word ch_reserved; 1094 Elf64_Xword ch_size; 1095 Elf64_Xword ch_addralign; 1096} Elf64_Chdr; 1097.Ed 1098.Pp 1099.Bl -tag -width "ch_addralign" -compact 1100.It Dv ch_type 1101The compression algorithm used. 1102A value of 1103.Dv ELFCOMPRESS_ZLIB 1104indicates that the data is compressed using 1105.Xr zlib 3 . 1106A value of 1107.Dv ELFCOMPRESS_ZSTD 1108indicates that the data is compressed using 1109Zstandard. 1110.It Dv ch_size 1111The size, in bytes, of the uncompressed section data. 1112This corresponds to the 1113.Sy sh_size 1114field of a section header containing uncompressed data. 1115.It Dv ch_addralign 1116The address alignment of the uncompressed section data. 1117This corresponds to the 1118.Sy sh_addralign 1119field of a section header containing uncompressed data. 1120.El 1121.Pp 1122String table sections hold null-terminated character sequences, commonly 1123called strings. 1124The object file uses these strings to represent symbol 1125and section names. 1126One references a string as an index into the string 1127table section. 1128The first byte, which is index zero, is defined to hold 1129a null character. 1130Similarly, a string table's last byte is defined to 1131hold a null character, ensuring null termination for all strings. 1132.Pp 1133An object file's symbol table holds information needed to locate and 1134relocate a program's symbolic definitions and references. 1135A symbol table 1136index is a subscript into this array. 1137.Bd -literal -offset indent 1138typedef struct { 1139 Elf32_Word st_name; 1140 Elf32_Addr st_value; 1141 Elf32_Word st_size; 1142 unsigned char st_info; 1143 unsigned char st_other; 1144 Elf32_Half st_shndx; 1145} Elf32_Sym; 1146.Ed 1147.Bd -literal -offset indent 1148typedef struct { 1149 Elf64_Word st_name; 1150 unsigned char st_info; 1151 unsigned char st_other; 1152 Elf64_Half st_shndx; 1153 Elf64_Addr st_value; 1154 Elf64_Xword st_size; 1155} Elf64_Sym; 1156.Ed 1157.Pp 1158.Bl -tag -width "st_value" -compact 1159.It Dv st_name 1160This member holds an index into the object file's symbol string table, 1161which holds character representations of the symbol names. 1162If the value 1163is non-zero, it represents a string table index that gives the symbol 1164name. 1165Otherwise, the symbol table has no name. 1166.It Dv st_value 1167This member gives the value of the associated symbol. 1168.It Dv st_size 1169Many symbols have associated sizes. 1170This member holds zero if the symbol 1171has no size or an unknown size. 1172.It Dv st_info 1173This member specifies the symbol's type and binding attributes: 1174.Pp 1175.Bl -tag -width "STT_SECTION" -compact 1176.It Dv STT_NOTYPE 1177The symbol's type is not defined. 1178.It Dv STT_OBJECT 1179The symbol is associated with a data object. 1180.It Dv STT_FUNC 1181The symbol is associated with a function or other executable code. 1182.It Dv STT_SECTION 1183The symbol is associated with a section. 1184Symbol table entries of 1185this type exist primarily for relocation and normally have 1186.Sy STB_LOCAL 1187bindings. 1188.It Dv STT_FILE 1189By convention the symbol's name gives the name of the source file 1190associated with the object file. 1191A file symbol has 1192.Sy STB_LOCAL 1193bindings, its section index is 1194.Sy SHN_ABS , 1195and it precedes the other 1196.Sy STB_LOCAL 1197symbols of the file, if it is present. 1198.It Dv STT_LOPROC 1199This value up to and including 1200.Sy STT_HIPROC 1201are reserved for processor-specific semantics. 1202.It Dv STT_HIPROC 1203This value down to and including 1204.Sy STT_LOPROC 1205are reserved for processor-specific semantics. 1206.El 1207.Pp 1208.Bl -tag -width "STB_GLOBAL" -compact 1209.It Dv STB_LOCAL 1210Local symbols are not visible outside the object file containing their 1211definition. 1212Local symbols of the same name may exist in multiple file 1213without interfering with each other. 1214.It Dv STB_GLOBAL 1215Global symbols are visible to all object files being combined. 1216One file's 1217definition of a global symbol will satisfy another file's undefined 1218reference to the same symbol. 1219.It Dv STB_WEAK 1220Weak symbols resemble global symbols, but their definitions have lower 1221precedence. 1222.It Dv STB_LOPROC 1223This value up to and including 1224.Sy STB_HIPROC 1225are reserved for processor-specific semantics. 1226.It Dv STB_HIPROC 1227This value down to and including 1228.Sy STB_LOPROC 1229are reserved for processor-specific semantics. 1230.Pp 1231There are macros for packing and unpacking the binding and type fields: 1232.Pp 1233.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1234.It Xo 1235.Fn ELF32_ST_BIND info 1236.Xc 1237or 1238.Fn ELF64_ST_BIND info 1239extract a binding from an st_info value. 1240.It Xo 1241.Fn ELF64_ST_TYPE info 1242.Xc 1243or 1244.Fn ELF32_ST_TYPE info 1245extract a type from an st_info value. 1246.It Xo 1247.Fn ELF32_ST_INFO bind type 1248.Xc 1249or 1250.Fn ELF64_ST_INFO bind type 1251convert a binding and a type into an st_info value. 1252.El 1253.El 1254.Pp 1255.It Dv st_other 1256This member currently holds zero and has no defined meaning. 1257.It Dv st_shndx 1258Every symbol table entry is 1259.Dq defined 1260in relation to some section. 1261This member holds the relevant section 1262header table index. 1263.El 1264.Pp 1265Relocation is the process of connecting symbolic references with 1266symbolic definitions. 1267Relocatable files must have information that 1268describes how to modify their section contents, thus allowing executable 1269and shared object files to hold the right information for a process' 1270program image. 1271Relocation entries are these data. 1272.Pp 1273Relocation structures that do not need an addend: 1274.Bd -literal -offset indent 1275typedef struct { 1276 Elf32_Addr r_offset; 1277 Elf32_Word r_info; 1278} Elf32_Rel; 1279.Ed 1280.Bd -literal -offset indent 1281typedef struct { 1282 Elf64_Addr r_offset; 1283 Elf64_Xword r_info; 1284} Elf64_Rel; 1285.Ed 1286.Pp 1287Relocation structures that need an addend: 1288.Bd -literal -offset indent 1289typedef struct { 1290 Elf32_Addr r_offset; 1291 Elf32_Word r_info; 1292 Elf32_Sword r_addend; 1293} Elf32_Rela; 1294.Ed 1295.Bd -literal -offset indent 1296typedef struct { 1297 Elf64_Addr r_offset; 1298 Elf64_Xword r_info; 1299 Elf64_Sxword r_addend; 1300} Elf64_Rela; 1301.Ed 1302.Pp 1303.Bl -tag -width "r_offset" -compact 1304.It Dv r_offset 1305This member gives the location at which to apply the relocation action. 1306For a relocatable file, the value is the byte offset from the beginning 1307of the section to the storage unit affected by the relocation. 1308For an 1309executable file or shared object, the value is the virtual address of 1310the storage unit affected by the relocation. 1311.It Dv r_info 1312This member gives both the symbol table index with respect to which the 1313relocation must be made and the type of relocation to apply. 1314Relocation 1315types are processor-specific. 1316When the text refers to a relocation 1317entry's relocation type or symbol table index, it means the result of 1318applying 1319.Sy ELF_[32|64]_R_TYPE 1320or 1321.Sy ELF[32|64]_R_SYM , 1322respectively to the entry's 1323.Sy r_info 1324member. 1325.It Dv r_addend 1326This member specifies a constant addend used to compute the value to be 1327stored into the relocatable field. 1328.El 1329.Ss Note Section 1330ELF note sections consist of entries with the following format: 1331.Bl -column -offset indent "namesz" "32 bits" "Null-terminated originator name" 1332.Sy Field Ta Sy Size Ta Sy Description 1333.It Va namesz Ta 32 bits Ta Size of "name" 1334.It Va descsz Ta 32 bits Ta Size of "desc" 1335.It Va type Ta 32 bits Ta OS-dependent note type 1336.It Va name Ta Va namesz Ta Null-terminated originator name 1337.It Va desc Ta Va descsz Ta OS-dependent note data 1338.El 1339.Pp 1340The 1341.Va name 1342and 1343.Va desc 1344fields are padded to ensure 4-byte alignemnt. 1345.Va namesz 1346and 1347.Va descsz 1348specify the unpadded length. 1349.Pp 1350.Fx 1351defines the following ELF note types 1352.Po with corresponding interpretation of 1353.Va desc Pc : 1354.Bl -tag -width 4n 1355.It Dv NT_FREEBSD_ABI_TAG Pq Value: 1 1356Indicates the OS ABI version in a form of a 32-bit integer containing expected 1357ABI version 1358.Po i.e., 1359.Dv __FreeBSD_version Pc . 1360.It Dv NT_FREEBSD_NOINIT_TAG Pq Value: 2 1361Indicates that the C startup does not call initialization routines, and thus 1362.Xr rtld 1 1363must do so. 1364.Va desc 1365is ignored. 1366.It Dv NT_FREEBSD_ARCH_TAG Pq Value: 3 1367Contains the MACHINE_ARCH that the executable was built for. 1368.It Dv NT_FREEBSD_FEATURE_CTL Pq Value: 4 1369Contains a bitmask of mitigations and features to enable: 1370.Bl -tag -width 4n 1371.It NT_FREEBSD_FCTL_ASLR_DISABLE Pq Value: 0x01 1372Request that address randomization (ASLR) not be performed. 1373See 1374.Xr security 7 . 1375.It NT_FREEBSD_FCTL_PROTMAX_DISABLE Pq Value: 0x02 1376Request that 1377.Xr mmap 2 1378calls not set PROT_MAX to the initial value of the 1379.Fa prot 1380argument. 1381.It NT_FREEBSD_FCTL_STKGAP_DISABLE Pq Value: 0x04 1382Disable stack gap. 1383.It NT_FREEBSD_FCTL_WXNEEDED Pq Value: 0x08 1384Indicate that the binary requires mappings that are simultaneously 1385writeable and executable. 1386.El 1387.El 1388.Sh SEE ALSO 1389.Xr as 1 , 1390.Xr gdb 1 , 1391.Xr ld 1 , 1392.Xr objdump 1 , 1393.Xr readelf 1 , 1394.Xr execve 2 , 1395.Xr zlib 3 , 1396.Xr ar 5 , 1397.Xr core 5 1398.Rs 1399.%A Hewlett Packard 1400.%B Elf-64 Object File Format 1401.Re 1402.Rs 1403.%A Santa Cruz Operation 1404.%B System V Application Binary Interface 1405.Re 1406.Rs 1407.%A Unix System Laboratories 1408.%T Object Files 1409.%B "Executable and Linking Format (ELF)" 1410.Re 1411.Sh HISTORY 1412The ELF header files made their appearance in 1413.Fx 2.2.6 . 1414ELF in itself first appeared in 1415.At V . 1416The ELF format is an adopted standard. 1417.Sh AUTHORS 1418This manual page was written by 1419.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org 1420with inspiration from BSDi's 1421.Bsx 1422.Nm 1423manpage. 1424