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diff --git a/man5/elf.5 b/man5/elf.5 new file mode 100644 index 000000000..7d9671457 --- /dev/null +++ b/man5/elf.5 @@ -0,0 +1,1369 @@ +.\" $OpenBSD: elf.5,v 1.12 2003/10/27 20:23:58 jmc Exp $ +.\"Copyright (c) 1999 Jeroen Ruigrok van der Werven +.\"All rights reserved. +.\" +.\"Redistribution and use in source and binary forms, with or without +.\"modification, are permitted provided that the following conditions +.\"are met: +.\"1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\"2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" +.\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND +.\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\"ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE +.\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\"SUCH DAMAGE. +.\" +.\" $FreeBSD: src/share/man/man5/elf.5,v 1.21 2001/10/01 16:09:23 ru Exp $ +.\" +.\" Slightly adapted - aeb, 2004-01-01 +.Dd July 31, 1999 +.Dt ELF 5 +.Os +.Sh NAME +.Nm elf +.Nd format of ELF executable binary files +.Sh SYNOPSIS +.\" .Fd #include <elf_abi.h> +.Fd #include <elf.h> +.Sh DESCRIPTION +The header file +.\" .Aq Pa elf_abi.h +.Aq Pa elf.h +defines the format of ELF executable binary files. +Amongst these files are +normal executable files, relocatable object files, core files and shared +libraries. +.Pp +An executable file using the ELF file format consists of an ELF header, +followed by a program header table or a section header table, or both. +The ELF header is always at offset zero of the file. +The program header +table and the section header table's offset in the file are defined in the +ELF header. +The two tables describe the rest of the particularities of +the file. +.Pp +.\" Applications which wish to process ELF binary files for their native +.\" architecture only should include +.\" .Aq Pa elf_abi.h +.\" in their source code. +.\" These applications should need to refer to +.\" all the types and structures by their generic names +.\" .Dq Elf_xxx +.\" and to the macros by +.\" .Dq ELF_xxx . +.\" Applications written this way can be compiled on any architecture, +.\" regardless of whether the host is 32-bit or 64-bit. +.\" .Pp +.\" Should an application need to process ELF files of an unknown +.\" architecture, then the application needs to explicitly use either +.\" .Dq Elf32_xxx +.\" or +.\" .Dq Elf64_xxx +.\" type and structure names. +.\" Likewise, the macros need to be identified by +.\" .Dq ELF32_xxx +.\" or +.\" .Dq ELF64_xxx . +.\" .Pp +This header file describes the above mentioned headers as C structures +and also includes structures for dynamic sections, relocation sections and +symbol tables. +.Pp +The following types are used for N-bit architectures (N=32,64, +ElfN stands for Elf32 or Elf64, uintN_t stands for uint32_t or uint64_t): +.Bd -literal -offset indent +ElfN_Addr Unsigned program address, uintN_t +ElfN_Off Unsigned file offset, uintN_t +ElfN_Section Unsigned section index, uint16_t +ElfN_Versym Unsigned version symbol information, uint16_t +Elf_Byte unsigned char +ElfN_Half uint16_t +ElfN_Sword int32_t +ElfN_Word uint32_t +ElfN_Sxword int64_t +ElfN_Xword uint64_t +.\" Elf32_Size Unsigned object size +.Ed +.Pp +(Note: The *BSD terminology is a bit different. There Elf64_Half is +twice as large as Elf32_Half, and Elf64Quarter is used for uint16_t. +In order to avoid confusion these types are replaced by explicit ones +in the below.) +.Pp +All data structures that the file format defines follow the +.Dq natural +size and alignment guidelines for the relevant class. +If necessary, +data structures contain explicit padding to ensure 4-byte alignment +for 4-byte objects, to force structure sizes to a multiple of 4, etc. +.Pp +The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: +.Bd -literal -offset indent +#define EI_NIDENT 16 + +typedef struct { + unsigned char e_ident[EI_NIDENT]; + uint16_t e_type; + uint16_t e_machine; + uint32_t e_version; + ElfN_Addr e_entry; + ElfN_Off e_phoff; + ElfN_Off e_shoff; + uint32_t e_flags; + uint16_t e_ehsize; + uint16_t e_phentsize; + uint16_t e_phnum; + uint16_t e_shentsize; + uint16_t e_shnum; + uint16_t e_shstrndx; +} ElfN_Ehdr; +.Ed +.Pp +The fields have the following meanings: +.Bl -tag -width "e_phentsize" -offset indent +.It Dv e_ident +This array of bytes specifies to interpret the file, +independent of the processor or the file's remaining contents. +Within this array everything is named by macros, which start with +the prefix +.Sy EI_ +and may contain values which start with the prefix +.Sy ELF . +The following macros are defined: +.Bl -tag -width "EI_VERSION" \" EI_ABIVERSION +.It Dv EI_MAG0 +The first byte of the magic number. +It must be filled with +.Sy ELFMAG0 . +(0: 0x7f) +.It Dv EI_MAG1 +The second byte of the magic number. +It must be filled with +.Sy ELFMAG1 . +(1: 'E') +.It Dv EI_MAG2 +The third byte of the magic number. +It must be filled with +.Sy ELFMAG2 . +(2: 'L') +.It Dv EI_MAG3 +The fourth byte of the magic number. +It must be filled with +.Sy ELFMAG3 . +(3: 'F') +.It Dv EI_CLASS +The fifth byte identifies the architecture for this binary: +.Pp +.Bl -tag -width "ELFCLASSNONE" -compact +.It Dv ELFCLASSNONE +This class is invalid. +.It Dv ELFCLASS32 +This defines the 32-bit architecture. +It supports machines with files +and virtual address spaces up to 4 Gigabytes. +.It Dv ELFCLASS64 +This defines the 64-bit architecture. +.El +.It Dv EI_DATA +The sixth byte specifies the data encoding of the processor-specific +data in the file. +Currently these encodings are supported: +.Pp +.Bl -tag -width "ELFDATA2LSB" -compact +.It Dv ELFDATANONE +Unknown data format. +.It Dv ELFDATA2LSB +Two's complement, little-endian. +.It Dv ELFDATA2MSB +Two's complement, big-endian. +.El +.It Dv EI_VERSION +The version number of the ELF specification: +.Pp +.Bl -tag -width "EV_CURRENT" -compact +.It Dv EV_NONE +Invalid version. +.It Dv EV_CURRENT +Current version. +.El +.It Dv EI_OSABI +This byte identifies the operating system +and ABI to which the object is targeted. +Some fields in other ELF structures have flags +and values that have platform specific meanings; +the interpretation of those fields is determined by the value of this byte. +E.g.: +.Pp +.Bl -tag -width "ELFOSABI_STANDALONE" -compact +.It Dv ELFOSABI_SYSV +UNIX System V ABI. +.\" 0 +.\" synonym: ELFOSABI_NONE +.It Dv ELFOSABI_HPUX +HP-UX ABI. +.\" 1 +.It Dv ELFOSABI_NETBSD +NetBSD ABI. +.\" 2 +.It Dv ELFOSABI_LINUX +Linux ABI. +.\" 3 +.\" .It Dv ELFOSABI_HURD +.\" Hurd ABI. +.\" 4 +.\" .It Dv ELFOSABI_86OPEN +.\" 86Open Common IA32 ABI. +.\" 5 +.It Dv ELFOSABI_SOLARIS +Solaris ABI. +.\" 6 +.\" .It Dv ELFOSABI_MONTEREY +.\" Monterey project ABI. Now replaced by +.\" ELFOSABI_AIX +.\" 7 +.It Dv ELFOSABI_IRIX +IRIX ABI. +.\" 8 +.It Dv ELFOSABI_FREEBSD +FreeBSD ABI. +.\" 9 +.It Dv ELFOSABI_TRU64 +TRU64 UNIX ABI. +.\" 10 +.\" ELFOSABI_MODESTO +.\" 11 +.\" ELFOSABI_OPENBSD +.\" 12 +.It Dv ELFOSABI_ARM +ARM architecture ABI. +.\" 97 +.It Dv ELFOSABI_STANDALONE +Stand-alone (embedded) ABI. +.\" 255 +.El +.It Dv EI_ABIVERSION +This byte identifies the version of the ABI +to which the object is targeted. +This field is used to distinguish among incompatible versions of an ABI. +The interpretation of this version number +is dependent on the ABI identified by the EI_OSABI field. +Applications conforming to this specification use the value 0. +.It Dv EI_PAD +Start of padding. +These bytes are reserved and set to zero. +Programs +which read them should ignore them. +The value for EI_PAD will change in +the future if currently unused bytes are given meanings. +.It Dv EI_BRAND +Start of architecture identification. +.It Dv EI_NIDENT +The size of the e_ident array. +.El +.Pp +.It Dv e_type +This member of the structure identifies the object file type: +.Pp +.Bl -tag -width "ET_NONE" -compact +.It Dv ET_NONE +An unknown type. +.It Dv ET_REL +A relocatable file. +.It Dv ET_EXEC +An executable file. +.It Dv ET_DYN +A shared object. +.It Dv ET_CORE +A core file. +.El +.Pp +.It Dv e_machine +This member specifies the required architecture for an individual file. +E.g.: +.Pp +.Bl -tag -width "EM_MIPS_RS4_BE" -compact +.It Dv EM_NONE +An unknown machine. +.\" 0 +.It Dv EM_M32 +AT&T WE 32100. +.\" 1 +.It Dv EM_SPARC +Sun Microsystems SPARC. +.\" 2 +.It Dv EM_386 +Intel 80386. +.\" 3 +.It Dv EM_68K +Motorola 68000. +.\" 4 +.It Dv EM_88K +Motorola 88000. +.\" 5 +.\" .It Dv EM_486 +.\" Intel 80486. +.\" 6 +.It Dv EM_860 +Intel 80860. +.\" 7 +.It Dv EM_MIPS +MIPS RS3000 (big-endian only). +.\" 8 +.\" EM_S370 +.\" 9 +.\" .It Dv EM_MIPS_RS4_BE +.\" MIPS RS4000 (big-endian only). Deprecated. +.\" 10 +.\" EM_MIPS_RS3_LE (MIPS R3000 little-endian) +.\" 10 +.It Dv EM_PARISC +HPPA. +.\" 15 +.It Dv EM_SPARC32PLUS +SPARC with enhanced instruction set. +.\" 18 +.It Dv EM_PPC +PowerPC. +.\" 20 +.\" EM_PPC64 +.\" 21 +.\" EM_S390 +.\" 22 +.\" EM_SH +.\" 42 +.It Dv EM_SPARCV9 +SPARC v9 64-bit. +.\" 43 +.\" EM_IA_64 +.\" 50 +.\" EM_X86_64 +.\" 62 +.It Dv EM_VAX +DEC Vax. +.\" 75 +.\" EM_CRIS +.\" 76 +.\" .It Dv EM_ALPHA +.\" Compaq [DEC] Alpha. +.\" .It Dv EM_ALPHA_EXP +.\" Compaq [DEC] Alpha with enhanced instruction set. +.El +.Pp +.It Dv e_version +This member identifies the file version: +.Pp +.Bl -tag -width "EV_CURRENT" -compact +.It Dv EV_NONE +Invalid version. +.It Dv EV_CURRENT +Current version. +.El +.It Dv e_entry +This member gives the virtual address to which the system first transfers +control, thus starting the process. +If the file has no associated entry +point, this member holds zero. +.It Dv e_phoff +This member holds the program header table's file offset in bytes. +If +the file has no program header table, this member holds zero. +.It Dv e_shoff +This member holds the section header table's file offset in bytes. +If the +file has no section header table this member holds zero. +.It Dv e_flags +This member holds processor-specific flags associated with the file. +Flag names take the form EF_`machine_flag'. +Currently no flags have been defined. +.It Dv e_ehsize +This member holds the ELF header's size in bytes. +.It Dv e_phentsize +This member holds the size in bytes of one entry in the file's program header +table; all entries are the same size. +.It Dv e_phnum +This member holds the number of entries in the program header +table. +Thus the product of +.Sy e_phentsize +and +.Sy e_phnum +gives the table's size +in bytes. +If a file has no program header, +.Sy e_phnum +holds the value zero. +.It Dv e_shentsize +This member holds a sections header's size in bytes. +A section header is one +entry in the section header table; all entries are the same size. +.It Dv e_shnum +This member holds the number of entries in the section header table. +Thus +the product of +.Sy e_shentsize +and +.Sy e_shnum +gives the section header table's size in bytes. +If a file has no section +header table, +.Sy e_shnum +holds the value of zero. +.It Dv e_shstrndx +This member holds the section header table index of the entry associated +with the section name string table. +If the file has no section name string +table, this member holds the value +.Sy SHN_UNDEF . +.Bl -tag -width "SHN_LORESERVE" +.It Dv SHN_UNDEF +This value marks an undefined, missing, irrelevant, or otherwise meaningless +section reference. +For example, a symbol +.Dq defined +relative to section number +.Sy SHN_UNDEF +is an undefined symbol. +.It Dv SHN_LORESERVE +This value specifies the lower bound of the range of reserved indices. +.It Dv SHN_LOPROC +Values greater than or equal to +.Sy SHN_HIPROC +are reserved for processor-specific semantics. +.It Dv SHN_HIPROC +Values less than or equal to +.Sy SHN_LOPROC +are reserved for processor-specific semantics. +.It Dv SHN_ABS +This value specifies absolute values for the corresponding reference. +For +example, symbols defined relative to section number +.Sy SHN_ABS +have absolute values and are not affected by relocation. +.It Dv SHN_COMMON +Symbols defined relative to this section are common symbols, such as Fortran +COMMON or unallocated C external variables. +.It Dv SHN_HIRESERVE +This value specifies the upper bound of the range of reserved +indices between +.Sy SHN_LORESERVE +and +.Sy SHN_HIRESERVE , +inclusive; the values do +not reference the section header table. +That is, the section header table +does +.Em not +contain entries for the reserved indices. +.El +.El +.Pp +An executable or shared object file's program header table is an array of +structures, each describing a segment or other information the system needs +to prepare the program for execution. +An object file +.Em segment +contains one or more +.Em sections . +Program headers are meaningful only for executable and shared object files. +A file specifies its own program header size with the ELF header's +.Sy e_phentsize +and +.Sy e_phnum +members. +The ELF program header is described by the type Elf32_Phdr or Elf64_Phdr +depending on the architecture: +.Bd -literal -offset indent +typedef struct { + uint32_t p_type; + Elf32_Off p_offset; + Elf32_Addr p_vaddr; + Elf32_Addr p_paddr; + uint32_t p_filesz; + uint32_t p_memsz; + uint32_t p_flags; + uint32_t p_align; +} Elf32_Phdr; +.Ed +.Bd -literal -offset indent +typedef struct { + uint32_t p_type; + uint32_t p_flags; + Elf64_Off p_offset; + Elf64_Addr p_vaddr; + Elf64_Addr p_paddr; + uint64_t p_filesz; + uint64_t p_memsz; + uint64_t p_align; +} Elf64_Phdr; +.Ed +.Pp +The main difference between the 32-bit and the 64-bit program header lies +in the location of the +.Sy p_flags +member in the total struct. +.Bl -tag -width "p_offset" -offset indent +.It Dv p_type +This member of the Phdr struct tells what kind of segment this array +element describes or how to interpret the array element's information. +.Bl -tag -width "PT_DYNAMIC" +.It Dv PT_NULL +The array element is unused and the other members' values are undefined. +This lets the program header have ignored entries. +.It Dv PT_LOAD +The array element specifies a loadable segment, described by +.Sy p_filesz +and +.Sy p_memsz . +The bytes from the file are mapped to the beginning of the memory +segment. +If the segment's memory size +.Pq Sy p_memsz +is larger than the file size +.Pq Sy p_filesz , +the +.Dq extra +bytes are defined to hold the value 0 and to follow the segment's +initialized area. +The file size may not be larger than the memory size. +Loadable segment entries in the program header table appear in ascending +order, sorted on the +.Sy p_vaddr +member. +.It Dv PT_DYNAMIC +The array element specifies dynamic linking information. +.It Dv PT_INTERP +The array element specifies the location and size of a null-terminated +path name to invoke as an interpreter. +This segment type is meaningful +only for executable files (though it may occur for shared objects). +However it may not occur more than once in a file. +If it is present, it must precede any loadable segment entry. +.It Dv PT_NOTE +The array element specifies the location and size for auxiliary information. +.It Dv PT_SHLIB +This segment type is reserved but has unspecified semantics. +Programs that +contain an array element of this type do not conform to the ABI. +.It Dv PT_PHDR +The array element, if present, specifies the location and size of the program +header table itself, both in the file and in the memory image of the program. +This segment type may not occur more than once in a file. +Moreover, it may +only occur if the program header table is part of the memory image of the +program. +If it is present, it must precede any loadable segment entry. +.It Dv PT_LOPROC +Values greater than or equal to +.Sy PT_HIPROC +are reserved for processor-specific semantics. +.It Dv PT_HIPROC +Values less than or equal to +.Sy PT_LOPROC +are reserved for processor-specific semantics. +.El +.Pp +.It Dv p_offset +This member holds the offset from the beginning of the file at which +the first byte of the segment resides. +.It Dv p_vaddr +This member holds the virtual address at which the first byte of the +segment resides in memory. +.It Dv p_paddr +On systems for which physical addressing is relevant, this member is +reserved for the segment's physical address. +Under +.Bx +this member is +not used and must be zero. +.It Dv p_filesz +This member holds the number of bytes in the file image of the segment. +It may be zero. +.It Dv p_memsz +This member holds the number of bytes in the memory image of the segment. +It may be zero. +.It Dv p_flags +This member holds flags relevant to the segment: +.Pp +.Bl -tag -width "PF_X" -compact +.It Dv PF_X +An executable segment. +.It Dv PF_W +A writable segment. +.It Dv PF_R +A readable segment. +.El +.Pp +A text segment commonly has the flags +.Sy PF_X +and +.Sy PF_R . +A data segment commonly has +.Sy PF_X , +.Sy PF_W +and +.Sy PF_R . +.It Dv p_align +This member holds the value to which the segments are aligned in memory +and in the file. +Loadable process segments must have congruent values for +.Sy p_vaddr +and +.Sy p_offset , +modulo the page size. +Values of zero and one mean no alignment is required. +Otherwise, +.Sy p_align +should be a positive, integral power of two, and +.Sy p_vaddr +should equal +.Sy p_offset , +modulo +.Sy p_align . +.El +.Pp +A file's section header table lets one locate all the file's sections. +The +section header table is an array of Elf32_Shdr or Elf64_Shdr structures. +The +ELF header's +.Sy e_shoff +member gives the byte offset from the beginning of the file to the section +header table. +.Sy e_shnum +holds the number of entries the section header table contains. +.Sy e_shentsize +holds the size in bytes of each entry. +.Pp +A section header table index is a subscript into this array. +Some section +header table indices are reserved. +An object file does not have sections for +these special indices: +.Bl -tag -width "SHN_LORESERVE" +.It Dv SHN_UNDEF +This value marks an undefined, missing, irrelevant or otherwise meaningless +section reference. +.It Dv SHN_LORESERVE +This value specifies the lower bound of the range of reserved indices. +.It Dv SHN_LOPROC +Values greater than or equal to +.Sy SHN_HIPROC +are reserved for processor-specific semantics. +.It Dv SHN_HIPROC +Values less than or equal to +.Sy SHN_LOPROC +are reserved for processor-specific semantics. +.It Dv SHN_ABS +This value specifies the absolute value for the corresponding reference. +For +example, a symbol defined relative to section number +.Sy SHN_ABS +has an absolute value and is not affected by relocation. +.It Dv SHN_COMMON +Symbols defined relative to this section are common symbols, such as FORTRAN +COMMON or unallocated C external variables. +.It Dv SHN_HIRESERVE +This value specifies the upper bound of the range of reserved indices. +The +system reserves indices between +.Sy SHN_LORESERVE +and +.Sy SHN_HIRESERVE , +inclusive. +The section header table does not contain entries for the +reserved indices. +.El +.Pp +The section header has the following structure: +.Bd -literal -offset indent +typedef struct { + uint32_t sh_name; + uint32_t sh_type; + uint32_t sh_flags; + Elf32_Addr sh_addr; + Elf32_Off sh_offset; + uint32_t sh_size; + uint32_t sh_link; + uint32_t sh_info; + uint32_t sh_addralign; + uint32_t sh_entsize; +} Elf32_Shdr; +.Ed +.Bd -literal -offset indent +typedef struct { + uint32_t sh_name; + uint32_t sh_type; + uint64_t sh_flags; + Elf64_Addr sh_addr; + Elf64_Off sh_offset; + uint64_t sh_size; + uint32_t sh_link; + uint32_t sh_info; + uint64_t sh_addralign; + uint64_t sh_entsize; +} Elf64_Shdr; +.Ed +.Bl -tag -width "sh_addralign" +.It Dv sh_name +This member specifies the name of the section. +Its value is an index +into the section header string table section, giving the location of +a null-terminated string. +.It Dv sh_type +This member categorizes the section's contents and semantics. +.Bl -tag -width "SHT_PROGBITS" +.It Dv SHT_NULL +This value marks the section header as inactive. +It does not +have an associated section. +Other members of the section header +have undefined values. +.It Dv SHT_PROGBITS +This section holds information defined by the program, whose +format and meaning are determined solely by the program. +.It Dv SHT_SYMTAB +This section holds a symbol table. +Typically, +.Sy SHT_SYMTAB +provides symbols for link editing, though it may also be used +for dynamic linking. +As a complete symbol table, it may contain +many symbols unnecessary for dynamic linking. +An object file can +also contain a +.Sy SHN_DYNSYM +section. +.It Dv SHT_STRTAB +This section holds a string table. +An object file may have multiple +string table sections. +.It Dv SHT_RELA +This section holds relocation entries with explicit addends, such +as type +.Sy Elf32_Rela +for the 32-bit class of object files. +An object may have multiple +relocation sections. +.It Dv SHT_HASH +This section holds a symbol hash table. +An object participating in +dynamic linking must contain a symbol hash table. +An object file may +have only one hash table. +.It Dv SHT_DYNAMIC +This section holds information for dynamic linking. +An object file may +have only one dynamic section. +.It Dv SHT_NOTE +This section holds information that marks the file in some way. +.It Dv SHT_NOBITS +A section of this type occupies no space in the file but otherwise +resembles +.Sy SHN_PROGBITS . +Although this section contains no bytes, the +.Sy sh_offset +member contains the conceptual file offset. +.It Dv SHT_REL +This section holds relocation offsets without explicit addends, such +as type +.Sy Elf32_Rel +for the 32-bit class of object files. +An object file may have multiple +relocation sections. +.It Dv SHT_SHLIB +This section is reserved but has unspecified semantics. +.It Dv SHT_DYNSYM +This section holds a minimal set of dynamic linking symbols. +An +object file can also contain a +.Sy SHN_SYMTAB +section. +.It Dv SHT_LOPROC +This value up to and including +.Sy SHT_HIPROC +is reserved for processor-specific semantics. +.It Dv SHT_HIPROC +This value down to and including +.Sy SHT_LOPROC +is reserved for processor-specific semantics. +.It Dv SHT_LOUSER +This value specifies the lower bound of the range of indices reserved for +application programs. +.It Dv SHT_HIUSER +This value specifies the upper bound of the range of indices reserved for +application programs. +Section types between +.Sy SHT_LOUSER +and +.Sy SHT_HIUSER +may be used by the application, without conflicting with current or future +system-defined section types. +.El +.Pp +.It Dv sh_flags +Sections support one-bit flags that describe miscellaneous attributes. +If a flag bit is set in +.Sy sh_flags , +the attribute is +.Dq on +for the section. +Otherwise, the attribute is +.Dq off +or does not apply. +Undefined attributes are set to zero. +.Pp +.Bl -tag -width "SHF_EXECINSTR" -compact +.It Dv SHF_WRITE +This section contains data that should be writable during process +execution. +.It Dv SHF_ALLOC +This section occupies memory during process execution. +Some control +sections do not reside in the memory image of an object file. +This +attribute is off for those sections. +.It Dv SHF_EXECINSTR +This section contains executable machine instructions. +.It Dv SHF_MASKPROC +All bits included in this mask are reserved for processor-specific +semantics. +.El +.Pp +.It Dv sh_addr +If this section appears in the memory image of a process, this member +holds the address at which the section's first byte should reside. +Otherwise, the member contains zero. +.It Dv sh_offset +This member's value holds the byte offset from the beginning of the file +to the first byte in the section. +One section type, +.Sy SHT_NOBITS , +occupies no space in the file, and its +.Sy sh_offset +member locates the conceptual placement in the file. +.It Dv sh_size +This member holds the section's size in bytes. +Unless the section type +is +.Sy SHT_NOBITS , +the section occupies +.Sy sh_size +bytes in the file. +A section of type +.Sy SHT_NOBITS +may have a non-zero size, but it occupies no space in the file. +.It Dv sh_link +This member holds a section header table index link, whose interpretation +depends on the section type. +.It Dv sh_info +This member holds extra information, whose interpretation depends on the +section type. +.It Dv sh_addralign +Some sections have address alignment constraints. +If a section holds a +doubleword, the system must ensure doubleword alignment for the entire +section. +That is, the value of +.Sy sh_addr +must be congruent to zero, modulo the value of +.Sy sh_addralign . +Only zero and positive integral powers of two are allowed. +Values of zero +or one mean the section has no alignment constraints. +.It Dv sh_entsize +Some sections hold a table of fixed-sized entries, such as a symbol table. +For such a section, this member gives the size in bytes for each entry. +This member contains zero if the section does not hold a table of +fixed-size entries. +.El +.Pp +Various sections hold program and control information: +.Bl -tag -width ".shstrtab" +.It .bss +This section holds uninitialized data that contributes to the program's +memory image. +By definition, the system initializes the data with zeros +when the program begins to run. +This section is of type +.Sy SHT_NOBITS . +The attribute types are +.Sy SHF_ALLOC +and +.Sy SHF_WRITE . +.It .comment +This section holds version control information. +This section is of type +.Sy SHT_PROGBITS . +No attribute types are used. +.It .ctors +This section holds initialized pointers to the C++ constructor functions. +This section is of type +.Sy SHT_PROGBITS . +The attribute types are +.Sy SHF_ALLOC +and +.Sy SHF_WRITE . +.It .data +This section holds initialized data that contribute to the program's +memory image. +This section is of type +.Sy SHT_PROGBITS . +The attribute types are +.Sy SHF_ALLOC +and +.Sy SHF_WRITE . +.It .data1 +This section holds initialized data that contribute to the program's +memory image. +This section is of type +.Sy SHT_PROGBITS . +The attribute types are +.Sy SHF_ALLOC +and +.Sy SHF_WRITE . +.It .debug +This section holds information for symbolic debugging. +The contents +are unspecified. +This section is of type +.Sy SHT_PROGBITS . +No attribute types are used. +.It .dtors +This section holds initialized pointers to the C++ destructor functions. +This section is of type +.Sy SHT_PROGBITS . +The attribute types are +.Sy SHF_ALLOC +and +.Sy SHF_WRITE . +.It .dynamic +This section holds dynamic linking information. +The section's attributes +will include the +.Sy SHF_ALLOC +bit. +Whether the +.Sy SHF_WRITE +bit is set is processor-specific. +This section is of type +.Sy SHT_DYNAMIC . +See the attributes above. +.It .dynstr +This section holds strings needed for dynamic linking, most commonly +the strings that represent the names associated with symbol table entries. +This section is of type +.Sy SHT_STRTAB . +The attribute type used is +.Sy SHF_ALLOC . +.It .dynsym +This section holds the dynamic linking symbol table. +This section is of type +.Sy SHT_DYNSYM . +The attribute used is +.Sy SHF_ALLOC . +.It .fini +This section holds executable instructions that contribute to the process +termination code. +When a program exits normally the system arranges to +execute the code in this section. +This section is of type +.Sy SHT_PROGBITS . +The attributes used are +.Sy SHF_ALLOC +and +.Sy SHF_EXECINSTR . +.It .got +This section holds the global offset table. +This section is of type +.Sy SHT_PROGBITS . +The attributes are processor-specific. +.It .hash +This section holds a symbol hash table. +This section is of type +.Sy SHT_HASH . +The attribute used is +.Sy SHF_ALLOC . +.It .init +This section holds executable instructions that contribute to the process +initialization code. +When a program starts to run the system arranges to +execute the code in this section before calling the main program entry point. +This section is of type +.Sy SHT_PROGBITS . +The attributes used are +.Sy SHF_ALLOC +and +.Sy SHF_EXECINSTR . +.It .interp +This section holds the pathname of a program interpreter. +If the file has +a loadable segment that includes the section, the section's attributes will +include the +.Sy SHF_ALLOC +bit. +Otherwise, that bit will be off. +This section is of type +.Sy SHT_PROGBITS . +.It .line +This section holds line number information for symbolic debugging, which +describes the correspondence between the program source and the machine code. +The contents are unspecified. +This section is of type +.Sy SHT_PROGBITS . +No attribute types are used. +.It .note +This section holds information in the +.Dq Note Section +format described below. +This section is of type +.Sy SHT_NOTE . +No attribute types are used. +.Ox +native executables usually contain a +.Sy .note.openbsd.ident +section to identify themselves, for the kernel to bypass any compatibility +ELF binary emulation tests when loading the file. +.It .plt +This section holds the procedure linkage table. +This section is of type +.Sy SHT_PROGBITS . +The attributes are processor-specific. +.It .relNAME +This section holds relocation information as described below. +If the file +has a loadable segment that includes relocation, the section's attributes +will include the +.Sy SHF_ALLOC +bit. +Otherwise the bit will be off. +By convention, +.Dq NAME +is supplied by the section to which the relocations apply. +Thus a relocation +section for +.Sy .text +normally would have the name +.Sy .rel.text . +This section is of type +.Sy SHT_REL . +.It .relaNAME +This section holds relocation information as described below. +If the file +has a loadable segment that includes relocation, the section's attributes +will include the +.Sy SHF_ALLOC +bit. +Otherwise the bit will be off. +By convention, +.Dq NAME +is supplied by the section to which the relocations apply. +Thus a relocation +section for +.Sy .text +normally would have the name +.Sy .rela.text . +This section is of type +.Sy SHT_RELA . +.It .rodata +This section holds read-only data that typically contributes to a +non-writable segment in the process image. +This section is of type +.Sy SHT_PROGBITS . +The attribute used is +.Sy SHF_ALLOC . +.It .rodata1 +This section holds read-only data that typically contributes to a +non-writable segment in the process image. +This section is of type +.Sy SHT_PROGBITS . +The attribute used is +.Sy SHF_ALLOC . +.It .shstrtab +This section holds section names. +This section is of type +.Sy SHT_STRTAB . +No attribute types are used. +.It .strtab +This section holds strings, most commonly the strings that represent the +names associated with symbol table entries. +If the file has a loadable +segment that includes the symbol string table, the section's attributes +will include the +.Sy SHF_ALLOC +bit. +Otherwise the bit will be off. +This section is of type +.Sy SHT_STRTAB . +.It .symtab +This section holds a symbol table. +If the file has a loadable segment +that includes the symbol table, the section's attributes will include +the +.Sy SHF_ALLOC +bit. +Otherwise the bit will be off. +This section is of type +.Sy SHT_SYMTAB . +.It .text +This section holds the +.Dq text , +or executable instructions, of a program. +This section is of type +.Sy SHT_PROGBITS . +The attributes used are +.Sy SHF_ALLOC +and +.Sy SHF_EXECINSTR . +.El +.Pp +String table sections hold null-terminated character sequences, commonly +called strings. +The object file uses these strings to represent symbol +and section names. +One references a string as an index into the string +table section. +The first byte, which is index zero, is defined to hold +a null character. +Similarly, a string table's last byte is defined to +hold a null character, ensuring null termination for all strings. +.Pp +An object file's symbol table holds information needed to locate and +relocate a program's symbolic definitions and references. +A symbol table +index is a subscript into this array. +.Bd -literal -offset indent +typedef struct { + uint32_t st_name; + Elf32_Addr st_value; + uint32_t st_size; + unsigned char st_info; + unsigned char st_other; + uint16_t st_shndx; +} Elf32_Sym; +.Ed +.Bd -literal -offset indent +typedef struct { + uint32_t st_name; + unsigned char st_info; + unsigned char st_other; + uint16_t st_shndx; + Elf64_Addr st_value; + uint64_t st_size; +} Elf64_Sym; +.Ed +.Bl -tag -width "st_value" +.It Dv st_name +This member holds an index into the object file's symbol string table, +which holds character representations of the symbol names. +If the value +is non-zero, it represents a string table index that gives the symbol +name. +Otherwise, the symbol table has no name. +.It Dv st_value +This member gives the value of the associated symbol. +.It Dv st_size +Many symbols have associated sizes. +This member holds zero if the symbol +has no size or an unknown size. +.It Dv st_info +This member specifies the symbol's type and binding attributes: +.Bl -tag -width "STT_SECTION" +.It Dv STT_NOTYPE +The symbol's type is not defined. +.It Dv STT_OBJECT +The symbol is associated with a data object. +.It Dv STT_FUNC +The symbol is associated with a function or other executable code. +.It Dv STT_SECTION +The symbol is associated with a section. +Symbol table entries of +this type exist primarily for relocation and normally have +.Sy STB_LOCAL +bindings. +.It Dv STT_FILE +By convention, the symbol's name gives the name of the source file +associated with the object file. +A file symbol has +.Sy STB_LOCAL +bindings, its section index is +.Sy SHN_ABS , +and it precedes the other +.Sy STB_LOCAL +symbols of the file, if it is present. +.It Dv STT_LOPROC +This value up to and including +.Sy STT_HIPROC +is reserved for processor-specific semantics. +.It Dv STT_HIPROC +This value down to and including +.Sy STT_LOPROC +is reserved for processor-specific semantics. +.El +.Bl -tag -width "STB_GLOBAL" +.It Dv STB_LOCAL +Local symbols are not visible outside the object file containing their +definition. +Local symbols of the same name may exist in multiple files +without interfering with each other. +.It Dv STB_GLOBAL +Global symbols are visible to all object files being combined. +One file's +definition of a global symbol will satisfy another file's undefined +reference to the same symbol. +.It Dv STB_WEAK +Weak symbols resemble global symbols, but their definitions have lower +precedence. +.It Dv STB_LOPROC +This value up to and including +.Sy STB_HIPROC +is reserved for processor-specific semantics. +.It Dv STB_HIPROC +This value down to and including +.Sy STB_LOPROC +is reserved for processor-specific semantics. +.Pp +There are macros for packing and unpacking the binding and type fields: +.Pp +.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact +.It Xo +.Fn ELF32_ST_BIND info +.Xc +or +.Fn ELF64_ST_BIND info +extract a binding from an st_info value. +.It Xo +.Fn ELF64_ST_TYPE info +.Xc +or +.Fn ELF32_ST_TYPE info +extract a type from an st_info value. +.It Xo +.Fn ELF32_ST_INFO bind type +.Xc +or +.Fn ELF64_ST_INFO bind type +convert a binding and a type into an st_info value. +.El +.El +.Pp +.It Dv st_other +This member currently holds zero and has no defined meaning. +.It Dv st_shndx +Every symbol table entry is +.Dq defined +in relation to some section. +This member holds the relevant section +header table index. +.El +.Pp +Relocation is the process of connecting symbolic references with +symbolic definitions. +Relocatable files must have information that +describes how to modify their section contents, thus allowing executable +and shared object files to hold the right information for a process' +program image. +Relocation entries are these data. +.Pp +Relocation structures that do not need an addend: +.Bd -literal -offset indent +typedef struct { + Elf32_Addr r_offset; + uint32_t r_info; +} Elf32_Rel; +.Ed +.Bd -literal -offset indent +typedef struct { + Elf64_Addr r_offset; + uint64_t r_info; +} Elf64_Rel; +.Ed +.Pp +Relocation structures that need an addend: +.Bd -literal -offset indent +typedef struct { + Elf32_Addr r_offset; + uint32_t r_info; + int32_t r_addend; +} Elf32_Rela; +.Ed +.Bd -literal -offset indent +typedef struct { + Elf64_Addr r_offset; + uint64_t r_info; + int64_t r_addend; +} Elf64_Rela; +.Ed +.Bl -tag -width "r_offset" +.It Dv r_offset +This member gives the location at which to apply the relocation action. +For a relocatable file, the value is the byte offset from the beginning +of the section to the storage unit affected by the relocation. +For an +executable file or shared object, the value is the virtual address of +the storage unit affected by the relocation. +.It Dv r_info +This member gives both the symbol table index with respect to which the +relocation must be made and the type of relocation to apply. +Relocation +types are processor-specific. +When the text refers to a relocation +entry's relocation type or symbol table index, it means the result of +applying +.Sy ELF_[32|64]_R_TYPE +or +.Sy ELF[32|64]_R_SYM , +respectively, to the entry's +.Sy r_info +member. +.It Dv r_addend +This member specifies a constant addend used to compute the value to be +stored into the relocatable field. +.El +.Sh SEE ALSO +.Xr as 1 , +.Xr gdb 1 , +.Xr ld 1 , +.Xr objdump 1 , +.Xr execve 2 , +.Xr core 5 +.Rs +.%A Hewlett-Packard +.%B Elf-64 Object File Format +.Re +.Rs +.%A Santa Cruz Operation +.%B System V Application Binary Interface +.Re +.Rs +.%A Unix System Laboratories +.%T Object Files +.%B "Executable and Linking Format (ELF)" +.Re +.Sh HISTORY +.Ox +ELF support first appeared in +.Ox 1.2 , +although not all supported platforms use it as the native +binary file format. +ELF in itself first appeared in +.At V . +The ELF format is an adopted standard. +.Sh AUTHORS +The original version of this manual page was written by +.An Jeroen Ruigrok van der Werven +.Aq asmodai@FreeBSD.org +with inspiration from BSDi's +.Bsx +.Nm elf +manpage. |