Originální popis anglicky:
elf —
format of
ELF executable binary files
Návod, kniha: File Formats Manual
#include <elf.h>
The header file ⟨
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.
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.
This header file describes the above mentioned headers as C structures and also
includes structures for dynamic sections, relocation sections and symbol
tables.
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):
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
(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.)
All data structures that the file format defines follow the
“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.
The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
#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;
The fields have the following meanings:
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
EI_ and may contain values which start with
the prefix ELF. The following macros are
defined:
EI_MAG0- The first byte of the magic number. It must be filled
with ELFMAG0. (0: 0x7f)
EI_MAG1- The second byte of the magic number. It must be filled
with ELFMAG1. (1: 'E')
EI_MAG2- The third byte of the magic number. It must be filled
with ELFMAG2. (2: 'L')
EI_MAG3- The fourth byte of the magic number. It must be filled
with ELFMAG3. (3: 'F')
EI_CLASS- The fifth byte identifies the architecture for this
binary:
ELFCLASSNONE- This class is invalid.
ELFCLASS32- This defines the 32-bit architecture. It supports
machines with files and virtual address spaces up to 4
Gigabytes.
ELFCLASS64- This defines the 64-bit architecture.
EI_DATA- The sixth byte specifies the data encoding of the
processor-specific data in the file. Currently these encodings are
supported:
ELFDATANONE- Unknown data format.
ELFDATA2LSB- Two's complement, little-endian.
ELFDATA2MSB- Two's complement, big-endian.
EI_VERSION- The version number of the ELF specification:
EV_NONE- Invalid version.
EV_CURRENT- Current version.
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.:
ELFOSABI_SYSV- UNIX System V ABI.
ELFOSABI_HPUX- HP-UX ABI.
ELFOSABI_NETBSD- NetBSD ABI.
ELFOSABI_LINUX- Linux ABI.
ELFOSABI_SOLARIS- Solaris ABI.
ELFOSABI_IRIX- IRIX ABI.
ELFOSABI_FREEBSD- FreeBSD ABI.
ELFOSABI_TRU64- TRU64 UNIX ABI.
ELFOSABI_ARM- ARM architecture ABI.
ELFOSABI_STANDALONE- Stand-alone (embedded) ABI.
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.
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.
EI_BRAND- Start of architecture identification.
EI_NIDENT- The size of the e_ident array.
e_type- This member of the structure identifies the object file
type:
ET_NONE- An unknown type.
ET_REL- A relocatable file.
ET_EXEC- An executable file.
ET_DYN- A shared object.
ET_CORE- A core file.
e_machine- This member specifies the required architecture for an
individual file. E.g.:
EM_NONE- An unknown machine.
EM_M32- AT&T WE 32100.
EM_SPARC- Sun Microsystems SPARC.
EM_386- Intel 80386.
EM_68K- Motorola 68000.
EM_88K- Motorola 88000.
EM_860- Intel 80860.
EM_MIPS- MIPS RS3000 (big-endian only).
EM_PARISC- HPPA.
EM_SPARC32PLUS- SPARC with enhanced instruction set.
EM_PPC- PowerPC.
EM_SPARCV9- SPARC v9 64-bit.
EM_VAX- DEC Vax.
e_version- This member identifies the file version:
EV_NONE- Invalid version.
EV_CURRENT- Current version.
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.
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.
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.
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.
e_ehsize- This member holds the ELF header's size in bytes.
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.
e_phnum- This member holds the number of entries in the program
header table. Thus the product of e_phentsize
and e_phnum gives the table's size in bytes.
If a file has no program header, e_phnum
holds the value zero.
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.
e_shnum- This member holds the number of entries in the section
header table. Thus the product of e_shentsize
and e_shnum gives the section header table's
size in bytes. If a file has no section header table,
e_shnum holds the value of zero.
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
SHN_UNDEF.
SHN_UNDEF- This value marks an undefined, missing, irrelevant, or
otherwise meaningless section reference. For example, a symbol
“defined” relative to section number
SHN_UNDEF is an undefined symbol.
SHN_LORESERVE- This value specifies the lower bound of the range of
reserved indices.
SHN_LOPROC- Values greater than or equal to
SHN_HIPROC are reserved for
processor-specific semantics.
SHN_HIPROC- Values less than or equal to
SHN_LOPROC are reserved for
processor-specific semantics.
SHN_ABS- This value specifies absolute values for the
corresponding reference. For example, symbols defined relative to
section number SHN_ABS have absolute
values and are not affected by relocation.
SHN_COMMON- Symbols defined relative to this section are common
symbols, such as Fortran COMMON or unallocated C external
variables.
SHN_HIRESERVE- This value specifies the upper bound of the range of
reserved indices between SHN_LORESERVE
and SHN_HIRESERVE, inclusive; the values
do not reference the section header table. That is, the section header
table does not contain entries for the
reserved indices.
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
segment contains one or more
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
e_phentsize and
e_phnum members. The ELF program header is
described by the type Elf32_Phdr or Elf64_Phdr depending on the architecture:
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;
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;
The main difference between the 32-bit and the 64-bit program header lies in the
location of the
p_flags member in the total
struct.
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.
PT_NULL- The array element is unused and the other members'
values are undefined. This lets the program header have ignored
entries.
PT_LOAD- The array element specifies a loadable segment,
described by p_filesz and
p_memsz. The bytes from the file are
mapped to the beginning of the memory segment. If the segment's memory
size (p_memsz) is larger than the file
size (p_filesz), the
“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
p_vaddr member.
PT_DYNAMIC- The array element specifies dynamic linking
information.
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.
PT_NOTE- The array element specifies the location and size for
auxiliary information.
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.
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.
PT_LOPROC- Values greater than or equal to
PT_HIPROC are reserved for
processor-specific semantics.
PT_HIPROC- Values less than or equal to
PT_LOPROC are reserved for
processor-specific semantics.
p_offset- This member holds the offset from the beginning of the file
at which the first byte of the segment resides.
p_vaddr- This member holds the virtual address at which the first
byte of the segment resides in memory.
p_paddr- On systems for which physical addressing is relevant, this
member is reserved for the segment's physical address. Under
BSD this member is not used and must be zero.
p_filesz- This member holds the number of bytes in the file image of
the segment. It may be zero.
p_memsz- This member holds the number of bytes in the memory image
of the segment. It may be zero.
p_flags- This member holds flags relevant to the segment:
PF_X- An executable segment.
PF_W- A writable segment.
PF_R- A readable segment.
A text segment commonly has the flags PF_X and
PF_R. A data segment commonly has
PF_X, PF_W and
PF_R.
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 p_vaddr and
p_offset, modulo the page size. Values of
zero and one mean no alignment is required. Otherwise,
p_align should be a positive, integral power
of two, and p_vaddr should equal
p_offset, modulo
p_align.
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
e_shoff member gives the byte offset
from the beginning of the file to the section header table.
e_shnum holds the number of entries the section
header table contains.
e_shentsize holds the size
in bytes of each entry.
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:
SHN_UNDEF- This value marks an undefined, missing, irrelevant or
otherwise meaningless section reference.
SHN_LORESERVE- This value specifies the lower bound of the range of
reserved indices.
SHN_LOPROC- Values greater than or equal to
SHN_HIPROC are reserved for
processor-specific semantics.
SHN_HIPROC- Values less than or equal to
SHN_LOPROC are reserved for
processor-specific semantics.
SHN_ABS- This value specifies the absolute value for the
corresponding reference. For example, a symbol defined relative to section
number SHN_ABS has an absolute value and is
not affected by relocation.
SHN_COMMON- Symbols defined relative to this section are common
symbols, such as FORTRAN COMMON or unallocated C external variables.
SHN_HIRESERVE- This value specifies the upper bound of the range of
reserved indices. The system reserves indices between
SHN_LORESERVE and
SHN_HIRESERVE, inclusive. The section header
table does not contain entries for the reserved indices.
The section header has the following structure:
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;
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;
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.
sh_type- This member categorizes the section's contents and
semantics.
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.
SHT_PROGBITS- This section holds information defined by the program,
whose format and meaning are determined solely by the program.
SHT_SYMTAB- This section holds a symbol table. Typically,
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
SHN_DYNSYM section.
SHT_STRTAB- This section holds a string table. An object file may
have multiple string table sections.
SHT_RELA- This section holds relocation entries with explicit
addends, such as type Elf32_Rela for the
32-bit class of object files. An object may have multiple relocation
sections.
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.
SHT_DYNAMIC- This section holds information for dynamic linking. An
object file may have only one dynamic section.
SHT_NOTE- This section holds information that marks the file in
some way.
SHT_NOBITS- A section of this type occupies no space in the file
but otherwise resembles SHN_PROGBITS.
Although this section contains no bytes, the
sh_offset member contains the conceptual
file offset.
SHT_REL- This section holds relocation offsets without explicit
addends, such as type Elf32_Rel for the
32-bit class of object files. An object file may have multiple
relocation sections.
SHT_SHLIB- This section is reserved but has unspecified
semantics.
SHT_DYNSYM- This section holds a minimal set of dynamic linking
symbols. An object file can also contain a
SHN_SYMTAB section.
SHT_LOPROC- This value up to and including
SHT_HIPROC is reserved for
processor-specific semantics.
SHT_HIPROC- This value down to and including
SHT_LOPROC is reserved for
processor-specific semantics.
SHT_LOUSER- This value specifies the lower bound of the range of
indices reserved for application programs.
SHT_HIUSER- This value specifies the upper bound of the range of
indices reserved for application programs. Section types between
SHT_LOUSER and
SHT_HIUSER may be used by the
application, without conflicting with current or future system-defined
section types.
sh_flags- Sections support one-bit flags that describe miscellaneous
attributes. If a flag bit is set in sh_flags,
the attribute is “on” for the section. Otherwise, the
attribute is “off” or does not apply. Undefined attributes
are set to zero.
SHF_WRITE- This section contains data that should be writable
during process execution.
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.
SHF_EXECINSTR- This section contains executable machine
instructions.
SHF_MASKPROC- All bits included in this mask are reserved for
processor-specific semantics.
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.
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,
SHT_NOBITS, occupies no space in the file,
and its sh_offset member locates the
conceptual placement in the file.
sh_size- This member holds the section's size in bytes. Unless the
section type is SHT_NOBITS, the section
occupies sh_size bytes in the file. A section
of type SHT_NOBITS may have a non-zero size,
but it occupies no space in the file.
sh_link- This member holds a section header table index link, whose
interpretation depends on the section type.
sh_info- This member holds extra information, whose interpretation
depends on the section type.
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
sh_addr must be congruent to zero, modulo the
value of sh_addralign. Only zero and positive
integral powers of two are allowed. Values of zero or one mean the section
has no alignment constraints.
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.
Various sections hold program and control information:
- .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
SHT_NOBITS. The attribute types are
SHF_ALLOC and
SHF_WRITE.
- .comment
- This section holds version control information. This
section is of type SHT_PROGBITS. No attribute
types are used.
- .ctors
- This section holds initialized pointers to the C++
constructor functions. This section is of type
SHT_PROGBITS. The attribute types are
SHF_ALLOC and
SHF_WRITE.
- .data
- This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS. The attribute types are
SHF_ALLOC and
SHF_WRITE.
- .data1
- This section holds initialized data that contribute to the
program's memory image. This section is of type
SHT_PROGBITS. The attribute types are
SHF_ALLOC and
SHF_WRITE.
- .debug
- This section holds information for symbolic debugging. The
contents are unspecified. This section is of type
SHT_PROGBITS. No attribute types are
used.
- .dtors
- This section holds initialized pointers to the C++
destructor functions. This section is of type
SHT_PROGBITS. The attribute types are
SHF_ALLOC and
SHF_WRITE.
- .dynamic
- This section holds dynamic linking information. The
section's attributes will include the
SHF_ALLOC bit. Whether the
SHF_WRITE bit is set is processor-specific.
This section is of type SHT_DYNAMIC. See the
attributes above.
- .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 SHT_STRTAB.
The attribute type used is SHF_ALLOC.
- .dynsym
- This section holds the dynamic linking symbol table. This
section is of type SHT_DYNSYM. The attribute
used is SHF_ALLOC.
- .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
SHT_PROGBITS. The attributes used are
SHF_ALLOC and
SHF_EXECINSTR.
- .got
- This section holds the global offset table. This section is
of type SHT_PROGBITS. The attributes are
processor-specific.
- .hash
- This section holds a symbol hash table. This section is of
type SHT_HASH. The attribute used is
SHF_ALLOC.
- .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
SHT_PROGBITS. The attributes used are
SHF_ALLOC and
SHF_EXECINSTR.
- .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
SHF_ALLOC bit. Otherwise, that bit will be
off. This section is of type
SHT_PROGBITS.
- .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 SHT_PROGBITS. No attribute types are
used.
- .note
- This section holds information in the “Note
Section” format described below. This section is of type
SHT_NOTE. No attribute types are used.
OpenBSD native executables usually contain a
.note.openbsd.ident section to identify
themselves, for the kernel to bypass any compatibility ELF binary
emulation tests when loading the file.
- .plt
- This section holds the procedure linkage table. This
section is of type SHT_PROGBITS. The
attributes are processor-specific.
- .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
SHF_ALLOC bit. Otherwise the bit will be off.
By convention, “NAME” is supplied by the section to which
the relocations apply. Thus a relocation section for
.text normally would have the name
.rel.text. This section is of type
SHT_REL.
- .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
SHF_ALLOC bit. Otherwise the bit will be off.
By convention, “NAME” is supplied by the section to which
the relocations apply. Thus a relocation section for
.text normally would have the name
.rela.text. This section is of type
SHT_RELA.
- .rodata
- This section holds read-only data that typically
contributes to a non-writable segment in the process image. This section
is of type SHT_PROGBITS. The attribute used
is SHF_ALLOC.
- .rodata1
- This section holds read-only data that typically
contributes to a non-writable segment in the process image. This section
is of type SHT_PROGBITS. The attribute used
is SHF_ALLOC.
- .shstrtab
- This section holds section names. This section is of type
SHT_STRTAB. No attribute types are used.
- .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 SHF_ALLOC bit.
Otherwise the bit will be off. This section is of type
SHT_STRTAB.
- .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 SHF_ALLOC bit. Otherwise the
bit will be off. This section is of type
SHT_SYMTAB.
- .text
- This section holds the “text”, or executable
instructions, of a program. This section is of type
SHT_PROGBITS. The attributes used are
SHF_ALLOC and
SHF_EXECINSTR.
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.
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.
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;
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;
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.
st_value- This member gives the value of the associated symbol.
st_size- Many symbols have associated sizes. This member holds zero
if the symbol has no size or an unknown size.
st_info- This member specifies the symbol's type and binding
attributes:
STT_NOTYPE- The symbol's type is not defined.
STT_OBJECT- The symbol is associated with a data object.
STT_FUNC- The symbol is associated with a function or other
executable code.
STT_SECTION- The symbol is associated with a section. Symbol table
entries of this type exist primarily for relocation and normally have
STB_LOCAL bindings.
STT_FILE- By convention, the symbol's name gives the name of the
source file associated with the object file. A file symbol has
STB_LOCAL bindings, its section index is
SHN_ABS, and it precedes the other
STB_LOCAL symbols of the file, if it is
present.
STT_LOPROC- This value up to and including
STT_HIPROC is reserved for
processor-specific semantics.
STT_HIPROC- This value down to and including
STT_LOPROC is reserved for
processor-specific semantics.
STB_LOCAL- Local symbols are not visible out
