ELF
—
executable and linking format
Because of the flexible nature of ELF, the structures describing it are
available both as 32bit and 64bit versions. This document uses the 32bit
versions, refer to <elf.h>
for
the corresponding 64bit versions.
The four main types of an ELF object file are:
- executable
- A file suitable for execution. It contains the information required for
creating a new process image.
- relocatable
- Contains the necessary information to be run through the link editor
ld(1) to create an executable or
a shared library.
- shared
- The shared object contains necessary information which can be used by
either the link editor ld(1) at
link time or by the dynamic loader
ld.elf_so(1) at run
time.
- core
- A file which describes the virtual address space and register state of a
process. Core files are typically used in conjunction with debuggers such
as gdb(1).
ELF files have a dual nature. The toolchain, including tools such
as the as(1) and linker
ld(1), treats them as a set of
sections described by their section headers. The system loader treats them
as a set of segments described by the program headers.
The general format of an ELF file is the following: The file
starts with an ELF header. This is followed by a table of program headers
(optional for relocatable and shared files). After this come the
sections/segments. The file ends with a table of section headers (optional
for executable files).
A segment can be considered to consist of several sections. For
example, all executable sections are typically packed into one loadable
segment which is read-only and executable (see p_flags
in the program header). This enables the system to map the entire file with
just a few operations, one for each loadable segment, instead of doing
numerous map operations for each section separately.
Each file is described by the ELF header:
typedef struct {
unsigned char e_ident[ELF_NIDENT];
Elf32_Half e_type;
Elf32_Half e_machine;
Elf32_Word e_version;
Elf32_Addr e_entry;
Elf32_Off e_phoff;
Elf32_Off e_shoff;
Elf32_Word e_flags;
Elf32_Half e_ehsize;
Elf32_Half e_phentsize;
Elf32_Half e_phnum;
Elf32_Half e_shentsize;
Elf32_Half e_shnum;
Elf32_Half e_shstrndx;
} Elf32_Ehdr;
- e_ident[]
- The array contains the following information in the indicated locations:
EI_MAG0
- The elements ranging from
EI_MAG0
to
EI_MAG3
contain the ELF magic number:
\0177ELF
EI_CLASS
- Contains the address size of the binary, either 32 or 64bit.
EI_DATA
- byte order
EI_VERSION
- Contains the ELF header version. This is currently always set to
1.
EI_OSABI
- Contains the operating system ABI identification. Note that even
though the definition
ELFOSABI_NETBSD
exists,
NetBSD uses
ELFOSABI_SYSV
here, since the
NetBSD ABI does not deviate from the
standard.
EI_ABIVERSION
- ABI version.
- e_type
- Contains the file type identification. It can be either
ET_REL
, ET_EXEC
,
ET_DYN
, or ET_CORE
for
relocatable, executable, shared, or core, respectively.
- e_machine
- Contains the machine type, e.g. SPARC, Alpha, MIPS, ...
- e_entry
- The program entry point if the file is executable.
- e_phoff
- The position of the program header table in the file or 0 if it doesn't
exist.
- e_shoff
- The position of the section header table in the file or 0 if it doesn't
exist.
- e_flags
- Contains processor-specific flags. For example, the SPARC port uses this
space to specify what kind of memory store ordering is required.
- e_ehsize
- The size of the ELF header.
- e_phentsize
- The size of an entry in the program header table. All entries are the same
size.
- e_phnum
- The number of entries in the program header table, or 0 if none
exists.
- e_shentsize
- The size of an entry in the section header table. All entries are the same
size.
- e_shnum
- The number of entries in the section header table, or 0 if none
exists.
- e_shstrndx
- Contains the index number of the section which contains the section name
strings.
Each ELF section in turn is described by the section header:
typedef struct {
Elf32_Word sh_name;
Elf32_Word sh_type;
Elf32_Word sh_flags;
Elf32_Addr sh_addr;
Elf32_Off sh_offset;
Elf32_Word sh_size;
Elf32_Word sh_link;
Elf32_Word sh_info;
Elf32_Word sh_addralign;
Elf32_Word sh_entsize;
} Elf32_Shdr;
- sh_name
- Contains an index to the position in the section header string section
where the name of the current section can be found.
- sh_type
- Contains the section type indicator. The more important possible values
are:
SHT_NULL
- Section is inactive. The other fields contain undefined values.
SHT_PROGBITS
- Section contains program information. It can be for example code,
data, or debugger information.
SHT_SYMTAB
- Section contains a symbol table. This section usually contains all the
symbols and is intended for the regular link editor
ld(1).
SHT_STRTAB
- Section contains a string table.
SHT_RELA
- Section contains relocation information with an explicit addend.
SHT_HASH
- Section contains a symbol hash table.
SHT_DYNAMIC
- Section contains dynamic linking information.
SHT_NOTE
- Section contains some special information. The format can be e.g.
vendor-specific.
SHT_NOBITS
- Sections contains information similar to
SHT_PROGBITS
, but takes up no space in the
file. This can be used for e.g. bss.
SHT_REL
- Section contains relocation information without an explicit
addend.
SHT_SHLIB
- This section type is reserved but has unspecified semantics.
SHT_DYNSYM
- Section contains a symbol table. This symbol table is intended for the
dynamic linker, and is kept as small as possible to conserve space,
since it must be loaded to memory at run time.
- sh_flags
- Contains the section flags, which can have the following values or any
combination of them:
SHF_WRITE
- Section is writable after it has been loaded.
SHF_ALLOC
- Section will occupy memory at run time.
SHF_EXECINSTR
- Section contains executable machine instructions.
- sh_addr
- Address to where the section will be loaded, or 0 if this section does not
reside in memory at run time.
- sh_offset
- The byte offset from the beginning of the file to the beginning of this
section. If the section is of type
SHT_NOBITS
,
this field specifies the conceptual placement in the file.
- sh_size
- The size of the section in the file for all types except
SHT_NOBITS
. For that type the value may differ
from zero, but the section will still always take up no space from the
file.
- sh_link
- Contains an index to the section header table. The interpretation depends
on the section type as follows:
SHT_REL
-
SHT_RELA
- Section index of the associated symbol table.
SHT_SYMTAB
-
SHT_DYNSYM
- Section index of the associated string table.
SHT_HASH
- Section index of the symbol table to which the hash table applies.
SHT_DYNAMIC
- Section index of of the string table by which entries in this section
are used.
- sh_info
- Contains extra information. The interpretation depends on the type as
follows:
SHT_REL
-
SHT_RELA
- Section index of the section to which the relocation information
applies.
SHT_SYMTAB
-
SHT_DYNSYM
- Contains a value one greater that the last local symbol table
index.
- sh_addralign
- Marks the section alignment requirement. If, for example, the section
contains a doubleword, the entire section must be doubleword aligned to
ensure proper alignment. Only 0 and integral powers of two are allowed.
Values 0 and 1 denote that the section has no alignment.
- sh_entsize
- Contains the entry size of an element for sections which are constructed
of a table of fixed-size entries. If the section does not hold a table of
fixed-size entries, this value is 0.
Every executable object must contain a program header. The program
header contains information necessary in constructing a process image.
typedef struct {
Elf32_Word p_type;
Elf32_Off p_offset;
Elf32_Addr p_vaddr;
Elf32_Addr p_paddr;
Elf32_Word p_filesz;
Elf32_Word p_memsz;
Elf32_Word p_flags;
Elf32_Word p_align;
} Elf32_Phdr;
- p_type
- Contains the segment type indicator. The possible values are:
PT_NULL
- Segment is inactive. The other fields contain undefined values.
PT_LOAD
- Segment is loadable. It is loaded to the address described by
p_vaddr. If p_memsz is
greater than p_filesz, the memory range from
(p_vaddr + p_filesz) to
(p_vaddr + p_memsz) is
zero-filled when the segment is loaded. p_filesz
can not be greater than p_memsz. Segments of
this type are sorted in the header table by
p_vaddr in ascending order.
PT_DYNAMIC
- Segment contains dynamic linking information.
PT_INTERP
- Segment contains a null-terminated path name to the interpreter. This
segment may be present only once in a file, and it must appear before
any loadable segments. This field will most likely contain the ELF
dynamic loader: /libexec/ld.elf_so
PT_NOTE
- Segment contains some special information. Format can be e.g.
vendor-specific.
PT_SHLIB
- This segment type is reserved but has unspecified semantics. Programs
which contain a segment of this type do not conform to the ABI, and
must indicate this by setting the appropriate ABI in the ELF header
EI_OSABI
field.
PT_PHDR
- The values in a program header of this type specify the
characteristics of the program header table itself. For example, the
p_vaddr field specifies the program header table
location in memory once the program is loaded. This field may not
occur more than once, may occur only if the program header table is
part of the file memory image, and must come before any loadable
segments.
- p_offset
- Contains the byte offset from the beginning of the file to the beginning
of this segment.
- p_vaddr
- Contains the virtual memory address to which this segment is loaded.
- p_paddr
- Contains the physical address to which this segment is loaded. This value
is usually ignored, but may be used while bootstrapping or in embedded
systems.
- p_filesz
- Contains the number of bytes this segment occupies in the file image.
- p_memsz
- Contains the number of bytes this segment occupies in the memory
image.
- p_flags
- Contains the segment flags, which specify the permissions for the segment
after it has been loaded. The following values or any combination of them
is acceptable:
PF_R
- Segment can be read.
PF_W
- Segment can be written.
PF_X
- Segment is executable.
- p_align
- Contains the segment alignment. Acceptable values are 0 and 1 for no
alignment, and integral powers of two. p_vaddr
should equal p_offset modulo
p_align.
The ELF object file format first appeared in AT&T
System V UNIX.