NetBSD/share/man/man5/elf.5

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.Dd November 18, 2006
.Dt ELF 5
.Os
.Sh NAME
.Nm ELF
.Nd executable and linking format
.Sh SYNOPSIS
.In elf.h
.Sh DESCRIPTION
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
.Aq Pa elf.h
for the corresponding 64bit versions.
.Pp
The four main types of an ELF object file are:
.Bl -tag -width "relocatable"
.It executable
A file suitable for execution. It contains the information required for
creating a new process image.
.It relocatable
Contains the necessary information to be run through the link editor
.Xr ld 1
to create an executable or a shared library.
.It shared
The shared object contains necessary information which can be used by
either the link editor
.Xr ld 1
at link time or by the dynamic loader
.Xr ld.elf_so 1
at run time.
.It 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
.Xr gdb 1 .
.El
.Pp
ELF files have a dual nature. The toolchain, including tools such as the
.Xr as 1
and linker
.Xr 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.
.Pp
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).
.Pp
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
.Fa 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.
.Pp
Each file is described by the ELF header:
.Bd -literal -offset indent
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;
.Ed
.Pp
.Bl -tag -width "e_phentsize"
.It Fa e_ident[]
The array contains the following information in the indicated locations:
.Bl -tag -width EI_ABIVERSION
.It Dv EI_MAG0
The elements ranging from
.Dv EI_MAG0
to
.Dv EI_MAG3
contain the ELF magic number: \\0177ELF
.It Dv EI_CLASS
Contains the address size of the binary, either 32 or 64bit.
.It Dv EI_DATA
byte order
.It Dv EI_VERSION
Contains the ELF header version. This is currently always set to 1.
.It Dv EI_OSABI
Contains the operating system ABI identification. Note that even though the
definition
.Dv ELFOSABI_NETBSD
exists,
.Nx
uses
.Dv ELFOSABI_SYSV
here, since the
.Nx
ABI does not deviate from the standard.
.It Dv EI_ABIVERSION
ABI version.
.El
.It Fa e_type
Contains the file type identification. It can be either
.Dv ET_REL ,
.Dv ET_EXEC ,
.Dv ET_DYN ,
or
.Dv ET_CORE
for relocatable, executable, shared, or core, respectively.
.It Fa e_machine
Contains the machine type, e.g. SPARC, Alpha, MIPS, ...
.It Fa e_entry
The program entry point if the file is executable.
.It Fa e_phoff
The position of the program header table in the file or 0 if it doesn't exist.
.It Fa e_shoff
The position of the section header table in the file or 0 if it doesn't exist.
.It Fa e_flags
Contains processor-specific flags. For example, the SPARC port uses this
space to specify what kind of memory store ordering is required.
.It Fa e_ehsize
The size of the ELF header.
.It Fa e_phentsize
The size of an entry in the program header table. All entries are the same
size.
.It Fa e_phnum
The number of entries in the program header table, or 0 if none exists.
.It Fa e_shentsize
The size of an entry in the section header table. All entries are the same
size.
.It Fa e_shnum
The number of entries in the section header table, or 0 if none exists.
.It Fa e_shstrndx
Contains the index number of the section which contains the section
name strings.
.El
.Pp
Each ELF section in turn is described by the section header:
.Bd -literal -offset indent
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;
.Ed
.Pp
.Bl -tag -width "sh_addralign"
.It Fa sh_name
Contains an index to the position in the section header string section where
the name of the current section can be found.
.It Fa sh_type
Contains the section type indicator. The more important possible values are:
.Bl -tag -width "SHT_PROGBITS"
.It Dv SHT_NULL
Section is inactive. The other fields contain undefined values.
.It Dv SHT_PROGBITS
Section contains program information. It can be for example code, data,
or debugger information.
.It Dv SHT_SYMTAB
Section contains a symbol table. This section usually contains all the
symbols and is intended for the regular link editor
.Xr ld 1 .
.It Dv SHT_STRTAB
Section contains a string table.
.It Dv SHT_RELA
Section contains relocation information with an explicit addend.
.It Dv SHT_HASH
Section contains a symbol hash table.
.It Dv SHT_DYNAMIC
Section contains dynamic linking information.
.It Dv SHT_NOTE
Section contains some special information. The format can be e.g.
vendor-specific.
.It Dv SHT_NOBITS
Sections contains information similar to
.Dv SHT_PROGBITS ,
but takes up no space in the file. This can be used for e.g. bss.
.It Dv SHT_REL
Section contains relocation information without an explicit addend.
.It Dv SHT_SHLIB
This section type is reserved but has unspecified semantics.
.It Dv 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.
.El
.It Fa sh_flags
Contains the section flags, which can have the following values or any
combination of them:
.Bl -tag -width SHF_EXECINSTR
.It Dv SHF_WRITE
Section is writable after it has been loaded.
.It Dv SHF_ALLOC
Section will occupy memory at run time.
.It Dv SHF_EXECINSTR
Section contains executable machine instructions.
.El
.It Fa sh_addr
Address to where the section will be loaded, or 0 if this section does not
reside in memory at run time.
.It Fa sh_offset
The byte offset from the beginning of the file to the beginning of this
section. If the section is of type
.Dv SHT_NOBITS ,
this field specifies the conceptual placement in the file.
.It Fa sh_size
The size of the section in the file for all types except
.Dv SHT_NOBITS .
For that type the value may differ from zero, but the section will still
always take up no space from the file.
.It Fa sh_link
Contains an index to the section header table. The interpretation depends
on the section type as follows:
.Pp
.Bl -tag -compact -width SHT_DYNAMIC
.It Dv SHT_REL
.It Dv SHT_RELA
Section index of the associated symbol table.
.Pp
.It Dv SHT_SYMTAB
.It Dv SHT_DYNSYM
Section index of the associated string table.
.Pp
.It Dv SHT_HASH
Section index of the symbol table to which the hash table applies.
.Pp
.It Dv SHT_DYNAMIC
Section index of of the string table by which entries in this section are used.
.El
.It Fa sh_info
Contains extra information. The interpretation depends on the type as
follows:
.Pp
.Bl -tag -compact -width SHT_DYNSYM
.It Dv SHT_REL
.It Dv SHT_RELA
Section index of the section to which the relocation information applies.
.Pp
.It Dv SHT_SYMTAB
.It Dv SHT_DYNSYM
Contains a value one greater that the last local symbol table index.
.El
.It Fa 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.
.It Fa 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.
.El
.Pp
Every executable object must contain a program header. The program header
contains information necessary in constructing a process image.
.Bd -literal -offset indent
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;
.Ed
.Pp
.Bl -tag -width p_offset
.It Fa p_type
Contains the segment type indicator. The possible values are:
.Bl -tag -width PT_DYNAMIC
.It Dv PT_NULL
Segment is inactive. The other fields contain undefined values.
.It Dv PT_LOAD
Segment is loadable. It is loaded to the address described by
.Fa p_vaddr .
If
.Fa p_memsz
is greater than
.Fa p_filesz ,
the memory range from
.Po Fa p_vaddr
+
.Fa p_filesz Pc
to
.Po Fa p_vaddr
+
.Fa p_memsz Pc
is zero-filled when the segment is loaded.
.Fa p_filesz
can not be greater than
.Fa p_memsz .
Segments of this type are sorted in the header table by
.Fa p_vaddr
in ascending order.
.It Dv PT_DYNAMIC
Segment contains dynamic linking information.
.It Dv 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:
.Pa /libexec/ld.elf_so
.It Dv PT_NOTE
Segment contains some special information. Format can be e.g. vendor-specific.
.It Dv 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
.Dv EI_OSABI
field.
.It Dv PT_PHDR
The values in a program header of this type specify the characteristics
of the program header table itself. For example, the
.Fa 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.
.El
.It Fa p_offset
Contains the byte offset from the beginning of the file to the beginning
of this segment.
.It Fa p_vaddr
Contains the virtual memory address to which this segment is loaded.
.It Fa 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.
.It Fa p_filesz
Contains the number of bytes this segment occupies in the file image.
.It Fa p_memsz
Contains the number of bytes this segment occupies in the memory image.
.It Fa 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:
.Bl -tag -width PF_R
.It Dv PF_R
Segment can be read.
.It Dv PF_W
Segment can be written.
.It Dv PF_X
Segment is executable.
.El
.It Fa p_align
Contains the segment alignment. Acceptable values are 0 and 1 for no alignment,
and integral powers of two.
.Fa p_vaddr
should equal
.Fa p_offset
modulo
.Fa p_align .
.El
.Sh SEE ALSO
.Xr as 1 ,
.Xr gdb 1 ,
.Xr ld 1 ,
.Xr ld.elf_so 1 ,
.Xr execve 2 ,
.Xr nlist 3 ,
.Xr a.out 5 ,
.Xr core 5 ,
.Xr link 5 ,
.Xr stab 5
.Sh HISTORY
The ELF object file format first appeared in
.At V .