395 lines
12 KiB
Groff
395 lines
12 KiB
Groff
.\" $NetBSD: elf.5,v 1.3 2001/04/14 16:52:02 kleink Exp $
|
|
.\"
|
|
.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
|
|
.\" All rights reserved.
|
|
.\"
|
|
.\" This document is derived from work contributed to The NetBSD Foundation
|
|
.\" by Antti Kantee
|
|
.\"
|
|
.\" 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.
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the NetBSD
|
|
.\" Foundation, Inc. and its contributors.
|
|
.\" 4. Neither the name of The NetBSD Foundation nor the names of its
|
|
.\" contributors may be used to endorse or promote products derived
|
|
.\" from this software without specific prior written permission.
|
|
.\"
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
|
|
.\"
|
|
.Dd April 14, 2001
|
|
.Dt ELF 5
|
|
.Os
|
|
.Sh NAME
|
|
.Nm ELF
|
|
.Nd executable and linking format
|
|
.Sh SYNOPSIS
|
|
.Fd #include <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
|
|
An ELF object file can come in three different types:
|
|
.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.so 1
|
|
at run time.
|
|
.El
|
|
.Pp
|
|
ELF files have a dual nature. The assembler
|
|
.Xr as 1
|
|
and linker
|
|
.Xr ld 1
|
|
treat them as a set of sections described by then 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 or segments.
|
|
The file ends with a table of section headers (optional for executable
|
|
files).
|
|
.Pp
|
|
A segment can consist of several sections with related attributes. A file
|
|
usually has only a few loadable segments, maybe one read-only/executable for
|
|
code, one for read-only data and one for read/write data. Using segments the
|
|
system can quickly map all the loadable sections into memory with just a few
|
|
operations, instead of having to map 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 ELFMAG0
|
|
The elements ranging from
|
|
.Dv ELFMAG0
|
|
to
|
|
.Dv ELFMAG3
|
|
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.
|
|
.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.
|
|
.It Fa e_ehsize
|
|
The size of the ELF header.
|
|
.It Fa e_phentsize
|
|
The size of an entry in the program header table.
|
|
.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.
|
|
.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. The relocation information located
|
|
here is absolute.
|
|
.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 relative relocation information.
|
|
.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 a 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.
|
|
.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
|
|
that contain a segment of this type do not conform to the ABI.
|
|
.It Dv PT_PHDR
|
|
Values of the other fields, such as size and offset, specify values for the
|
|
program header table. Only one segment of this kind may exist, and
|
|
it 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. Since
|
|
.Nx
|
|
doesn't use physical addressing, this value is ignored.
|
|
.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_R
|
|
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.so 1 ,
|
|
.Xr execve 2 ,
|
|
.Xr nlist 3 ,
|
|
.Xr a.out 5 ,
|
|
.Xr core 5
|
|
.Xr link 5 ,
|
|
.Xr stab 5 .
|
|
.Sh HISTORY
|
|
.Pa ELF
|
|
first appeared in
|
|
.At V .
|