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.\" Id: elf.3 3927 2021-03-07 17:22:22Z jkoshy
.\"
.Dd March 7, 2021
.Dt ELF 3
.Os
.Sh NAME
.Nm elf
.Nd API for manipulating ELF objects
.Sh LIBRARY
.Lb libelf
.Sh SYNOPSIS
.In libelf.h
.Sh DESCRIPTION
The
.Lb libelf
provides functions that allow an application to read and manipulate
ELF object files, and to read
.Xr ar 1
archives.
The library allows the manipulation of ELF objects in a byte ordering
and word-size independent way, allowing an application to read and
create ELF objects for 32 and 64 bit architectures and for little-
and big-endian machines.
The library is capable of processing ELF objects that use extended
section numbering.
.Pp
This manual page serves to provide an overview of the functionality in
the ELF library.
Further information may found in the manual pages for individual
.Xr ELF 3
functions that comprise the library.
.Ss ELF Concepts
As described in
.Xr elf 5 ,
ELF files contain several data structures that are laid out in a
specific way.
ELF files begin with an
.Dq Executable Header ,
and may contain an optional
.Dq Program Header Table ,
and optional data in the form of ELF
.Dq sections .
A
.Dq Section Header Table
describes the content of the data in these sections.
.Pp
ELF objects have an associated
.Dq "ELF class"
which denotes the natural machine word size for the architecture
the object is associated with.
Objects for 32 bit architectures have an ELF class of
.Dv ELFCLASS32 .
Objects for 64 bit architectures have an ELF class of
.Dv ELFCLASS64 .
.Pp
ELF objects also have an associated
.Dq endianness
which denotes the endianness of the machine architecture associated
with the object.
This may be
.Dv ELFDATA2LSB
for little-endian architectures and
.Dv ELFDATA2MSB
for big-endian architectures.
.Pp
ELF objects are also associated with an API version number.
This version number determines the layout of the individual components
of an ELF file and the semantics associated with these.
.Ss Data Representation And Translation
The
.Xr ELF 3
library distinguishes between
.Dq native
representations of ELF data structures and their
.Dq file
representations.
.Pp
An application would work with ELF data in its
.Dq native
representation, i.e., using the native byteorder and alignment mandated
by the processor the application is running on.
The
.Dq file
representation of the same data could use a different byte ordering
and follow different constraints on object alignment than these native
constraints.
.Pp
Accordingly, the
.Xr ELF 3
library offers translation facilities
.Xr ( elf32_xlatetof 3 ,
.Xr elf32_xlatetom 3 ,
.Xr elf64_xlatetof 3
and
.Xr elf64_xlatetom 3 )
to and from these representations.
It also provides higher-level APIs
.Xr ( gelf_xlatetof 3 ,
.Xr gelf_xlatetom 3 )
that retrieve and store data from the ELF object in a class-agnostic
manner.
.Ss Library Working Version
Conceptually, there are three version numbers associated with an
application using the ELF library to manipulate ELF objects:
.Bl -bullet -compact -offset indent
.It
The ELF version that the application was compiled against.
This version determines the ABI expected by the application.
.It
The ELF version of the ELF object being manipulated by the
application through the ELF library.
.It
The ELF version (or set of versions) supported by the ELF library itself.
.El
.Pp
In order to facilitate working with ELF objects of differing versions,
the ELF library requires the application to call the
.Fn elf_version
function before invoking many of its operations, in order to inform
the library of the application's desired working version.
.Pp
In the current implementation, all three versions have to be
.Dv EV_CURRENT .
.Ss Namespace use
The ELF library uses the following prefixes:
.Bl -tag -width "ELF_F_*"
.It Dv elf_
Used for class-independent functions.
.It Dv elf32_
Used for functions working with 32 bit ELF objects.
.It Dv elf64_
Used for functions working with 64 bit ELF objects.
.It Dv Elf_
Used for class-independent data types.
.It Dv ELF_C_
Used for command values used in a few functions.
These symbols are defined as members of the
.Vt Elf_Cmd
enumeration.
.It Dv ELF_E_
Used for error numbers.
.It Dv ELF_F_
Used for flags.
.It Dv ELF_K_
These constants define the kind of file associated with an ELF
descriptor.
See
.Xr elf_kind 3 .
The symbols are defined by the
.Vt Elf_Kind
enumeration.
.It Dv ELF_T_
These values are defined by the
.Vt Elf_Type
enumeration, and denote the types of ELF data structures
that can be present in an ELF object.
.El
.Pp
In addition, the library uses symbols with prefixes
.Dv _ELF
and
.Dv _libelf
for its internal use.
.Ss Descriptors
Applications communicate with the library using descriptors.
These are:
.Bl -tag -width ".Vt Elf_Data"
.It Vt Elf
An
.Vt Elf
descriptor represents an ELF object or an
.Xr ar 1
archive.
It is allocated using one of the
.Fn elf_begin
or
.Fn elf_memory
functions.
An
.Vt Elf
descriptor can be used to read and write data to an ELF file.
An
.Vt Elf
descriptor can be associated with zero or more
.Vt Elf_Scn
section descriptors.
.Pp
Given an ELF descriptor, the application may retrieve the ELF
object's class-dependent
.Dq "Executable Header"
structures using the
.Fn elf32_getehdr
or
.Fn elf64_getehdr
functions.
A new Ehdr structure may be allocated using the
.Fn elf64_newehdr
or
.Fn elf64_newehdr
functions.
.Pp
The
.Dq "Program Header Table"
associated with an ELF descriptor may be allocated using the
.Fn elf32_getphdr
or
.Fn elf64_getphdr
functions.
A new program header table may be allocated or an existing table
resized using the
.Fn elf32_newphdr
or
.Fn elf64_newphdr
functions.
.Pp
The
.Vt Elf
structure is opaque and has no members visible to the
application.
.It Vt Elf_Data
An
.Vt Elf_Data
data structure describes an individual chunk of a ELF file as
represented in memory.
It has the following application-visible members:
.Bl -tag -width ".Vt unsigned int d_version" -compact
.It Vt "uint64_t d_align"
The in-file alignment of the data buffer within its containing ELF section.
This value must be non-zero and a power of two.
.It Vt "void *d_buf"
A pointer to data in memory.
.It Vt "uint64_t d_off"
The offset within the containing section where this descriptor's data
would be placed.
This field will be computed by the library unless the application
requests full control of the ELF object's layout.
.It Vt "uint64_t d_size"
The number of bytes of data in this descriptor.
.It Vt "Elf_Type d_type"
The ELF type (see below) of the data in this descriptor.
.It Vt "unsigned int d_version"
The operating version for the data in this buffer.
.El
.Pp
.Vt Elf_Data
descriptors are usually used in conjunction with
.Vt Elf_Scn
descriptors.
.It Vt Elf_Scn
.Vt Elf_Scn
descriptors represent sections in an ELF object.
These descriptors are opaque and contain no application modifiable
fields.
.Pp
The
.Vt Elf_Scn
descriptor for a specific section in an ELF object can be
retrieved using the
.Fn elf_getscn
function.
The sections contained in an ELF object can be traversed using the
.Fn elf_nextscn
function.
New sections are allocated using the
.Fn elf_newscn
function.
.Pp
The
.Vt Elf_Data
descriptors associated with a given section can be retrieved
using the
.Fn elf_getdata
function.
New data descriptors can be added to a section
descriptor using the
.Fn elf_newdata
function.
The untranslated
.Dq file
representation of data in a section can be retrieved using the
.Fn elf_rawdata
function.
.El
.Ss Supported Elf Types
The following ELF datatypes are supported by the library.
.Pp
.Bl -tag -width ".Dv ELF_T_SYMINFO" -compact
.It Dv ELF_T_ADDR
Machine addresses.
.It Dv ELF_T_BYTE
Byte data.
The library will not attempt to translate byte data.
.It Dv ELF_T_CAP
Software and hardware capability records.
.It Dv ELF_T_DYN
Records used in a section of type
.Dv SHT_DYNAMIC .
.It Dv ELF_T_EHDR
ELF executable header.
.It Dv ELF_T_GNUHASH
GNU-style hash tables.
.It Dv ELF_T_HALF
16-bit unsigned words.
.It Dv ELF_T_LWORD
64 bit unsigned words.
.It Dv ELF_T_MOVE
ELF Move records.
.\".It Dv ELF_T_MOVEP
.\" As yet unsupported.
.It Dv ELF_T_NOTE
ELF Note structures.
.It Dv ELF_T_OFF
File offsets.
.It Dv ELF_T_PHDR
ELF program header table entries.
.It Dv ELF_T_REL
ELF relocation entries.
.It Dv ELF_T_RELA
ELF relocation entries with addends.
.It Dv ELF_T_SHDR
ELF section header entries.
.It Dv ELF_T_SWORD
Signed 32-bit words.
.It Dv ELF_T_SXWORD
Signed 64-bit words.
.It Dv ELF_T_SYMINFO
ELF symbol information.
.It Dv ELF_T_SYM
ELF symbol table entries.
.It Dv ELF_T_VDEF
Symbol version definition records.
.It Dv ELF_T_VNEED
Symbol version requirement records.
.It Dv ELF_T_WORD
Unsigned 32-bit words.
.It Dv ELF_T_XWORD
Unsigned 64-bit words.
.El
.Pp
The symbol
.Dv ELF_T_NUM
denotes the number of Elf types known to the library.
.Pp
The following table shows the mapping between ELF section types
defined in
.Xr elf 5
and the types supported by the library.
.Bl -column ".Dv SHT_PREINIT_ARRAY" ".Dv ELF_T_SYMINFO"
.It Em Section Type Ta Em "Library Type" Ta Em Description
.It Dv SHT_DYNAMIC Ta Dv ELF_T_DYN Ta Xo
.Sq .dynamic
section entries.
.Xc
.It Dv SHT_DYNSYM Ta Dv ELF_T_SYM Ta Symbols for dynamic linking.
.It Dv SHT_FINI_ARRAY Ta Dv ELF_T_ADDR Ta Termination function pointers.
.It Dv SHT_GNU_HASH Ta Dv ELF_T_GNUHASH Ta GNU hash sections.
.It Dv SHT_GNU_LIBLIST Ta Dv ELF_T_WORD Ta List of libraries to be pre-linked.
.It Dv SHT_GNU_verdef Ta Dv ELF_T_VDEF Ta Symbol version definitions.
.It Dv SHT_GNU_verneed Ta Dv ELF_T_VNEED Ta Symbol versioning requirements.
.It Dv SHT_GNU_versym Ta Dv ELF_T_HALF Ta Version symbols.
.It Dv SHT_GROUP Ta Dv ELF_T_WORD Ta Section group marker.
.It Dv SHT_HASH Ta Dv ELF_T_HASH Ta Symbol hashes.
.It Dv SHT_INIT_ARRAY Ta Dv ELF_T_ADDR Ta Initialization function pointers.
.It Dv SHT_NOBITS Ta Dv ELF_T_BYTE Ta Xo
Empty sections.
See
.Xr elf 5 .
.Xc
.It Dv SHT_NOTE Ta Dv ELF_T_NOTE Ta ELF note records.
.It Dv SHT_PREINIT_ARRAY Ta Dv ELF_T_ADDR Ta Pre-initialization function pointers.
.It Dv SHT_PROGBITS Ta Dv ELF_T_BYTE Ta Machine code.
.It Dv SHT_REL Ta Dv ELF_T_REL Ta ELF relocation records.
.It Dv SHT_RELA Ta Dv ELF_T_RELA Ta Relocation records with addends.
.It Dv SHT_STRTAB Ta Dv ELF_T_BYTE Ta String tables.
.It Dv SHT_SYMTAB Ta Dv ELF_T_SYM Ta Symbol tables.
.It Dv SHT_SYMTAB_SHNDX Ta Dv ELF_T_WORD Ta Used with extended section numbering.
.It Dv SHT_SUNW_dof Ta Dv ELF_T_BYTE Ta Xo
Used by
.Xr dtrace 1 .
.Xc
.It Dv SHT_SUNW_move Ta Dv ELF_T_MOVE Ta ELF move records.
.It Dv SHT_SUNW_syminfo Ta Dv ELF_T_SYMINFO Ta Additional symbol flags.
.It Dv SHT_SUNW_verdef Ta Dv ELF_T_VDEF Ta Xo
Same as
.Dv SHT_GNU_verdef .
.Xc
.It Dv SHT_SUNW_verneed Ta Dv ELF_T_VNEED Ta Xo
Same as
.Dv SHT_GNU_verneed .
.Xc
.It Dv SHT_SUNW_versym Ta Dv ELF_T_HALF Ta Xo
Same as
.Dv SHT_GNU_versym .
.Xc
.El
.Pp
Section types in the range
.Dv [ SHT_LOOS ,
.Dv SHT_HIUSER ]
are otherwise considered to be of type
.Dv ELF_T_BYTE .
.Ss Functional Grouping
This section contains a brief overview of the available functionality
in the ELF library.
Each function listed here is described further in its own manual page.
.Bl -tag -width indent
.It "Archive Access"
.Bl -tag -compact -width indent
.It Fn elf_getarsym
Retrieve the archive symbol table.
.It Fn elf_getarhdr
Retrieve the archive header for an object.
.It Fn elf_getbase
Retrieve the offset of a member inside an archive.
.It Fn elf_next
Iterate through an
.Xr ar 1
archive.
.It Fn elf_rand
Random access inside an
.Xr ar 1
archive.
.El
.It "Data Structures"
.Bl -tag -compact -width indent
.It Fn elf_getdata
Retrieve translated data for an ELF section.
.It Fn elf_getscn
Retrieve the section descriptor for a named section.
.It Fn elf_ndxscn
Retrieve the index for a section.
.It Fn elf_newdata
Add a new
.Vt Elf_Data
descriptor to an ELF section.
.It Fn elf_newscn
Add a new section descriptor to an ELF descriptor.
.It Fn elf_nextscn
Iterate through the sections in an ELF object.
.It Fn elf_rawdata
Retrieve untranslated data for an ELF section.
.It Fn elf_rawfile
Return a pointer to the untranslated file contents for an ELF object.
.It Fn elf32_getehdr , Fn elf64_getehdr
Retrieve the Executable Header in an ELF object.
.It Fn elf32_getphdr , Fn elf64_getphdr
Retrieve the Program Header Table in an ELF object.
.It Fn elf32_getshdr , Fn elf64_getshdr
Retrieve the ELF section header associated with an
.Vt Elf_Scn
descriptor.
.It Fn elf32_newehdr , Fn elf64_newehdr
Allocate an Executable Header in an ELF object.
.It Fn elf32_newphdr , Fn elf64_newphdr
Allocate or resize the Program Header Table in an ELF object.
.El
.It "Data Translation"
.Bl -tag -compact -width indent
.It Fn elf32_xlatetof , Fn elf64_xlatetof
Translate an ELF data structure from its native representation to its
file representation.
.It Fn elf32_xlatetom , Fn elf64_xlatetom
Translate an ELF data structure from its file representation to a
native representation.
.El
.It "Error Reporting"
.Bl -tag -compact -width indent
.It Fn elf_errno
Retrieve the current error.
.It Fn elf_errmsg
Retrieve a human readable description of the current error.
.El
.It "Initialization"
.Bl -tag -compact -width indent
.It Fn elf_begin
Opens an
.Xr ar 1
archive or ELF object given a file descriptor.
.It Fn elf_end
Close an ELF descriptor and release all its resources.
.It Fn elf_memory
Opens an
.Xr ar 1
archive or ELF object present in a memory arena.
.It Fn elf_version
Sets the operating version.
.El
.It "IO Control"
.Bl -tag -width ".Fn elf_setshstrndx" -compact
.It Fn elf_cntl
Manage the association between and ELF descriptor and its underlying file.
.It Fn elf_flagdata
Mark an
.Vt Elf_Data
descriptor as dirty.
.It Fn elf_flagehdr
Mark the ELF Executable Header in an ELF descriptor as dirty.
.It Fn elf_flagphdr
Mark the ELF Program Header Table in an ELF descriptor as dirty.
.It Fn elf_flagscn
Mark an
.Vt Elf_Scn
descriptor as dirty.
.It Fn elf_flagshdr
Mark an ELF Section Header as dirty.
.It Fn elf_setshstrndx
Set the index of the section name string table for the ELF object.
.It Fn elf_update
Recompute ELF object layout and optionally write the modified object
back to the underlying file.
.El
.It "Queries"
.Bl -tag -width ".Fn elf_getshstrndx" -compact
.It Fn elf32_checksum , Fn elf64_checkum
Compute checksum of an ELF object.
.It Fn elf_getident
Retrieve the identification bytes for an ELF object.
.It Fn elf_getphdrnum
Retrieve the number of program headers in an ELF object.
.It Fn elf_getshdrnum
Retrieve the number of sections in an ELF object.
.It Fn elf_getshdrstrndx
Retrieve the section index of the section name string table in
an ELF object.
.It Fn elf_hash
Compute the ELF hash value of a string.
.It Fn elf_kind
Query the kind of object associated with an ELF descriptor.
.It Fn elf32_fsize , Fn elf64_fsize
Return the size of the file representation of an ELF type.
.El
.El
.Ss Controlling ELF Object Layout
In the usual mode of operation, library will compute section
offsets and alignments based on the contents of an ELF descriptor's
sections without need for further intervention by the
application.
.Pp
However, if the application wishes to take complete charge of the
layout of the ELF file, it may set the
.Dv ELF_F_LAYOUT
flag on an ELF descriptor using
.Xr elf_flagelf 3 ,
following which the library will use the data offsets and alignments
specified by the application when laying out the file.
Application control of file layout is described further in the
.Xr elf_update 3
manual page.
.Pp
Gaps in between sections will be filled with the fill character
set by function
.Fn elf_fill .
.Ss Error Handling
In case an error is encountered, these library functions set an
internal error number and signal the presence of the error by
returning a special return value.
The application can check the
current error number by calling
.Xr elf_errno 3 .
A human readable description of the recorded error is available by
calling
.Xr elf_errmsg 3 .
.Ss Memory Management Rules
The library keeps track of all
.Vt Elf_Scn
and
.Vt Elf_Data
descriptors associated with an ELF descriptor and recovers them
when the descriptor is closed using
.Xr elf_end 3 .
Thus the application must not call
.Xr free 3
on data structures allocated by the ELF library.
.Pp
Conversely the library will not
free data that it has not allocated.
As an example, an application may call
.Xr elf_newdata 3
to allocate a new
.Vt Elf_Data
descriptor and can set the
.Va d_off
member of the descriptor to point to a region of memory allocated
using
.Xr malloc 3 .
It is the applications responsibility to free this arena, though the
library will reclaim the space used by the
.Vt Elf_Data
descriptor itself.
.Sh SEE ALSO
.Xr gelf 3 ,
.Xr ar 5 ,
.Xr elf 5
.Sh HISTORY
The original
.Nm
API was developed for
.At V .
The current implementation of the API appeared in
.Fx 7.0
and
.Nx 6.0 .
.Sh AUTHORS
The ELF library was written by
.An Joseph Koshy Aq Mt jkoshy@FreeBSD.org .