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$Id: elf.3 3743 2019-06-12 19:36:30Z jkoshy $
.Dd June 12, 2019 .Dt ELF 3 .Os .Sh NAME .Nm elf .Nd API for manipulating ELF objects .Sh LIBRARY .Lb libelf .Sh SYNOPSIS n 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.
p 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.
p 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 .
p 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.
p 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.
p 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.
p 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: l -bullet -compact -offset indent t The ELF version that the application was compiled against. This version determines the ABI expected by the application. t The ELF version of the ELF object being manipulated by the application through the ELF library. t The ELF version (or set of versions) supported by the ELF library itself. .El
p 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.
p In the current implementation, all three versions have to be .Dv EV_CURRENT . .Ss Namespace use The ELF library uses the following prefixes: l -tag -width "ELF_F_*" t Dv elf_ Used for class-independent functions. t Dv elf32_ Used for functions working with 32 bit ELF objects. t Dv elf64_ Used for functions working with 64 bit ELF objects. t Dv Elf_ Used for class-independent data types. t Dv ELF_C_ Used for command values used in a few functions. These symbols are defined as members of the .Vt Elf_Cmd enumeration. t Dv ELF_E_ Used for error numbers. t Dv ELF_F_ Used for flags. t 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. t 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
p 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: l -tag -width ".Vt Elf_Data" t 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.
p 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.
p 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.
p The .Vt Elf structure is opaque and has no members visible to the application. t 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: l -tag -width ".Vt unsigned int d_version" -compact t 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. t Vt "void *d_buf" A pointer to data in memory. t 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. t Vt "uint64_t d_size" The number of bytes of data in this descriptor. t Vt "Elf_Type d_type" The ELF type (see below) of the data in this descriptor. t Vt "unsigned int d_version" The operating version for the data in this buffer. .El
p .Vt Elf_Data descriptors are usually used in conjunction with .Vt Elf_Scn descriptors. t Vt Elf_Scn .Vt Elf_Scn descriptors represent sections in an ELF object. These descriptors are opaque and contain no application modifiable fields.
p 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.
p 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.
p
l -tag -width ".Dv ELF_T_SYMINFO" -compact t Dv ELF_T_ADDR Machine addresses.
t Dv ELF_T_BYTE Byte data.
The library will not attempt to translate byte data.
t Dv ELF_T_CAP Software and hardware capability records.
t Dv ELF_T_DYN Records used in a section of type
.Dv SHT_DYNAMIC .
t Dv ELF_T_EHDR ELF executable header.
t Dv ELF_T_GNUHASH GNU-style hash tables.
t Dv ELF_T_HALF 16-bit unsigned words.
t Dv ELF_T_LWORD 64 bit unsigned words.
t Dv ELF_T_MOVE ELF Move records.
.It Dv ELF_T_MOVEP
As yet unsupported.
t Dv ELF_T_NOTE ELF Note structures.
t Dv ELF_T_OFF File offsets.
t Dv ELF_T_PHDR ELF program header table entries.
t Dv ELF_T_REL ELF relocation entries.
t Dv ELF_T_RELA ELF relocation entries with addends.
t Dv ELF_T_SHDR ELF section header entries.
t Dv ELF_T_SWORD Signed 32-bit words.
t Dv ELF_T_SXWORD Signed 64-bit words.
t Dv ELF_T_SYMINFO ELF symbol information.
t Dv ELF_T_SYM ELF symbol table entries.
t Dv ELF_T_VDEF Symbol version definition records.
t Dv ELF_T_VNEED Symbol version requirement records.
t Dv ELF_T_WORD Unsigned 32-bit words.
t Dv ELF_T_XWORD Unsigned 64-bit words.
.El
p The symbol .Dv ELF_T_NUM denotes the number of Elf types known to the library.
p The following table shows the mapping between ELF section types defined in .Xr elf 5 and the types supported by the library. l -column ".Dv SHT_PREINIT_ARRAY" ".Dv ELF_T_SYMINFO" t Em Section Type Ta Em "Library Type" Ta Em Description t Dv SHT_DYNAMIC Ta Dv ELF_T_DYN Ta Xo .Sq .dynamic section entries. .Xc t Dv SHT_DYNSYM Ta Dv ELF_T_SYM Ta Symbols for dynamic linking. t Dv SHT_FINI_ARRAY Ta Dv ELF_T_ADDR Ta Termination function pointers. t Dv SHT_GNU_HASH Ta Dv ELF_T_GNUHASH Ta GNU hash sections. t Dv SHT_GNU_LIBLIST Ta Dv ELF_T_WORD Ta List of libraries to be pre-linked. t Dv SHT_GNU_verdef Ta Dv ELF_T_VDEF Ta Symbol version definitions. t Dv SHT_GNU_verneed Ta Dv ELF_T_VNEED Ta Symbol versioning requirements. t Dv SHT_GNU_versym Ta Dv ELF_T_HALF Ta Version symbols. t Dv SHT_GROUP Ta Dv ELF_T_WORD Ta Section group marker. t Dv SHT_HASH Ta Dv ELF_T_HASH Ta Symbol hashes. t Dv SHT_INIT_ARRAY Ta Dv ELF_T_ADDR Ta Initialization function pointers. t Dv SHT_NOBITS Ta Dv ELF_T_BYTE Ta Xo Empty sections. See .Xr elf 5 . .Xc t Dv SHT_NOTE Ta Dv ELF_T_NOTE Ta ELF note records. t Dv SHT_PREINIT_ARRAY Ta Dv ELF_T_ADDR Ta Pre-initialization function pointers. t Dv SHT_PROGBITS Ta Dv ELF_T_BYTE Ta Machine code. t Dv SHT_REL Ta Dv ELF_T_REL Ta ELF relocation records. t Dv SHT_RELA Ta Dv ELF_T_RELA Ta Relocation records with addends. t Dv SHT_STRTAB Ta Dv ELF_T_BYTE Ta String tables. t Dv SHT_SYMTAB Ta Dv ELF_T_SYM Ta Symbol tables. t Dv SHT_SYMTAB_SHNDX Ta Dv ELF_T_WORD Ta Used with extended section numbering. t Dv SHT_SUNW_dof Ta Dv ELF_T_BYTE Ta Xo Used by .Xr dtrace 1 . .Xc t Dv SHT_SUNW_move Ta Dv ELF_T_MOVE Ta ELF move records. t Dv SHT_SUNW_syminfo Ta Dv ELF_T_SYMINFO Ta Additional symbol flags. t Dv SHT_SUNW_verdef Ta Dv ELF_T_VDEF Ta Xo Same as .Dv SHT_GNU_verdef . .Xc t Dv SHT_SUNW_verneed Ta Dv ELF_T_VNEED Ta Xo Same as .Dv SHT_GNU_verneed . .Xc t Dv SHT_SUNW_versym Ta Dv ELF_T_HALF Ta Xo Same as .Dv SHT_GNU_versym . .Xc .El
p 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. l -tag -width indent t "Archive Access" l -tag -compact -width indent t Fn elf_getarsym Retrieve the archive symbol table. t Fn elf_getarhdr Retrieve the archive header for an object. t Fn elf_getbase Retrieve the offset of a member inside an archive. t Fn elf_next Iterate through an .Xr ar 1 archive. t Fn elf_rand Random access inside an .Xr ar 1 archive. .El t "Data Structures" l -tag -compact -width indent t Fn elf_getdata Retrieve translated data for an ELF section. t Fn elf_getscn Retrieve the section descriptor for a named section. t Fn elf_ndxscn Retrieve the index for a section. t Fn elf_newdata Add a new .Vt Elf_Data descriptor to an ELF section. t Fn elf_newscn Add a new section descriptor to an ELF descriptor. t Fn elf_nextscn Iterate through the sections in an ELF object. t Fn elf_rawdata Retrieve untranslated data for an ELF section. t Fn elf_rawfile Return a pointer to the untranslated file contents for an ELF object. t Fn elf32_getehdr , Fn elf64_getehdr Retrieve the Executable Header in an ELF object. t Fn elf32_getphdr , Fn elf64_getphdr Retrieve the Program Header Table in an ELF object. t Fn elf32_getshdr , Fn elf64_getshdr Retrieve the ELF section header associated with an .Vt Elf_Scn descriptor. t Fn elf32_newehdr , Fn elf64_newehdr Allocate an Executable Header in an ELF object. t Fn elf32_newphdr , Fn elf64_newphdr Allocate or resize the Program Header Table in an ELF object. .El t "Data Translation" l -tag -compact -width indent t Fn elf32_xlatetof , Fn elf64_xlatetof Translate an ELF data structure from its native representation to its file representation. t Fn elf32_xlatetom , Fn elf64_xlatetom Translate an ELF data structure from its file representation to a native representation. .El t "Error Reporting" l -tag -compact -width indent t Fn elf_errno Retrieve the current error. t Fn elf_errmsg Retrieve a human readable description of the current error. .El t "Initialization" l -tag -compact -width indent t Fn elf_begin Opens an .Xr ar 1 archive or ELF object given a file descriptor. t Fn elf_end Close an ELF descriptor and release all its resources. t Fn elf_memory Opens an .Xr ar 1 archive or ELF object present in a memory arena. t Fn elf_version Sets the operating version. .El t "IO Control" l -tag -width ".Fn elf_setshstrndx" -compact t Fn elf_cntl Manage the association between and ELF descriptor and its underlying file. t Fn elf_flagdata Mark an .Vt Elf_Data descriptor as dirty. t Fn elf_flagehdr Mark the ELF Executable Header in an ELF descriptor as dirty. t Fn elf_flagphdr Mark the ELF Program Header Table in an ELF descriptor as dirty. t Fn elf_flagscn Mark an .Vt Elf_Scn descriptor as dirty. t Fn elf_flagshdr Mark an ELF Section Header as dirty. t Fn elf_setshstrndx Set the index of the section name string table for the ELF object. t Fn elf_update Recompute ELF object layout and optionally write the modified object back to the underlying file. .El t "Queries" l -tag -width ".Fn elf_getshstrndx" -compact t Fn elf32_checksum , Fn elf64_checkum Compute checksum of an ELF object. t Fn elf_getident Retrieve the identification bytes for an ELF object. t Fn elf_getphdrnum Retrieve the number of program headers in an ELF object. t Fn elf_getshdrnum Retrieve the number of sections in an ELF object. t Fn elf_getshdrstrndx Retrieve the section index of the section name string table in an ELF object. t Fn elf_hash Compute the ELF hash value of a string. t Fn elf_kind Query the kind of object associated with an ELF descriptor. t 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.
p 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.
p 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 an 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.
p 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 . .Sh AUTHORS The ELF library was written by .An Joseph Koshy Aq Mt jkoshy@FreeBSD.org .