Annotation of src/share/man/man5/elf.5, Revision 1.15
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1.13 pooka 30: .Dd November 18, 2006
1.1 pooka 31: .Dt ELF 5
32: .Os
33: .Sh NAME
34: .Nm ELF
35: .Nd executable and linking format
36: .Sh SYNOPSIS
1.10 wiz 37: .In elf.h
1.1 pooka 38: .Sh DESCRIPTION
39: Because of the flexible nature of ELF, the structures describing it are
40: available both as 32bit and 64bit versions. This document uses the 32bit
41: versions, refer to
1.15 ! joerg 42: .In elf.h
1.1 pooka 43: for the corresponding 64bit versions.
44: .Pp
1.8 thorpej 45: The four main types of an ELF object file are:
1.1 pooka 46: .Bl -tag -width "relocatable"
47: .It executable
48: A file suitable for execution. It contains the information required for
49: creating a new process image.
50: .It relocatable
51: Contains the necessary information to be run through the link editor
52: .Xr ld 1
53: to create an executable or a shared library.
54: .It shared
55: The shared object contains necessary information which can be used by
56: either the link editor
57: .Xr ld 1
58: at link time or by the dynamic loader
1.4 pooka 59: .Xr ld.elf_so 1
1.1 pooka 60: at run time.
1.8 thorpej 61: .It core
62: A file which describes the virtual address space and register state
63: of a process. Core files are typically used in conjunction with
64: debuggers such as
65: .Xr gdb 1 .
1.1 pooka 66: .El
67: .Pp
1.4 pooka 68: ELF files have a dual nature. The toolchain, including tools such as the
1.1 pooka 69: .Xr as 1
70: and linker
1.4 pooka 71: .Xr ld 1 ,
72: treats them as a set of sections described by their section headers. The system
1.1 pooka 73: loader treats them as a set of segments described by the program headers.
74: .Pp
75: The general format of an ELF file is the following: The file starts with an
76: ELF header. This is followed by a table of program headers (optional for
1.4 pooka 77: relocatable and shared files). After this come the sections/segments.
1.1 pooka 78: The file ends with a table of section headers (optional for executable
79: files).
80: .Pp
1.4 pooka 81: A segment can be considered to consist of several sections. For example,
82: all executable sections are typically packed into one loadable segment
83: which is read-only and executable (see
84: .Fa p_flags
85: in the program header). This enables the system to map the entire file with
86: just a few operations, one for each loadable segment, instead of doing
87: numerous map operations for each section separately.
1.1 pooka 88: .Pp
89: Each file is described by the ELF header:
90: .Bd -literal -offset indent
91: typedef struct {
92: unsigned char e_ident[ELF_NIDENT];
93: Elf32_Half e_type;
94: Elf32_Half e_machine;
95: Elf32_Word e_version;
96: Elf32_Addr e_entry;
97: Elf32_Off e_phoff;
98: Elf32_Off e_shoff;
99: Elf32_Word e_flags;
100: Elf32_Half e_ehsize;
101: Elf32_Half e_phentsize;
102: Elf32_Half e_phnum;
103: Elf32_Half e_shentsize;
104: Elf32_Half e_shnum;
105: Elf32_Half e_shstrndx;
106: } Elf32_Ehdr;
107: .Ed
108: .Pp
109: .Bl -tag -width "e_phentsize"
110: .It Fa e_ident[]
111: The array contains the following information in the indicated locations:
112: .Bl -tag -width EI_ABIVERSION
1.12 pooka 113: .It Dv EI_MAG0
1.5 wiz 114: The elements ranging from
1.12 pooka 115: .Dv EI_MAG0
1.1 pooka 116: to
1.12 pooka 117: .Dv EI_MAG3
1.1 pooka 118: contain the ELF magic number: \\0177ELF
119: .It Dv EI_CLASS
120: Contains the address size of the binary, either 32 or 64bit.
121: .It Dv EI_DATA
122: byte order
123: .It Dv EI_VERSION
124: Contains the ELF header version. This is currently always set to 1.
125: .It Dv EI_OSABI
1.4 pooka 126: Contains the operating system ABI identification. Note that even though the
127: definition
128: .Dv ELFOSABI_NETBSD
129: exists,
130: .Nx
131: uses
132: .Dv ELFOSABI_SYSV
133: here, since the
134: .Nx
135: ABI does not deviate from the standard.
1.1 pooka 136: .It Dv EI_ABIVERSION
137: ABI version.
138: .El
139: .It Fa e_type
140: Contains the file type identification. It can be either
141: .Dv ET_REL ,
142: .Dv ET_EXEC ,
143: .Dv ET_DYN ,
144: or
145: .Dv ET_CORE
146: for relocatable, executable, shared, or core, respectively.
147: .It Fa e_machine
148: Contains the machine type, e.g. SPARC, Alpha, MIPS, ...
149: .It Fa e_entry
150: The program entry point if the file is executable.
151: .It Fa e_phoff
152: The position of the program header table in the file or 0 if it doesn't exist.
153: .It Fa e_shoff
154: The position of the section header table in the file or 0 if it doesn't exist.
155: .It Fa e_flags
1.4 pooka 156: Contains processor-specific flags. For example, the SPARC port uses this
157: space to specify what kind of memory store ordering is required.
1.1 pooka 158: .It Fa e_ehsize
159: The size of the ELF header.
160: .It Fa e_phentsize
1.4 pooka 161: The size of an entry in the program header table. All entries are the same
162: size.
1.1 pooka 163: .It Fa e_phnum
164: The number of entries in the program header table, or 0 if none exists.
165: .It Fa e_shentsize
1.4 pooka 166: The size of an entry in the section header table. All entries are the same
167: size.
1.1 pooka 168: .It Fa e_shnum
169: The number of entries in the section header table, or 0 if none exists.
170: .It Fa e_shstrndx
171: Contains the index number of the section which contains the section
172: name strings.
173: .El
174: .Pp
175: Each ELF section in turn is described by the section header:
176: .Bd -literal -offset indent
177: typedef struct {
178: Elf32_Word sh_name;
179: Elf32_Word sh_type;
180: Elf32_Word sh_flags;
181: Elf32_Addr sh_addr;
182: Elf32_Off sh_offset;
183: Elf32_Word sh_size;
184: Elf32_Word sh_link;
185: Elf32_Word sh_info;
186: Elf32_Word sh_addralign;
187: Elf32_Word sh_entsize;
188: } Elf32_Shdr;
189: .Ed
190: .Pp
191: .Bl -tag -width "sh_addralign"
192: .It Fa sh_name
193: Contains an index to the position in the section header string section where
194: the name of the current section can be found.
195: .It Fa sh_type
196: Contains the section type indicator. The more important possible values are:
197: .Bl -tag -width "SHT_PROGBITS"
198: .It Dv SHT_NULL
199: Section is inactive. The other fields contain undefined values.
200: .It Dv SHT_PROGBITS
201: Section contains program information. It can be for example code, data,
202: or debugger information.
203: .It Dv SHT_SYMTAB
204: Section contains a symbol table. This section usually contains all the
205: symbols and is intended for the regular link editor
206: .Xr ld 1 .
207: .It Dv SHT_STRTAB
208: Section contains a string table.
209: .It Dv SHT_RELA
1.5 wiz 210: Section contains relocation information with an explicit addend.
1.1 pooka 211: .It Dv SHT_HASH
212: Section contains a symbol hash table.
213: .It Dv SHT_DYNAMIC
214: Section contains dynamic linking information.
215: .It Dv SHT_NOTE
216: Section contains some special information. The format can be e.g.
217: vendor-specific.
218: .It Dv SHT_NOBITS
219: Sections contains information similar to
220: .Dv SHT_PROGBITS ,
221: but takes up no space in the file. This can be used for e.g. bss.
222: .It Dv SHT_REL
1.4 pooka 223: Section contains relocation information without an explicit addend.
1.1 pooka 224: .It Dv SHT_SHLIB
225: This section type is reserved but has unspecified semantics.
226: .It Dv SHT_DYNSYM
227: Section contains a symbol table. This symbol table is intended for the
228: dynamic linker, and is kept as small as possible to conserve space, since
229: it must be loaded to memory at run time.
230: .El
231: .It Fa sh_flags
232: Contains the section flags, which can have the following values or any
233: combination of them:
234: .Bl -tag -width SHF_EXECINSTR
235: .It Dv SHF_WRITE
236: Section is writable after it has been loaded.
237: .It Dv SHF_ALLOC
238: Section will occupy memory at run time.
239: .It Dv SHF_EXECINSTR
240: Section contains executable machine instructions.
241: .El
242: .It Fa sh_addr
243: Address to where the section will be loaded, or 0 if this section does not
244: reside in memory at run time.
245: .It Fa sh_offset
246: The byte offset from the beginning of the file to the beginning of this
247: section. If the section is of type
248: .Dv SHT_NOBITS ,
249: this field specifies the conceptual placement in the file.
250: .It Fa sh_size
251: The size of the section in the file for all types except
252: .Dv SHT_NOBITS .
253: For that type the value may differ from zero, but the section will still
254: always take up no space from the file.
255: .It Fa sh_link
256: Contains an index to the section header table. The interpretation depends
257: on the section type as follows:
258: .Pp
259: .Bl -tag -compact -width SHT_DYNAMIC
260: .It Dv SHT_REL
261: .It Dv SHT_RELA
262: Section index of the associated symbol table.
263: .Pp
264: .It Dv SHT_SYMTAB
265: .It Dv SHT_DYNSYM
266: Section index of the associated string table.
267: .Pp
268: .It Dv SHT_HASH
269: Section index of the symbol table to which the hash table applies.
270: .Pp
271: .It Dv SHT_DYNAMIC
272: Section index of of the string table by which entries in this section are used.
273: .El
274: .It Fa sh_info
275: Contains extra information. The interpretation depends on the type as
276: follows:
277: .Pp
278: .Bl -tag -compact -width SHT_DYNSYM
279: .It Dv SHT_REL
280: .It Dv SHT_RELA
281: Section index of the section to which the relocation information applies.
282: .Pp
283: .It Dv SHT_SYMTAB
1.5 wiz 284: .It Dv SHT_DYNSYM
1.1 pooka 285: Contains a value one greater that the last local symbol table index.
286: .El
287: .It Fa sh_addralign
288: Marks the section alignment requirement. If, for example, the section contains
289: a doubleword, the entire section must be doubleword aligned to ensure proper
290: alignment. Only 0 and integral powers of two are allowed. Values 0 and 1
291: denote that the section has no alignment.
292: .It Fa sh_entsize
1.11 uebayasi 293: Contains the entry size of an element for sections which are constructed
1.1 pooka 294: of a table of fixed-size entries. If the section does not hold a table of
295: fixed-size entries, this value is 0.
296: .El
297: .Pp
298: Every executable object must contain a program header. The program header
299: contains information necessary in constructing a process image.
300: .Bd -literal -offset indent
301: typedef struct {
302: Elf32_Word p_type;
303: Elf32_Off p_offset;
304: Elf32_Addr p_vaddr;
305: Elf32_Addr p_paddr;
306: Elf32_Word p_filesz;
307: Elf32_Word p_memsz;
308: Elf32_Word p_flags;
309: Elf32_Word p_align;
310: } Elf32_Phdr;
311: .Ed
312: .Pp
313: .Bl -tag -width p_offset
314: .It Fa p_type
315: Contains the segment type indicator. The possible values are:
316: .Bl -tag -width PT_DYNAMIC
317: .It Dv PT_NULL
318: Segment is inactive. The other fields contain undefined values.
319: .It Dv PT_LOAD
320: Segment is loadable. It is loaded to the address described by
321: .Fa p_vaddr .
322: If
323: .Fa p_memsz
324: is greater than
325: .Fa p_filesz ,
326: the memory range from
1.5 wiz 327: .Po Fa p_vaddr
1.1 pooka 328: +
329: .Fa p_filesz Pc
330: to
331: .Po Fa p_vaddr
332: +
333: .Fa p_memsz Pc
334: is zero-filled when the segment is loaded.
335: .Fa p_filesz
336: can not be greater than
337: .Fa p_memsz .
338: Segments of this type are sorted in the header table by
339: .Fa p_vaddr
340: in ascending order.
341: .It Dv PT_DYNAMIC
342: Segment contains dynamic linking information.
343: .It Dv PT_INTERP
344: Segment contains a null-terminated path name to the interpreter. This segment
345: may be present only once in a file, and it must appear before any loadable
1.4 pooka 346: segments. This field will most likely contain the ELF dynamic loader:
1.13 pooka 347: .Pa /libexec/ld.elf_so
1.1 pooka 348: .It Dv PT_NOTE
349: Segment contains some special information. Format can be e.g. vendor-specific.
350: .It Dv PT_SHLIB
351: This segment type is reserved but has unspecified semantics. Programs
1.4 pooka 352: which contain a segment of this type do not conform to the ABI, and must
353: indicate this by setting the appropriate ABI in the ELF header
354: .Dv EI_OSABI
355: field.
1.1 pooka 356: .It Dv PT_PHDR
1.4 pooka 357: The values in a program header of this type specify the characteristics
358: of the program header table itself. For example, the
359: .Fa p_vaddr
360: field specifies the program header table location in memory once the
361: program is loaded. This field may not occur more than once, may occur only
362: if the program header table is part of the file memory image, and must
363: come before any loadable segments.
1.1 pooka 364: .El
365: .It Fa p_offset
366: Contains the byte offset from the beginning of the file to the beginning
367: of this segment.
368: .It Fa p_vaddr
369: Contains the virtual memory address to which this segment is loaded.
370: .It Fa p_paddr
1.4 pooka 371: Contains the physical address to which this segment is loaded. This value
372: is usually ignored, but may be used while bootstrapping or in embedded
373: systems.
1.1 pooka 374: .It Fa p_filesz
375: Contains the number of bytes this segment occupies in the file image.
376: .It Fa p_memsz
377: Contains the number of bytes this segment occupies in the memory image.
378: .It Fa p_flags
379: Contains the segment flags, which specify the permissions for the segment
380: after it has been loaded. The following values or any combination of them
381: is acceptable:
382: .Bl -tag -width PF_R
383: .It Dv PF_R
384: Segment can be read.
1.9 wiz 385: .It Dv PF_W
1.1 pooka 386: Segment can be written.
387: .It Dv PF_X
388: Segment is executable.
389: .El
390: .It Fa p_align
391: Contains the segment alignment. Acceptable values are 0 and 1 for no alignment,
392: and integral powers of two.
393: .Fa p_vaddr
394: should equal
395: .Fa p_offset
396: modulo
397: .Fa p_align .
398: .El
399: .Sh SEE ALSO
400: .Xr as 1 ,
401: .Xr gdb 1 ,
402: .Xr ld 1 ,
1.4 pooka 403: .Xr ld.elf_so 1 ,
1.1 pooka 404: .Xr execve 2 ,
405: .Xr nlist 3 ,
406: .Xr a.out 5 ,
1.6 wiz 407: .Xr core 5 ,
1.1 pooka 408: .Xr link 5 ,
1.6 wiz 409: .Xr stab 5
1.1 pooka 410: .Sh HISTORY
1.8 thorpej 411: The ELF object file format first appeared in
1.1 pooka 412: .At V .
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