Courses/Computer Science/CPSC 355.W2014/Lecture Notes/ELFCont
The ELF Format / Code and Data Container
Continue examination of the ELF structure (via readelf(1) and objdump(1)) and how to create one (without C or gcc).
When you give a source file to a compiler, it produces an executable artifact in a certain format. It turns your code and data into a format the OS will recognize and feed to the CPU.
Let us first consider a great work of beauty and art:
https://code.google.com/p/corkami/wiki/ELF101
We continue with our simple 'fx' program and this time ask readelf to just dump the section header table (with is nicer and easier to read than readelf -t )
(eye@mordor l10)$ readelf -S -W fx There are 38 section headers, starting at offset 0x1138: Section Headers: [Nr] Name Type Addr Off Size ES Flg Lk Inf Al [ 0] NULL 00000000 000000 000000 00 0 0 0 [ 1] .interp PROGBITS 08048134 000134 000013 00 A 0 0 1 [ 2] .note.ABI-tag NOTE 08048148 000148 000020 00 A 0 0 4 [ 3] .note.gnu.build-id NOTE 08048168 000168 000024 00 A 0 0 4 [ 4] .gnu.hash GNU_HASH 0804818c 00018c 000024 04 A 5 0 4 [ 5] .dynsym DYNSYM 080481b0 0001b0 000070 10 A 6 1 4 [ 6] .dynstr STRTAB 08048220 000220 00005b 00 A 0 0 1 [ 7] .gnu.version VERSYM 0804827c 00027c 00000e 02 A 5 0 2 [ 8] .gnu.version_r VERNEED 0804828c 00028c 000020 00 A 6 1 4 [ 9] .rel.dyn REL 080482ac 0002ac 000010 08 A 5 0 4 [10] .rel.plt REL 080482bc 0002bc 000020 08 A 5 12 4 [11] .init PROGBITS 080482dc 0002dc 000030 00 AX 0 0 4 [12] .plt PROGBITS 0804830c 00030c 000050 04 AX 0 0 4 [13] .text PROGBITS 08048360 000360 0001ec 00 AX 0 0 16 [14] .fini PROGBITS 0804854c 00054c 00001c 00 AX 0 0 4 [15] .rodata PROGBITS 08048568 000568 000018 00 A 0 0 4 [16] .eh_frame_hdr PROGBITS 08048580 000580 000024 00 A 0 0 4 [17] .eh_frame PROGBITS 080485a4 0005a4 00007c 00 A 0 0 4 [18] .ctors PROGBITS 08049620 000620 000008 00 WA 0 0 4 [19] .dtors PROGBITS 08049628 000628 000008 00 WA 0 0 4 [20] .jcr PROGBITS 08049630 000630 000004 00 WA 0 0 4 [21] .dynamic DYNAMIC 08049634 000634 0000c8 08 WA 6 0 4 [22] .got PROGBITS 080496fc 0006fc 000004 04 WA 0 0 4 [23] .got.plt PROGBITS 08049700 000700 00001c 04 WA 0 0 4 [24] .data PROGBITS 0804971c 00071c 000008 00 WA 0 0 4 [25] .bss NOBITS 08049740 000724 000048 00 WA 0 0 32 [26] .comment PROGBITS 00000000 000724 000058 01 MS 0 0 1 [27] .debug_aranges PROGBITS 00000000 00077c 000020 00 0 0 1 [28] .debug_pubnames PROGBITS 00000000 00079c 00002e 00 0 0 1 [29] .debug_info PROGBITS 00000000 0007ca 000357 00 0 0 1 [30] .debug_abbrev PROGBITS 00000000 000b21 00011b 00 0 0 1 [31] .debug_line PROGBITS 00000000 000c3c 0000c0 00 0 0 1 [32] .debug_frame PROGBITS 00000000 000cfc 000034 00 0 0 4 [33] .debug_str PROGBITS 00000000 000d30 00022f 01 MS 0 0 1 [34] .debug_pubtypes PROGBITS 00000000 000f5f 00006f 00 0 0 1 [35] .shstrtab STRTAB 00000000 000fce 000169 00 0 0 1 [36] .symtab SYMTAB 00000000 001728 0004d0 10 37 53 4 [37] .strtab STRTAB 00000000 001bf8 00022c 00 0 0 1 Key to Flags: W (write), A (alloc), X (execute), M (merge), S (strings) I (info), L (link order), G (group), x (unknown) O (extra OS processing required) o (OS specific), p (processor specific) (eye@mordor l10)$
The number of sections may be surprising to you, especially since the program we wrote was very small in terms of both code and data. And so you have another lesson in how the compiler interposes on your programming experience: it is generating quite a bit of additional information *not* present in your source file.
One of the reasons there are so many sections is because we compiled the program with the -g option, which produces "debugging" information like type information, line numbers, etc. (sections 27 to 34).
