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| -rw-r--r-- | elf.ml | 195 |
1 files changed, 195 insertions, 0 deletions
diff --git a/elf.ml b/elf.ml new file mode 100644 index 0000000..0c82a73 --- /dev/null +++ b/elf.ml @@ -0,0 +1,195 @@ +(* This is a module to spit simple ELF + object files that can afterwards be + linked to build an application. +*) + +let hash s = + (* The ELF hash function. *) + let open Int64 in (* I doubt this is necessary... *) + let rec f p h = + if p = String.length s then to_int h else + let h = shift_left h 4 in + let h = add h (of_int (int_of_char s.[p])) in + let g = logand h (of_int 0xf0000000) in + let h = logxor h (shift_right g 24) in + f (p+1) (logand h (of_int 0x0fffffff)) in + f 0 (of_int 0) + +let le n x = + (* Make a string of bytes in little endian convention. *) + let b = Bytes.create n in + let rec f i x = + if i = n then () else + let d = char_of_int (x land 0xff) in + Bytes.set b i d; + f (i+1) (x lsr 8) in + f 0 x; Bytes.to_string b + +let stt_NOTYPE = 0 +let stt_OBJECT = 1 +let stt_FUNC = 2 + +let stb_LOCAL = 0 +let stb_GLOBAL = 16 +let stb_WEAK = 32 + +let sht_NULL = le 4 0 +let sht_PROGBITS = le 4 1 +let sht_SYMTAB = le 4 2 +let sht_STRTAB = le 4 3 +let sht_RELA = le 4 4 +let sht_NOTE = le 4 7 +let sht_NOBITS = le 4 8 + +let shf_WRITE = 1 +let shf_ALLOC = 2 +let shf_EXECINSTR = 4 + +let barebones_elf oc text = + let header = String.concat "" + [ "\x7fELF" (* e_ident, magic *) + ; "\x02" (* e_ident, ELFCLASS64 *) + ; "\x01" (* e_ident, ELFDATA2LSB *) + ; "\x01" (* e_indent, EV_CURRENT *) + ; "\x00" (* e_ident, ELFOSABI_SYSV *) + ; "\x00" (* e_ident, ABI version *) + ; "\x00\x00\x00\x00\x00\x00\x00" (* e_ident, padding *) + ; "\x01\x00" (* e_type, ET_REL *) + ; "\x3e\x00" (* e_machine, EM_X86_64 *) + ; "\x01\x00\x00\x00" (* e_version, EV_CURRENT *) + ; "\x00\x00\x00\x00\x00\x00\x00\x00" (* e_entry, unused *) + ; "\x00\x00\x00\x00\x00\x00\x00\x00" (* e_phoff, unused *) + ; "\x40\x00\x00\x00\x00\x00\x00\x00" (* e_shoff, 64 *) + ; "\x00\x00\x00\x00" (* e_flags, 0 *) + ; "\x40\x00" (* e_hsize, 64 *) + ; "\x00\x00" (* e_phentsize, 0 *) + ; "\x00\x00" (* e_phnum, 0 *) + ; "\x40\x00" (* e_shentsize, 64 *) + ; "\x07\x00" (* e_shnum, 7 *) + ; "\x06\x00" (* e_shstrndx, 6 *) + ] in + + (* We will use the following section organization. + 1- .text PROGBITS + 2- .data PROGBITS + 3- .bss NOBITS + 4- .rela RELA + 5- .symtab SYMTAB + 6- .strtab STRTAB + *) + + let adds s x = (String.length s, s ^ x ^ "\x00") in + (* section names *) + let textstr, strtab = adds "\x00" ".text" in + let datastr, strtab = adds strtab ".data" in + let bssstr , strtab = adds strtab ".bss" in + let relastr, strtab = adds strtab ".rela" in + let symtstr, strtab = adds strtab ".symt" in + let strtstr, strtab = adds strtab ".strt" in + (* function names *) + let mainstr, strtab = adds strtab "main" in + + let symtab = String.concat "" + [ le 0x18 0 (* first symbol is reserved *) + ; le 4 mainstr (* st_name *) + ; le 1 (stt_FUNC lor stb_GLOBAL) (* st_info *) + ; "\x00" (* st_other *) + ; le 2 1 (* st_shndx, .text *) + ; le 8 0 (* st_value, offset in .text section *) + ; le 8 (String.length text) (* st_size *) + ] in + + let textoff = 64 + 7 * 64 in + let dataoff = textoff + String.length text in + let bssoff = dataoff + 0 in + let relaoff = bssoff + 0 in + let symtoff = relaoff + 0 in + let strtoff = symtoff + String.length symtab in + + let sh = String.concat "" + [ le 64 0 (* first section header is reserved *) + (* .text *) + ; le 4 textstr (* sh_name *) + ; sht_PROGBITS (* sh_type *) + ; le 8 (shf_ALLOC lor shf_EXECINSTR) (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 textoff (* sh_offset *) + ; le 8 (String.length text) (* sh_size *) + ; le 4 0 (* sh_link *) + ; le 4 0 (* sh_info *) + ; le 8 1 (* sh_addralign *) + ; le 8 0 (* sh_entsize *) + (* .data *) + ; le 4 datastr (* sh_name *) + ; sht_PROGBITS (* sh_type *) + ; le 8 (shf_ALLOC lor shf_WRITE) (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 dataoff (* sh_offset *) + ; le 8 0 (* sh_size *) + ; le 4 0 (* sh_link *) + ; le 4 0 (* sh_info *) + ; le 8 1 (* sh_addralign *) + ; le 8 0 (* sh_entsize *) + (* .bss *) + ; le 4 bssstr (* sh_name *) + ; sht_NOBITS (* sh_type *) + ; le 8 (shf_ALLOC lor shf_WRITE) (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 bssoff (* sh_offset *) + ; le 8 0 (* sh_size *) + ; le 4 0 (* sh_link *) + ; le 4 0 (* sh_info *) + ; le 8 1 (* sh_addralign *) + ; le 8 0 (* sh_entsize *) + (* .rela *) + ; le 4 relastr (* sh_name *) + ; sht_RELA (* sh_type *) + ; le 8 0 (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 relaoff (* sh_offset *) + ; le 8 0 (* sh_size *) + ; le 4 5 (* sh_link, symtab index *) + ; le 4 1 (* sh_info, text section *) + ; le 8 8 (* sh_addralign *) + ; le 8 0x18 (* sh_entsize *) + (* .symtab *) + ; le 4 symtstr (* sh_name *) + ; sht_SYMTAB (* sh_type *) + ; le 8 0 (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 symtoff (* sh_offset *) + ; le 8 (String.length symtab) (* sh_size *) + ; le 4 6 (* sh_link, strtab index *) + ; le 4 1 (* sh_info, first non-local symbol *) + ; le 8 8 (* sh_addralign *) + ; le 8 0x18 (* sh_entsize *) + (* .strtab *) + ; le 4 strtstr (* sh_name *) + ; sht_STRTAB (* sh_type *) + ; le 8 0 (* sh_flags *) + ; le 8 0 (* sh_addr *) + ; le 8 strtoff (* sh_offset *) + ; le 8 (String.length strtab) (* sh_size *) + ; le 4 0 (* sh_link *) + ; le 4 0 (* sh_info *) + ; le 8 1 (* sh_addralign *) + ; le 8 0x18 (* sh_entsize *) + ] in + + List.iter (output_string oc) + [ header + ; sh + ; text + ; symtab + ; strtab + ] + + +let _ = + let oc = open_out "test.o" in + let text = String.concat "" + [ "\xb8\x2a\x00\x00\x00" (* mov 42, %eax *) + ; "\xc3" (* retq *) + ; "\xc3\xc3" (* padding *) + ] in + barebones_elf oc text |