A Real Object File Format

Transcription

1 A Real Object File Format Computer Science and Engineering College of Engineering The Ohio State University Lecture 28

2 Linking and Loading in Practice Real object files have multiple segments Text: read-only (instructions) Data: read/write (storage) Why distinguish between R and RW segments? Multiple processes executing same program Load one copy of read-only segment(s), and share it Each process gets its own copy of read/write segments When linking multiple object files: Group R (RW) segments together

3 Linking with Text/Data Segm ts text-2 data-2 text-1 data-1 text-3 data-2 text-1 text-2 text-3 data-1 data-2 data-2

4 Example: Instructions/Data Prog.ORIG.EXT Funny Start LD r1,count LEA r0,msg Loop TRAP x22 ADD r1,r1,#-1 BRP Loop JMP Funny ;etc A 0001 E 0 9 B 0002 F F Count.FILL #4 Msg.STRZ "hello" Array.BLKW #30.END Start 009A B C D C 009E C

5 Example: Instructions/Data Prog.TEXT.EXT Funny Start LD r1,count LEA r0,msg Loop TRAP x22 ADD r1,r1,#-1 BRP Loop JMP Funny ;etc....data Count.FILL #4 Msg.STRZ "hello" Array.BLKW #30 text segment data segment

6 Example: Instructions/Data Prog.TEXT.EXT Funny Start LD r1,count LEA r0,msg Loop TRAP x22 ADD r1,r1,#-1 BRP Loop JMP Funny ;etc....data Count.FILL #4 Msg.STRZ "hello" Array.BLKW # E F F C C

7 Object File Format: UNIX a.out header text data text reloc n data reloc n symbol table string table text data bss heap stack

8 Example: Object File 00000F3F E C E001F F C6F6F F756E74004D e6e7900

9 Example: Object File 00000F3F E C E001 F022127F C6F 6F F756E74 004D e6e79 00

10 Giving Structure to a Binary File Binary file contains unstructured data, eg: Equivalently (as hex: 4 bits digit): 00000F3F E E E001F02212 Equivalently (as ASCII chars: 8 bits char):?. ^Hh"à ð" How do we distinguish between text, modification, entry, etc records? Ans:

11 UNIX a.out: Header Structure All object files begin the same way: int a_magic; //magic number int a_text; //text seg size int a_data; //data seg size int a_bss; //uninit data size int a_syms; //symbol table size int a_entry; //entry point int a_trsize; //text reloc n size int a_drsize; //data reloc n size Here an int is 4 bytes, so header record is the first 4 * 8 = 32 bytes

12 Giving Structure to our Example Header = First 32 bytes 00000F3F E C E001F Adding spaces to distinguish 8 ints 00000F3F E E E001 F022127F Interpretation: 00000F3F magic number (#3903, object file type) E text segment size (#302 bytes) initialized data size (#20 bytes) E uninitialized data size (#30 bytes) symbol table size (#72 bytes) entry point (address x0) text relocation size (#80 bytes) data relocation size (#0 bytes) 2200E001F022127F

13 UNIX a.out: Text/Data Records Similar to our text records, except: No "T" flag No addresses From the header in our example: #302 bytes of stuff to load into memory as instructions (read-only) 2200E001F022127F #20 bytes (ie 10 words) of stuff to load into memory as data (read/write) #30 bytes (ie 15 words) of memory needs to be reserved for uninitialized data (read/write)

14 UNIX a.out: Relocation Record Handles both relocation and external symbols One entry (8 bytes) for each location to be patched 4 bytes: address 3 bytes: index 1 byte: flags Address: location (offset within segment) to patch Flags: 2 bits for length of patch item (1, 2, 4, or 8 bytes) 1 bit ( extern ) to indicate reason for patching On: External symbol (ie "X" record) Off: Relocation (ie "M" record) Index: Meaning depends on extern flag On: index is symbol number from symbol table Off: index indicates segment (text, data, bss)

15 Example: Instructions/Data Prog.TEXT.EXT Funny Start LD r1,count LEA r0,msg Loop TRAP x22 ADD r1,r1,#-1 BRP Loop JMP Funny.END Start.DATA Count.FILL #4 Msg.STRZ "hello" Array.BLKW # E F F C C Data-relative Data-relative Text-relative Funny

16 UNIX a.out: Symbol Table Each entry (12 bytes) describes 1 symbol 4 bytes: name offset 4 bytes: type / spare / debug info (2) 4 bytes: value Name offset: pointer into string table Allows arbitrarily long symbol names (null terminate) Type byte: low bit is external (global) flag Text/data/bss: relative symbol (to that segment) Abs: absolute value (may or may not be external) Undefined: external bit must be on

17 Symbol Table and String Table Symbol table: Each entry is 12 bytes 04, text, 00 (Start = 0, relative to text) 0A, text, 02 (Loop = 2, relative to text) 0F, data, 00 (Count = 0, relative to data) 15, data, 01 (Msg = 1, relative to data) 19, bss, 00 (Array = 0, relative to bss) 1F, extn (Funny is external) String table: One long string, with nulls First four bytes: String table length = 37 (ie ) Remainder: Null-terminated symbol names C6F6F Result %StartLoopCountMsgArrayFunny

18 Evolution of Object File Formats Magic numbers are often used to distinguish file types PDF (ie %PDF ) GIF ( GIF89a ) PNG E 47 0D 0A 1A 0A (nonascii + PNG + newlines to detect conversion problems + EOF to halt output on MSDOS) A.out (PDP-11 machine instruction for unconditional branch past the header information) Variations zmagic, qmagic, now obsolete ELF: Executable and linking format Magic number 7f 45 4c 46 (ie ".ELF") Better support for shared libraries Position independent libraries

19 Summary Distinguish R and RW segments Header structures binary object file Size information for constituent parts Combine relocation and external records Symbol table Name: offset into string table Value, other information String table: One long concatenation of all symbol names