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GPL Disassembling

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Meanwhile I wrote a perl script, which does all the jobs. Also it lookup references and add this to the disassemble output.

Disassembling with GNU/GPL toolsEdit

The gnu/gpl tools are not made for analysing alien binary dumps because we usually have the source code if we need to debug. This is not really an replacement for IDA but for me it's was sufficient.

Installing software is not explained in this tutorials.

Prerequisites:

  • You have a raw binary firmware dump to look at. I'll use here "dump.bin"

In this toybox we have:

arm-elf-objcopy | arm-linux-gnu-objcopy
arm-elf-objdump | arm-linux-gnu-objdump

Here we go:

strings -t x dump.bin > dump.strings
hexdump -C dump.bin > dump.hex

arm-linux-gnu-objdump -m arm -b binary -D dump.bin > dump.dis

However, theres a problem: all files start with an offset of 0x00. Here comes my renumber.pl script:

strings -t x dump.bin | ./renumber.pl 0xff810000 > dump.strings
hexdump -C dump.bin |./renumber.pl 0xff810000 > dump.hex

(Not sure if the objdump line above is a typo, but on Ubuntu, the equivalent command is probably arm-elf-objdump rather than arm-linux-gnu-objdump Andyhull 02:55, May 1, 2012 (UTC))

Before we disassemble the dump, we pack it into elf format. This meat is good for feeding gdb and the IDA demo version ;)

arm-linux-gnu-objcopy --change-addresses=0xff810000 -I binary -O elf32-littlearm -B arm dump.bin dump.elf
arm-linux-gnu-objcopy --set-section-flags .data=code dump.elf

Verify the elf file:

arm-linux-gnu-objdump -x dump.elf

Disassemble:

arm-linux-gnu-objdump -d dump.elf > dump.dis

So finally we have 3 ascii files to stare at:

  • dump.dis
  • dump.strings
  • dump.hex

and

  • dump.elf for gdb and qemu

Putting all togetherEdit

Meanwhile I wrote GPL:disassemble.pl perl script, which does all the jobs. Also it lookup references and add this to the disassemble output.

disassemble.pl 0xff810000 dump.bin

e.g. output:

NSTUB(Capture.Create, 0xff938368):
ff938368: 	e92d4010 	stmdb	sp!, {r4, lr}
ff93836c: 	e59f0020 	ldr	r0, [pc, #32]	; ff938394: (ffac13cc) 
ff938370: 	ebfcc3fd 	bl	ff86936c <_binary_dump_bin_start+0x5936c -847876>
ff938374: 	eb01cf03 	bl	ff9abf88 <_binary_dump_bin_start+0x19bf88 +474132>
ff938378: 	e3a00000 	mov	r0, #0	; 0x0
ff93837c: 	e8bd8010 	ldmia	sp!, {r4, pc}
// this is obviously an entry point, because      ^^ is a "return"
ff938380: 	e24f1020 	sub	r1, pc, #32	; ff938368: (e92d4010) 
ff938384: 	e28f000c 	add	r0, pc, #12	; ff938398: (74706143)  *"Capture.Create"
ff938388: 	eafcc355 	b	ff8690e4 <_binary_dump_bin_start+0x590e4 -848548>
// another
ff93838c: 	e28f0004 	add	r0, pc, #4	; ff938398: (74706143)  *"Capture.Create"
ff938390: 	eafcc355 	b	ff8690ec <_binary_dump_bin_start+0x590ec -848548>
// this is data, referenced from 0xff93836c followed by some text
ff938394: 	ffac13cc 	undefined instruction 0xffac13cc
"Capture.Create":
ff938398: 	74706143 	ldrvcbt	r6, [r0], #-323
ff93839c: 	2e657275 	mcrcs	2, 3, r7, cr5, cr5, {3}
ff9383a0: 	61657243 	cmnvs	r5, r3, asr #4
ff9383a4: 	00006574 	andeq	r6, r0, r4, ror r5

Note: The entire disassembled file is shown as instructions, including strings and numeric constants. Strings are identified where referenced, as shown above, but the corresponding address still has disassembled (nonsense) instructions. If the instructions you are looking at don't make any sense, they are probably data.

using gcc/gasEdit

Another way to create an elf file with symbols from chdk's stub files: forum However, the disassemble script makes a better format but this one is very good for gdb+qemu ;)

Next lesson: run the dump in Qemu

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