objdump and ARM vs Thumb

10
votes

I'm trying to disassemble an object built for ARM with gcc. Unfortunately, objdump is trying to guess whether the code is ARM and Thumb, and is getting it wrong: it thinks my code is Thumb when it's actually ARM.

I see that objdump has an option to force it to interpret all instructions as Thumb (-Mforce-thumb), but it doesn't have one to force ARM mode!

This seems like a really weird omission to me, and it's seriously hampering my ability to get work done (I'm on an embedded device and my only means of debugging is to look at the disassembly). I've tried various approaches, including trying to tell objdump to use an ARM architecture that doesn't support Thumb, but nothing seems to work. Any ideas?

(And yes, I know that the instructions really are ARM...)

2
assemblyarmobjdumpbinutilsthumb
are you stripping the binary (elf) files, maybe the info was there and then removed. Or maybe you have to add a switch to include the debug info into the binary. – old_timer
It's objdump ... -Mno-force-thumb today. Can't say if this was present back in 2011. Essentially it's either ARM or Thumb, so when you say no-force-thumb the instruction set is ARM. – JSmyth

2 Answers

5
votes

arm-linux-gnueabi-objdump -marm -b binary -D does the trick for me. It doesn't preserve the symbol information though, so it's not quite what you are looking for, but it gives you the disassemble, which is a start.

-1
votes

This sounds like a bug in either compiler/linker or objdump. Normally the symbols should be marked properly if they're Thumb or ARM and disassembled accordingly.

That said, there's a few things you can try:

  1. interpret file as plain binary (-b binary -D) - this will disable symbol checking and will disassemble whole file as ARM. Downside: lots of garbage in the output, no nice addresses.
  2. use IDA Pro. Even if it guesses incorrectly, you can always override its decision. Also, it's a much nicer disassembly environment :)