How do I disassemble raw MIPS code?

Question

Similarly to How do I disassemble raw x86 code?, but then for the MIPS architecture: how do I disassemble raw MIPS code with objdump? I want to check the instructions in a vmlinux image, but to do so I now have to:

: > x.c
mipsel-linux-gnu-gcc -c -o x.o x.c
mipsel-linux-gnu-objcopy --add-section raw=vmlinux x.o
mipsel-linux-gnu-objcopy --remove-section .comment x.o
mipsel-linux-gnu-objdump -D x.o | less

Is there an easier way to do it? I've tried the below to no avail:

mipsel-linux-gnu-objdump -b elf32-tradlittlemips -mmips -Mgpr-names=O32,cp0-names=mips1,cp0-names=mips1,hwr-names=mips1,reg-names=mips1 -D vmlinux | less

It just spits out:

mipsel-linux-gnu-objdump: vmlinux: File format not recognized

If it helps, here is the output of some commands:

$ file x.o
x.o: ELF 32-bit LSB relocatable, MIPS, MIPS-I version 1 (SYSV), with unknown capability 0xf41 = 0x756e6700, with unknown capability 0x70100 = 0x1040000, not stripped
$ mipsel-linux-gnu-objdump -p x.o

x.o:     file format elf32-tradlittlemips
private flags = 1006: [abi=O32] [mips1] [not 32bitmode] [PIC] [CPIC]

The target is an AR7 CPU.

Answer 1

Hmm, it seems easier than that. -b elf32-tradlittlemips does not work because the file is not an ELF executable, but binary. So, the correct option to be used is -b binary. The other option, -mmips makes objdump recognize the file as binary for MIPS. Since the target machine is little endian, I also had to add -EL to make the output match the output for x.o.

-mmips only includes the basic instruction set. The AR7 has a MIPS32 processor which has more instructions than just mips. To decode these newer MIPS32 instructions, use -mmips:isa32. A list of available ISAs can be listed with objdump -i -m.

The final command becomes:

mipsel-linux-gnu-objdump -b binary -mmips:isa32 -EL -D vmlinux

This would show registers like $3 instead of their names. To adjust that, I used the next additional options which are mentioned in mipsel-linux-gnu-objdump --help:

-Mgpr-names=32,cp0-names=mips32,cp0-names=mips32,hwr-names=mips32,reg-names=mips32

I chose for mips32 after reading:

• http://www.linux-mips.org/wiki/AR7
• http://www.linux-mips.org/wiki/Instruction_Set_Architecture