How to deal with this "error" instructions generated by objdump?

Question

So basically I am using objdump to disassemble a binary from GNU Coreutils, on 32-bit Linux x86.

In the disassembled code, I found one "broken" instruction like this:

 804b4db:       ff 24 85 e4 09 05 08    jmp    *0x80509e4(,%eax,4)

It seems like a disassemble error?

And, by digging into the section info, I figure out that 0x80509e4 inside the .rodata section.

So does it mean that 0x80509e4 is a jump table?

@perror Basically I think it should be a typical jump table instruction, am I right? — lllllllllllll, May 8, 2014 at 2:36
So, Igor answered, but the instruction is not broken at all. It just follow a strange syntax. When left empty, the argument is assumed to be 1 in memory addressing. And, yes, this is most likely a jump table. — perror, May 9, 2014 at 7:48

Igor Skochinsky · Accepted Answer · 2014-05-08 10:00:26Z

7

This is just the ugly AT&T syntax. In Intel syntax it's:

jmp dword ptr [eax*4+0x80509e4]

And yes, it's most likely a jump table.

You can switch objdump to Intel syntax by adding -M intel to the command line.

answered May 8, 2014 at 10:00

Igor Skochinsky♦

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