I'm trying to reverse engineer some assembly, and I've gotten to this point:
40073f: 89 45 fc mov %eax,-0x4(%rbp)
400742: 83 7d fc 05 cmpl $0x5,-0x4(%rbp)
400746: 77 37 ja 40077f <f51+0x85>
400748: 8b 45 fc mov -0x4(%rbp),%eax
40074b: 48 8b 04 c5 28 09 40 mov 0x400928(,%rax,8),%rax
400752: 00
400753: ff e0 jmpq *%rax
400755: b8 11 00 00 00 mov $0x11,%eax
40075a: eb 28 jmp 400784 <f51+0x8a>
40075c: b8 12 00 00 00 mov $0x12,%eax
400761: eb 21 jmp 400784 <f51+0x8a>
400763: b8 13 00 00 00 mov $0x13,%eax
400768: eb 1a jmp 400784 <f51+0x8a>
40076a: b8 14 00 00 00 mov $0x14,%eax
40076f: eb 13 jmp 400784 <f51+0x8a>
400771: b8 15 00 00 00 mov $0x15,%eax
400776: eb 0c jmp 400784 <f51+0x8a>
400778: b8 16 00 00 00 mov $0x16,%eax
40077d: eb 05 jmp 400784 <f51+0x8a>
40077f: b8 0a 00 00 00 mov $0xa,%eax
400784: 5d pop %rbp
400785: c3 retq
I can see that what I'm looking at here is a switch statement where the default case is when -0x4(%rbp) > 5, but I'm confused about a few instructions:
Is 40074b just going to a spot on the jump table and pushing that instruction into rax so we can jump to the proper spot in the switch case after that (which is line 400753)?
In which case I have no idea how to interpret our different cases. If my understanding is correct the jump table starts at address 400928, going there I see:
400928: 55 push %rbp
400929: 07 (bad)
40092a: 40 00 00 add %al,(%rax)
40092d: 00 00 add %al,(%rax)
40092f: 00 5c 07 40 add %bl,0x40(%rdi,%rax,1)
400933: 00 00 add %al,(%rax)
400935: 00 00 add %al,(%rax)
400937: 00 63 07 add %ah,0x7(%rbx)
40093a: 40 00 00 add %al,(%rax)
40093d: 00 00 add %al,(%rax)
40093f: 00 6a 07 add %ch,0x7(%rdx)
400942: 40 00 00 add %al,(%rax)
400945: 00 00 add %al,(%rax)
400947: 00 71 07 add %dh,0x7(%rcx)
40094a: 40 00 00 add %al,(%rax)
40094d: 00 00 add %al,(%rax)
40094f: 00 78 07 add %bh,0x7(%rax)
400952: 40 00 00 add %al,(%rax)
400955: 00 00 add %al,(%rax)
At this point I have little idea of what I'm looking at. Presumably with line 400753 we jump to somewhere in this table, but then what? Or is my understanding completely off?
-D
or something to disassemble that non-code. Now you know why your tools didn't want to disassemble it for you :P – Peter Cordes-d
wouldn't disassemble them. – Peter Cordes