Question

im playing with assembly language and gdb trying to modify a memory address:

[phone]f84e8000000 je 0x00058181

id like to change the second byte that reads 84 to 85 so that the instruction becomes jne. Then i do the following in gdb after breaking on code:

set {char}0x00058094=85

but i get the following “andnps %xmm0,%xmm5” instead of jne:

(gdb)disas
0x00058093 <-[SWBConditionalImplementations checkRegistration:preferences:callbacks:]+67>:  andnps %xmm0,%xmm5

Thanks for any help!

Answer 1

Your problem is that you are passing a decimal value, not a hex one, you should use set {char}0x00058094=0x85 or set {char}0x00058094=133 to do what you wanted, alternatively set *((char*)0x00058094) = 0x85 would also work.

disassembly makes this a little clearer:

0F55E8                             ANDNPS XMM5,XMM0

vs.

0F85 E8000000                      JNZ 004010F1

85 is 0x55, which is why you get the SIMD instruction and no the JNE you wanted.

(I’m a bit disappointed that I didn’t notice that sooner…)