For experimentation, while waiting for vendor to fix bug, wanted to try and eliminate a crash that occurs on occasion. Up till now I've only done patching replacing existing code, not trying to insert additional code.
.text:000000018000A260 cmp [rax], r12d <- RAX=0, crashes program .text:000000018000A263 jz short loc_18000A271 .text:000000018000A265 cmp dword ptr [rax], 6 .text:000000018000A268 jnz short loc_18000A276 .text:000000018000A26A cmp ecx, 40h
And further on:
.text:000000018000A21B mov rcx, [rsp+388h+var_350] .text:000000018000A220 call cs:WindowsDeleteString .text:000000018000A226 mov [rsp+388h+var_350], r15 .text:000000018000A22B mov rbx, [rsp+388h+var_348]
I want to insert some new instruction, changing to jmp to patched code
.text:000000018000A260 jmp <patched code>
Patched Code Idea - I can't find much suitable place to insert code - so was thinking of shortening some non-essential strings in .rdata section to insert this code - Is there any issue with this? Essentially what I am trying to do is if RAX = 0 , skip over the use of [rax]
cmp rax,0 jz .text:000000018000A21B ; The code point past using [rax] cmp [rax], r12d jmp .text:000000018000A263 ; Continue program execution normally
Now it seems IDA "Assemble" doesn't always work, for example cmp rax,0 it says "Invalid Operand" So I had to patch bytes instead
48 83 F8 00 = cmp rax,0
Is there a way to get the "assemble" in IDA to reference my jump locations, using the location references in IDA. Or is there a suggested method to calculate how to build my jmp/jz instructions.