I am learning IDA pro on a test binary and I'd like to return a static value from this function:

; __int64 __fastcall sub_B580(char *name)
sub_B580 proc near

pai= qword ptr -70h
req= addrinfo ptr -68h
service= byte ptr -2Fh
var_20= qword ptr -20h

; __unwind {
push    rbp
push    rbx
sub     rsp, 68h
mov     rax, fs:28h
mov     [rsp+78h+var_20], rax
xor     eax, eax
test    rdi, rdi
jz      loc_B676


Returning a static value from any function is quite easy. I assume that your calling convention takes return value in the (e|r)ax register.

So to return some value, just patch your function to look like

mov rax, <value>

Lets consider an example.

#include <stdio.h>

int test(int n) {
    int i, sm = 0;
    for (i = 1; i <= n; i++)
        sm += (i * i);
    return sm;

int main(int argc, char **argv) {
    int n;
    scanf("%d", &n);
    printf("%d\n", test(n));
    return 0;

When run this looks like

$ ./test
$ ./test

When compiled this looks like

.text:0000000000000720                 public test
.text:0000000000000720 test            proc near               ; CODE XREF: main+2C↓p
.text:0000000000000720 var_14          = dword ptr -14h
.text:0000000000000720 var_8           = dword ptr -8
.text:0000000000000720 var_4           = dword ptr -4
.text:0000000000000720 ; __unwind {
.text:0000000000000720                 push    rbp
.text:0000000000000721                 mov     rbp, rsp
.text:0000000000000724                 mov     [rbp+var_14], edi
.text:0000000000000727                 mov     [rbp+var_8], 0
.text:000000000000072E                 mov     [rbp+var_4], 1
.text:0000000000000735                 jmp     short loc_745
.text:0000000000000737 ; ---------------------------------------------------------------------------
.text:0000000000000737 loc_737:                                ; CODE XREF: test+2B↓j
.text:0000000000000737                 mov     eax, [rbp+var_4]
.text:000000000000073A                 imul    eax, [rbp+var_4]
.text:000000000000073E                 add     [rbp+var_8], eax
.text:0000000000000741                 add     [rbp+var_4], 1
.text:0000000000000745 loc_745:                                ; CODE XREF: test+15↑j
.text:0000000000000745                 mov     eax, [rbp+var_4]
.text:0000000000000748                 cmp     eax, [rbp+var_14]
.text:000000000000074B                 jle     short loc_737
.text:000000000000074D                 mov     eax, [rbp+var_8]
.text:0000000000000750                 pop     rbp
.text:0000000000000751                 retn
.text:0000000000000751 ; } // starts at 720
.text:0000000000000751 test            endp

Click on the function start at 0x720. Go to Edit -> Patch Program -> Assemble.

In the window opened write

mov eax, 99

and press OK.

It will patch the current push rbp.

Finally write


press OK and then Cancel to stop patching.

Go to Edit -> Patch Program -> Apply patches to input file

Save binary with the updated changes. Now it runs like

$ ./test
$ ./test
$ ./test
| improve this answer | |
  • Thank you so much. I had another question: I have a if condition that if met will call exit(0). Is there a way i can change exit(0) to continue; ? – bestupload Oct 19 '19 at 1:38
  • 1
    Please post it as other question. – sudhackar Oct 19 '19 at 2:38

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