Question

Is there any way to programmatically mock a function for a embedded c application, running on linux. In below example I want to mock main to call someBlah instead of someFunc in run-time.

#include <stdio.h>

void someFunc( void )
{
    printf("%s():%d\n",__func__,__LINE__);
}

void someBlah( void )
{
    printf("%s():%d\n",__func__,__LINE__);
}

int main(void)
{
    someFunc();
}

The program will be executing from ram in Linux so text segment should be modifiable. I know GDB works on some similar concept where breakpoints code locations are replaced by trap instructions.

Answer 1

You could always make some part of the text segment modifiable by an appropriate call to mprotect and overwrite some code with your own (e.g. by generating machine code with libjit, GNU lightning, … or manually).

But using function pointers is a cleaner way of doing that.

If the functions are inside a shared library, you could even overwrite its Procedure Linkage Table (see also the ABI spec, which depends upon the architecture – here is one for ARM)