How can I compile c code into a firmware patch and get the location of all the functions?

Question

I'm compiling some firmware patches. These are normally written in C then patched into a firmware image to change the behavior of a hardware device. The trick is that I will replace a function call in the original firmware to dispatch to my patch, do my trickery then call the original function and finally return.

Here is a simple example:

__attribute__ ((section (".patches")))

void clean_memory() __attribute__ ((section (".patches")));

void clean_memory() {
  int *i;
  for (i=(int *)0x80000;i<(int *)0x84000;i++)
    *(i)=0;
  asm("jsr      0x12345\n\t"); // call the original function
}

I'm compiling it with the following:

m68k-rtems-gcc -mcpu32 -Wl,--section-start=.patches=0x31000 -O2 -c patches.c
m68k-rtems-ld -nostdlib --oformat=binary -M patches.o -o patches.bin

I would really like to automate this process rather than hand-patching the file every time I make a modification to the patch.

How can I get a list of the offsets and lengths where each function in the patches.bin file exist? This is important when I patch the original function call since the offsets will change as the size of every function in the patch changes

Answer 1

This information should be contained in the mapfile which you are already generating with the -M option. This sends it to a default mapfile. You can specify your own with something like -Map patches.map.

Generating a cross reference table in the mapfile with -cref may also contain some useful information.