Can GDB change the assembly code of a running program?

Question

I want to add some extra funcionality to /bin/ls.
So I started it on gdb and added a breakpoint at the beginning.

Now question is: how can I change the code of a running program in memory? I can see the assembly code, but I'm not able to modify. How can I do it?

On Windows I can easily do this with olldbg for example. How about on Linux?

(I know that doing this I will only change the code of the process in memory. So then I can dump memory to a file, and then I'll have my changes saved in a binary file).

Thank you.

Answer 1

compile code command

Introduced around 7.9, it allows code compilation and injection. Documentation: https://sourceware.org/gdb/onlinedocs/gdb/Compiling-and-Injecting-Code.html

I have given a minimal example in this answer.

While it is not actual code modification, it does allow you to compile some code on the fly and run it once immediately, which might be enough.

And the GNU cauldron presentation suggests that actual code modification may be added later on as an extension to this feature, see slide 30 "Fix and continue".

There are a few constructs that did not work as I expected like return, so I've asked why at: In the GDB compile code command, what language constructs behave exactly as if they were present in the original source?