How to break on instruction with a specific opcode in GDB?

Question

I have the assembly code of some code that will be executed at a point in the program. I don\'t know the address of the code in memory.

Is it possible to make gdb br

Answer 1

No, this is not possible and it would also be very inefficient to implement.

Debugger's typically support two kinds of breakpoints:

• Hardware Breakpoints: The debugger asks the CPU to raise a special exception interrupt when some event occurs, like some location in memory is changed.
• Software Breakpoints: The debugger replaces the opcode at the breakpoint's address with a special "trap" instruction (int 3 / 0xcc on the x86 architecture).

Matching the current instruction's opcode would either require CPU support to insert a hardware breakpoint or the debugger needs to know the address to use a software breakpoint.

In theory, the debugger could just search the entire memory for the instruction's byte sequence, but since the byte sequence could also occur in the middle of an instruction or in data, it may get false positives.

Since assembly instructions are variable-length, control could jump to any arbitrary address or code could modify itself, it's also not trivial to disassemble an entire region of memory to find some particular instruction.

So basically, the only way of reliably finding the instruction in arbitrary assembly code would be by single-stepping on the instruction level. And this would be extremely expensive, even a trivial library call such as printf() could take minutes on today's hardware if you single-step every instruction.