Possible to trap write to address (x86 - linux)

Question

I want to be able to detect when a write to memory address occurs -- for example by setting a callback attached to an interrupt. Does anyone know how?

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Answer 1

mprotect does have a disadvantage: your memory must be page-boundary aligned. I had my problematic memory on the stack and was not able to use mprotect().

As Adam said, what you want is to manipulate the debug registers. On windows, I used this: http://www.morearty.com/code/breakpoint/ and it worked great. I also ported it to Mach-O (Mac OS X), and it worked great, too. It was also easy, because Mach-O has thread_set_state(), which is equivalent to SetThreadContext().

The Problem with linux is that it doesn't have such equivalents. I found ptrace, but I thought, this can't be it, there must be something simpler. But there isn't. Yet. I think they are working on a hw_breakpoint API for both kernel and user space. (see http://lwn.net/Articles/317153/)

But when I found this: http://blogs.oracle.com/nike/entry/memory_debugger_for_linux I gave it a try and it wasn't that bad. The ptrace method works by some "outside process" acting as a "debugger", attaching to your program, injecting new values for the debug registers, and terminating with your program continuing with a new hw breakpoint set. The thing is, you can create this "outside process" yourself by using fork(), (I had no success with a pthread), and doing these simple steps inline in your code.

The addwatchpoint code must be adapted to work with 64 bit linux, but that's just changing USER_DR7 etc. to offsetof(struct user, u_debugreg[7]). Another thing is that after a PTRACE_ATTACH, you have to wait for the debuggee to actually stop. But instead of retrying a POKEUSER in a busy loop, the correct thing to do would be a waitpid() on your pid.

The only catch with the ptrace method is that your program can have only one "debugger" attached at a time. So a ptrace attach will fail if your program is already running under gdb control. But just like the example code does, you can register a signal handler for SIGTRAP, run without gdb, and when you catch the signal, enter a busy loop waiting for gdb to attach. From there you can see who tried to write your memory.