Re: PATCH: Re: BUG: PTRACE_POKETEXT modifies memory in related processes

[Daniel Jacobowitz <dan@debian.org>]

On Fri, Feb 01, 2002 at 08:32:21PM -0800, Nathan Field wrote:
> > > I believe I have found a bug in recent 2.4 linux kernels (at least 2.4.17)
> > > running on x86 related to using ptrace to modify a processes memory. From
> [snip incorrect guess at cause of problem]
> >
> > I don't think you're correctly understanding the circumstances.  Are
> > you setting the breakpoint after they fork (seems unlikely, given this
> > test case - not much time to do so)?  Otherwise, the breakpoint is
> > simply being carried over by your forking.  Why you see it now and did
> > not before I don't know.
> 	Actually I do understand the circumstances. I am setting the
> breakpoint _AFTER_ the fork, when the parent and the child should exist in
> different memory spaces.
> 
> > Basically, GDB on Linux does not support fork very well.  It doesn't
> > show up terribly often, because exec() clears breakpoints.
> 	You are right on that, GDB sucks when debugging forks, which is
> why I have written my own debugger that can correctly handle forks. This
> is why I've come across this problem. Basically I detect a fork, do some
> magic, catch the child process before it continues to run and attach to
> it. Then I remove all the breakpoints from the child that were in the
> parent. The problem is that in 2.4.17 when I remove the breakpoints from
> the child they are also removed from the parent. I do know what I'm
> talking about here, I'm not trying to remove the breakpoints before the
> fork system call, it is after the fork system call (I actually remove the
> breakpoints after the fork library call has returned), and the bug is
> related to COW because I figured out exactly what's wrong and here's a
> patch that fixes it:

Sorry then.  You didn't give any of this context in the original
message.  Had you been using GDB - a reasonable assumption - my
explanation would have been accurate.

I've got the design mostly worked out to make GDB handle fork() better. 
I just need to settle on a few details and find some time to finish it.

> --- ptrace.c.orig	Fri Feb  1 20:17:18 2002
> +++ ptrace.c	Fri Feb  1 19:54:58 2002
> @@ -148,16 +148,16 @@
>  		int bytes, ret, offset;
>  		void *maddr;
> 
> -		ret = get_user_pages(current, mm, addr, 1,
> -				write, 1, &page, &vma);
> -		if (ret <= 0)
> -			break;
> -
>  		bytes = len;
>  		offset = addr & (PAGE_SIZE-1);
>  		if (bytes > PAGE_SIZE-offset)
>  			bytes = PAGE_SIZE-offset;
> 
> +		ret = get_user_pages(current, mm, addr, 1,
> +				write, 1, &page, &vma);
> +		if (ret <= 0)
> +			break;
> +
>  		flush_cache_page(vma, addr);
> 
>  		maddr = kmap(page);

Why is this first half even necessary?  I don't see that it makes any
difference.  Maybe I'm missing something.

> @@ -173,6 +173,7 @@
>  		put_page(page);
>  		len -= bytes;
>  		buf += bytes;
> +		addr += bytes;
>  	}
>  	up_read(&mm->mmap_sem);
>  	mmput(mm);
> 
> 
> Basically the kernel was calling get_user_pages on the same address, even
> as it moved through different addresses in memory. I have tested this
> change on my own 2.4.17 kernel and it now works correctly.

That'll do it all right.  Might want to forward this patch (at least
the latter bit) to Linus and Marcello to make sure they see it.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer