Re: [Precord RFA/RFC] Check Linux sys_brk release memory in process record and replay.

Hui Zhu
2009-06-14T09:26:09Z

On Sun, Jun 14, 2009 at 06:56, Michael Snyder<msnyder@vmware.com> wrote:
> Hui Zhu wrote:
>>
>> Ping.
>
> OK, my bad for taking so long to get to this... please allow me
> to summarize the problem, to check my own understanding
> (tell me if I'm wrong).
>

For that "nice people" words. I just want to make a joke. :)

> Currently linux-record.c does not know how to "undo" a sys_brk
> system call.  You (teawater) are concerned because if the child
> process calls sys_brk to free some memory, we cannot un-free it
> and therefore we may get into trouble by writing to the freed
> memory during replay.  Something like this:
>
>  1) child allocates memory X
>  2) child writes to memory X
>  3) child frees memory X
>  4) user asks for reverse-continue
>  5) gdb tries to revert the write that happened in step #2,
>     gets SIGSEGV because location has been freed.
>
> So far so good?
>
> Now, your proposal is that during the record mode, we will
> detect any sys_brk call that frees memory, and query the
> user whether to continue or give up.
>
> I'm not too crazy about that solution.  I think it's
> awkward, and drastic for a situation that may only be
> a problem later on (or not at all).  Let me throw out
> some other ideas:
>
> A) Is it possible to actually "reverse" a sys_brk call?
> Suppose we record the arguments, and when we want to reverse
> it, we just change an increase into a decrease and vice versa?
>
> B) Suppose we wait until an actual memory error occurs
> during replay, and THEN inform the user?  It will avoid
> warning him about something that may never happen.
>
> We could use catch_errors to trap the SIGSEGV, and then
> check to see if the error was caused by a write to memory
> above the BRK boundary.  You will still need to keep track
> of the BRK boundary, but you won't have that awkward early
> query to deal with.

The sys_brk just can increase and decrease data segment size. The
decrease behavior is very hard to replay.
I read some code of malloc and free in glibc. I found that most of
time, free will not call brk to release memory to system. Because it
is low efficiency.
So I think when brk release really happen, give user a query is a easy
way to handle it.

What do you think about it?

thanks,
Hui