Re: [reverse/record] adjust_pc_after_break in reverse execution mode?

Pedro Alves
2008-10-24T01:51:01Z

On Friday 24 October 2008 01:37:31, Michael Snyder wrote:
> > In sum, it appears that decr_pc_after_break doesn't matter when you have
> > continguous breakpoints, as long as you get from from B1's address to B2's
> > address by single-stepping.  All is good then, it appears!
>
> I agree, at least that is the conclusion I am leaning toward.
>

Not so fast! I knew I had to spend a little extra thinking about
it, 'cause I knew something was broken, just couldn't find what. :-)
*as long as you get from from B1's address to B2's address
by single-stepping* was a restriction that doesn't always apply.

Here's a test that will fail in forward record/replay mode, but not
in normal "play" mode.

volatile int global_foo = 0;

int
main (int argc, char **argv)
{
asm ("nop"); /* 1st insn */
asm ("nop"); /* 2nd insn */
asm ("nop"); /* 3rd insn */
asm ("nop"); /* 4th insn */
if (!global_foo)
goto ahead;
asm ("nop"); /* 5th insn */
asm ("nop"); /* 6th insn */
asm ("nop"); /* 7th insn */
asm ("nop"); /* 8th insn */ <<< break 1 here
ahead:
asm ("nop"); /* 9th insn */ <<< break 2 here
end:
return 0;
}

If you let the program reply until break 2 is hit, and assuming insn
8th and 9th are assembled as contiguous (they do on x86 -O0 for me), you'll
see that adjust_pc_after_break will indeed make it appear that breakpoint
1 was hit. Now, nops are nops, but real code could have something
else there...

/me goes back to bed.

--
Pedro Alves