Re: [PATCH] Fix problems with finishing a dummy function call on simulators.

Pedro Alves
2015-06-09T17:51:25Z

On 06/09/2015 04:00 PM, Luis Machado wrote:
> This is in line with what was done by Joel's patch here:
>
> https://sourceware.org/ml/gdb-patches/2014-11/msg00478.html
>
> And it also answers Pedro's question about whether this is specific to SPARC
> QEMU or not. This indeed seems to affect multiple QEMU targets and also other
> simulators (proprietary).

Sounds like a different issue, although related.

>
> I ran into this weird issue of not being able to "finish" an inferior function
> call. It looks as if the program is running away, but it really is stuck
> somewhere. "finish" still works fine for regular functions not called manually
> by GDB.

Sounds like that would fail on SPARC qemu as well.

>
> I tracked this failure down to GDB having both a bp_call_dummy and bp_finish in
> its breakpoint list. As a result of one not being considered permanent and the
> other considered permanent, GDB will not issue a Z packet to force the insertion
> of that location's breakpoint, confusing the simulator that does not know how
> to deal properly with these permanent breakpoints that GDB inserted beforehand.
>
> The attached patch fixes this, though i'm inclined to say we could probably
> check if both bp_call_dummy and bp_finish are present and force the
> insertion of that location's breakpoint. It isn't clear to me where exactly that
> check would go or if it would be cleaner than checking that information in
> the same function Joel used.
>
> I see no regressions on x86-64 and it fixes a bunch of failures for simulator
> targets we use (MIPS and PowerPC to name two).

If it happens that you "finish" from a normal function, and the finish
breakpoint ends up on top of a real permanent breakpoint, then this patch
will make us end up inserting a breakpoint on top of that permanent
breakpoint. I don't see what's special about finish breakpoints;
it's the address (dummy breakpoint location) that is special. It very much
sounds like that any kind of breakpoint that is placed on top of the dummy
breakpoint ends up with the same issue. E.g., if you stepi out of
the called function, with a software single-step breakpoint, sounds like
GDB will miss inserting the software step breakpoint because that's
at the same address as the dummy breakpoint.

As a data point, I assume that GDB is considering the non-permanent
dummy breakpoint a duplicate of the permanent finish breakpoint and
then none ends up inserted. Is that right?

Not exactly sure what to do here. Maybe we should stop considering
permanent and non-permanent breakpoints at the same address as
duplicates. That should result in GDB inserting the non-permanent
one, I think. Or we could get stop marking permanent breakpoints
as always inserted, and let normal breakpoints insert on top of
permanent breakpoints normally. See also:
https://sourceware.org/ml/gdb-patches/2015-03/msg00174.html

Thanks,
Pedro Alves