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Re: [PATCH 1/2] This patch fixes GDBServer's run control for single stepping
- From: Yao Qi <qiyaoltc at gmail dot com>
- To: Antoine Tremblay <antoine dot tremblay at ericsson dot com>
- Cc: Pedro Alves <palves at redhat dot com>, "gdb-patches\@sourceware.org" <gdb-patches at sourceware dot org>
- Date: Fri, 31 Mar 2017 17:31:35 +0100
- Subject: Re: [PATCH 1/2] This patch fixes GDBServer's run control for single stepping
- Authentication-results: sourceware.org; auth=none
- References: <20161129120702.9490-1-antoine.tremblay@ericsson.com> <20170127150139.GB24676@E107787-LIN> <wwokwpdg5vxa.fsf@ericsson.com> <CAH=s-PP-i3v_Fr=QeWt9BQeJzjCHtW79nGYpJ9hF-Bb=OBo89Q@mail.gmail.com> <wwokr33o5pkb.fsf@ericsson.com> <CAH=s-PO98nCE4UB9ag+V=M2mBnZT0FOeHV3d7mFMLYe1+v=mFg@mail.gmail.com> <wwok8tps8yo2.fsf@ericsson.com> <2255ed6f-a146-026c-f871-00e9a33dfcf0@redhat.com> <wwokwpcp7fvn.fsf@ericsson.com> <b5fb81d1-66fc-68c2-9785-ffa487de59e0@redhat.com> <wwoktw7t7bzy.fsf@ericsson.com> <CAH=s-PPx+SjoE0DkTKKNqg4Dr4zHFNt6QeC-XXT_LoXVh004iw@mail.gmail.com> <wwokh93s1he3.fsf@ericsson.com> <CAH=s-PPrB=s6d9Q07W=-b8Sz9umh6_Lj24PyO4x99Z3QrtfmzQ@mail.gmail.com> <wwokzig4l0i1.fsf@ericsson.com> <86d1cy4umo.fsf@gmail.com> <wwokvaqqlipt.fsf@ericsson.com>
Antoine Tremblay <antoine.tremblay@ericsson.com> writes:
> You have to add if the current instruction is an IT instruction in wich
> case the next instruction will be in an IT block.
>
Oh, you are right.
> Also if you have a conditional instruction that would evalutate to
> true and is not the last one, get_next_pcs may return an instruction
> after the IT block, arm_breakpoint_kind_from_current_state will be
> called from the IT block with that PC and return a THUMB2_KIND when it
> should not. See the else case in arm-get-next-pcs.c:~351
With the current PC and CPSR, it is not difficult to know whether
next_pc is still within IT block nor not, because all instructions in IT
block should be sequentially executed or skipped.
>
> My point was that we should use get_next_pc directly since it's the best
> place to detect if the next_pc is in the IT block. And the intent would
> be clear.
Yeah, we can record the information of breakpoint type in the return
value of get_next_pc, ...
>
> It would give something like the patch below. (Note the GDB part of this
> is missing but it works with GDBServer)
>
... but using extra bit in CORE_ADDR is not a good idea to me.
>> The problem of this patch is that we end up inserting different
>> kinds of breakpoints on the same instruction. For a given 32-bit thumb
>> instruction, GDB and GDBserver knows 32-bit thumb breakpoint instruction
>> is used for GDB breakpoint, but only GDBserver knows 16-bit thumb
>> breakpoint is used for GDBserver single-step breakpoint, so GDB will be
>> confused on this. I stopped here, and start to do something else.
>
> Humm but how will the GDBServer 16-bit breakpoint be reported to GDB ?
> Won't it always be hit and handled by GDBServer ?
>
> And if you have a GDB breakpoint on an instruction and GDBServer puts
> a single step breakpoint on that GDB breakpoint instruction, GDBServer
> still knows of the GDB and GDBServer breakpoint types.
>
> So how does GDB get confused ?
That was my conclusion at that point. I got some regressions in
gdb.threads/*.exp when I tested my patch (gdb running is on
x86_64-linux), but I can't remember more details.
I am also wondering that we can use some code in
arm_adjust_breakpoint_address about detecting BPADDR is within IT block
or not by scanning instructions backward, if none of two bytes (can be
16-bit thumb instruction or the 2nd half of 32-bit thumb instruction)
matches IT instruction, the PC is not within IT block.
--
Yao (齐尧)