* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
@ 2016-02-10 12:52 ` Luis Machado
0 siblings, 0 replies; 16+ messages in thread
From: Luis Machado @ 2016-02-10 12:52 UTC (permalink / raw)
To: Maciej W. Rozycki; +Cc: Maciej W. Rozycki, gdb-patches, linux-mips
On 02/09/2016 06:55 PM, Maciej W. Rozycki wrote:
> On Tue, 9 Feb 2016, Luis Machado wrote:
>
>> I'm finally getting back to this. Sorry for the delay.
>
> No problem, thanks for keeping a track of it.
>
>>> I'm not convinced however that it is safe to assume all SIGTRAPs come
>>> from breakpoints -- this signal is sent by the kernel for both BREAK and
>>> trap (multiple mnemonics, e.g. TEQ, TGEI, etc.) instructions which may
>>> have been placed throughout code for some reason, for example to serve as
>>> cheap assertion checks.
>>>
>>> Is there a separate check made afterwards like `bpstat_explains_signal'
>>> to validate the source of the signal here?
>>
>> In my specific example we are dealing with two breakpoint hits by different
>> threads. The first one is reported just fine and the one we have problems with
>> is the queued one that is reported afterwards when we attempt to move the
>> other thread.
>>
>> We do rely on bpstat_explains_signal when gdbserver/the remote target notifies
>> gdb about a stop. In the case of a queued breakpoint hit, it doesn't even get
>> reported back to gdb and is just ignored by gdbserver if it is recognized as a
>> breakpoint hit.
>
> I'm not sure I understand what's going on here, why is a breakpoint hit
> required to be ignored by gdbserver?
>
> From what you say I infer GDB controls a multi-threaded program and there
> it sets a software breakpoint which, by its nature, is global to all the
> threads. Then multiple threads hit the breakpoint simultaneously (or
> nearly simultaneously) and the hits are delivered to GDB one by one, via
> gdbserver. So why is GDB not prepared for that?
>
> It set the breakpoint itself so it should expect it to hit sometime, and
> if there are multiple threads, then there can be multiple hits at once or
> almost at once because, even in the all-stop mode, there is no guaranteed
> way to stop the threads all at once. The threads may be spread across
> different processors in an SMP system for example, all trapping literally
> at once -- and then the kernel queueing the signals according to its own
> internal schedule before delivering them to the debugger (that, from the
> kernel's point of view, being gdbserver in this case).
>
I went through the data again and i was partially mistaken about the
above. Here's a detailed description of the events that take place in
the reported situation.
1 - A breakpoint is inserted by GDB at some code that is executed by
multiple threads.
2 - Two threads, let us call them 1 and 2, happen to hit the same
software breakpoint, so both SIGTRAP's get sent by the kernel and
gdbserver picks one of them to process.
3 - gdbserver figures out this is from a breakpoint hit, since it knows
there is a breakpoint inserted at that PC, and sends a swbreak stop
reply back to gdb.
4 - GDB gets the swbreak stop reply and notifies the user about the
breakpoint hit for thread 1, displays the frame information etc.
Now, the user goes and deletes that specific breakpoint that triggered
the previous event and switches the context to thread 2 and then
attempts to continue execution.
5 - In gdbserver, thread 2 still has a pending SIGTRAP that was not yet
handled.
6 - gdbserver proceeds to handle it, sees it is a SIGTRAP but cannot map
that event back to a software breakpoint hit due to the removal of such
breakpoint and because gdbserver doesn't expect SI_KERNEL to mean
"software breakpoint hit".
7 - gdbserver then assumes this is a generic trap and reports it as such
to GDB, in a new stop reply.
8 - GDB receives the stop reply and displays a generic trace/breakpoint
SIGTRAP since it also cannot map the trap back to a software breakpoint,
i.e. bpstat_explains_signal returns 0 and random_signal is non-zero.
Patching gdbserver to recognize a si_code of SI_KERNEL as a software
breakpoint trap causes changes starting from 6.
6 - gdbserver proceeds to handle it, sees it is a SIGTRAP and that its
si_code is SI_KERNEL, meaning a software breakpoint hit now, even though
we can't recognize a breakpoint hit by checking for an underlying
breakpoint instruction.
7 - gdbserver sends a swbreak stop reply back to GDB.
8 - GDB receives the stop reply and notices it is a delayed breakpoint
hit. According to its logic, GDB discards this as useless and the
program continues its execution properly. Here random_signal is non-zero
and target_stopped_by_sw_breakpoint () returns 1 (because gdbserver told
GDB so).
The problem of forcing gdbserver to recognize all traps with
si_code==SI_KERNEL is that even hardcoded traps will be reported back to
GDB as a swbreak event, which is not ideal.
But currently there is no easy way to tell a software breakpoint hit and
a hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
>> With the proposed change, even the hardcoded traps will initially be
>> recognized as breakpoints, albeit maybe recognized as permanent breakpoints
>> for some opcodes. It may cause gdbserver to ignore a second hardcoded trap
>> hit, which is not desirable.
>
> So why does gdbserver have to be taught which breakpoints may have
> potentially been set by GDB and which may have not? Why not to deliver
> them all and leave it up to GDB to decide? I believe it will be the right
> thing to let GDB know that more than one thread has hit the same
> breakpoint.
>
> Did I miss anything? How is this situation handled in a native debug
> scenario?
>
I take it native debugging will display the same sympthoms because the
definitions of si_code are shared between GDB and gdbserver, from
nat/linux-ptrace.h. Native also uses a similar function to check for
breakpoint hits, namely gdb/linux-nat.c:check_stopped_by_breakpoint. I
did not test this with a native debugger though.
>>> Perhaps we should make it a part of the ABI and teach MIPS/Linux about
>>> the breakpoint encoding used by GDB, which is `BREAK 5' (aka BRK_SSTEPBP
>>> in kernel-speak, a misnomer I'm afraid), and make it set `si_code' to
>>> TRAP_BRKPT, as expected. This won't fix history of course, but at least
>>> it will make debugging a little bit easier to handle in the future.
>>> Cc-ing `linux-mips' for further input.
>>
>> This is the best solution in my opinion and will definitely make the
>> debugger's life easier if it can tell the difference between multiple
>> seemingly equivalent SIGTRAP's.
>>
>> Does this involve handling BRK_SSTEPBP inside
>> arch/mips/kernel/traps.c:do_trap_or_bp?
>
> No, as I noted in my reply to David elsewhere in this thread, this would
> have to be in `do_bp' instead, to exclude trap instructions (e.g. TEQ,
> etc.) from being treated as breakpoints. I can implement this change
> myself for you, but we need to agree first what the right solution for GDB
> is. So far it looks to me we'd be only papering over a problem elsewhere.
Hopefully the above makes what we're facing more clear.
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
@ 2016-02-15 23:50 ` Maciej W. Rozycki
0 siblings, 0 replies; 16+ messages in thread
From: Maciej W. Rozycki @ 2016-02-15 23:50 UTC (permalink / raw)
To: Luis Machado; +Cc: Maciej W. Rozycki, gdb-patches, linux-mips
On Wed, 10 Feb 2016, Luis Machado wrote:
> I went through the data again and i was partially mistaken about the above.
> Here's a detailed description of the events that take place in the reported
> situation.
>
> 1 - A breakpoint is inserted by GDB at some code that is executed by multiple
> threads.
> 2 - Two threads, let us call them 1 and 2, happen to hit the same software
> breakpoint, so both SIGTRAP's get sent by the kernel and gdbserver picks one
> of them to process.
> 3 - gdbserver figures out this is from a breakpoint hit, since it knows there
> is a breakpoint inserted at that PC, and sends a swbreak stop reply back to
> gdb.
> 4 - GDB gets the swbreak stop reply and notifies the user about the breakpoint
> hit for thread 1, displays the frame information etc.
>
> Now, the user goes and deletes that specific breakpoint that triggered the
> previous event and switches the context to thread 2 and then attempts to
> continue execution.
>
> 5 - In gdbserver, thread 2 still has a pending SIGTRAP that was not yet
> handled.
> 6 - gdbserver proceeds to handle it, sees it is a SIGTRAP but cannot map that
> event back to a software breakpoint hit due to the removal of such breakpoint
> and because gdbserver doesn't expect SI_KERNEL to mean "software breakpoint
> hit".
> 7 - gdbserver then assumes this is a generic trap and reports it as such to
> GDB, in a new stop reply.
> 8 - GDB receives the stop reply and displays a generic trace/breakpoint
> SIGTRAP since it also cannot map the trap back to a software breakpoint, i.e.
> bpstat_explains_signal returns 0 and random_signal is non-zero.
>
> Patching gdbserver to recognize a si_code of SI_KERNEL as a software
> breakpoint trap causes changes starting from 6.
>
> 6 - gdbserver proceeds to handle it, sees it is a SIGTRAP and that its si_code
> is SI_KERNEL, meaning a software breakpoint hit now, even though we can't
> recognize a breakpoint hit by checking for an underlying breakpoint
> instruction.
> 7 - gdbserver sends a swbreak stop reply back to GDB.
> 8 - GDB receives the stop reply and notices it is a delayed breakpoint hit.
> According to its logic, GDB discards this as useless and the program continues
> its execution properly. Here random_signal is non-zero and
> target_stopped_by_sw_breakpoint () returns 1 (because gdbserver told GDB so).
>
> The problem of forcing gdbserver to recognize all traps with
> si_code==SI_KERNEL is that even hardcoded traps will be reported back to GDB
> as a swbreak event, which is not ideal.
>
> But currently there is no easy way to tell a software breakpoint hit and a
> hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
Thanks for the detailed explanation.
FWIW, I maintain it's GDB that should be handling it. What if TRAP_BRKPT
is reported for a breakpoint that has not been set by GDB in the first
place and is still there in code? I take it either GDB or gdbserver, as
applicable, will just sit there looping indefinitely in an attempt to
discard the event while executing the breakpoint instruction over and over
again. There's nothing stopping the user from having a MIPS `BREAK 5'
instruction or say INT3 for the x86 target already present in the
executable being debugged.
What I think GDB ought to be doing here is caching addresses for recently
removed breakpoints and discarding spurious hits in that cache. It may
actually be worth verifying, before discarding such a hit, that there is
no breakpoint instruction there anymore in target memory too -- a clever
user might have set a breakpoint on a breakpoint instruction already there
in code!
It seems to me it'll be enough if the cache is only retained over a
single resumption step, per thread of course, as it does not appear to me
that the kernel might queue more than one software breakpoint signal at
once.
NB conceptually this is similar to handling spurious hardware interrupts
in the kernel, where the kernel interrogates all possible interrupt
sources for the reporting interrupt line, which might be a physical shared
wire, a shared vector, etc., before discarding them as invalid. Such
interrupts are sometimes delivered when the originating device pulls its
request away before it's been handled.
> > Did I miss anything? How is this situation handled in a native debug
> > scenario?
>
> I take it native debugging will display the same sympthoms because the
> definitions of si_code are shared between GDB and gdbserver, from
> nat/linux-ptrace.h. Native also uses a similar function to check for
> breakpoint hits, namely gdb/linux-nat.c:check_stopped_by_breakpoint. I did not
> test this with a native debugger though.
OK, noted. Thanks!
> > > > Perhaps we should make it a part of the ABI and teach MIPS/Linux
> > > > about
> > > > the breakpoint encoding used by GDB, which is `BREAK 5' (aka BRK_SSTEPBP
> > > > in kernel-speak, a misnomer I'm afraid), and make it set `si_code' to
> > > > TRAP_BRKPT, as expected. This won't fix history of course, but at least
> > > > it will make debugging a little bit easier to handle in the future.
> > > > Cc-ing `linux-mips' for further input.
> > >
> > > This is the best solution in my opinion and will definitely make the
> > > debugger's life easier if it can tell the difference between multiple
> > > seemingly equivalent SIGTRAP's.
> > >
> > > Does this involve handling BRK_SSTEPBP inside
> > > arch/mips/kernel/traps.c:do_trap_or_bp?
> >
> > No, as I noted in my reply to David elsewhere in this thread, this would
> > have to be in `do_bp' instead, to exclude trap instructions (e.g. TEQ,
> > etc.) from being treated as breakpoints. I can implement this change
> > myself for you, but we need to agree first what the right solution for GDB
> > is. So far it looks to me we'd be only papering over a problem elsewhere.
>
> Hopefully the above makes what we're facing more clear.
I think we could do this as well, so as to save GDB from the need to
verify truly irrelevant traps against its cache. Though I don't think it
buys us anything short-term as we'll have to continue support kernels
which have no notion of TRAP_BRKPT. So no need to rush doing this IMHO.
Maciej
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
@ 2016-02-15 23:50 ` Maciej W. Rozycki
0 siblings, 0 replies; 16+ messages in thread
From: Maciej W. Rozycki @ 2016-02-15 23:50 UTC (permalink / raw)
To: Luis Machado; +Cc: Maciej W. Rozycki, gdb-patches, linux-mips
On Wed, 10 Feb 2016, Luis Machado wrote:
> I went through the data again and i was partially mistaken about the above.
> Here's a detailed description of the events that take place in the reported
> situation.
>
> 1 - A breakpoint is inserted by GDB at some code that is executed by multiple
> threads.
> 2 - Two threads, let us call them 1 and 2, happen to hit the same software
> breakpoint, so both SIGTRAP's get sent by the kernel and gdbserver picks one
> of them to process.
> 3 - gdbserver figures out this is from a breakpoint hit, since it knows there
> is a breakpoint inserted at that PC, and sends a swbreak stop reply back to
> gdb.
> 4 - GDB gets the swbreak stop reply and notifies the user about the breakpoint
> hit for thread 1, displays the frame information etc.
>
> Now, the user goes and deletes that specific breakpoint that triggered the
> previous event and switches the context to thread 2 and then attempts to
> continue execution.
>
> 5 - In gdbserver, thread 2 still has a pending SIGTRAP that was not yet
> handled.
> 6 - gdbserver proceeds to handle it, sees it is a SIGTRAP but cannot map that
> event back to a software breakpoint hit due to the removal of such breakpoint
> and because gdbserver doesn't expect SI_KERNEL to mean "software breakpoint
> hit".
> 7 - gdbserver then assumes this is a generic trap and reports it as such to
> GDB, in a new stop reply.
> 8 - GDB receives the stop reply and displays a generic trace/breakpoint
> SIGTRAP since it also cannot map the trap back to a software breakpoint, i.e.
> bpstat_explains_signal returns 0 and random_signal is non-zero.
>
> Patching gdbserver to recognize a si_code of SI_KERNEL as a software
> breakpoint trap causes changes starting from 6.
>
> 6 - gdbserver proceeds to handle it, sees it is a SIGTRAP and that its si_code
> is SI_KERNEL, meaning a software breakpoint hit now, even though we can't
> recognize a breakpoint hit by checking for an underlying breakpoint
> instruction.
> 7 - gdbserver sends a swbreak stop reply back to GDB.
> 8 - GDB receives the stop reply and notices it is a delayed breakpoint hit.
> According to its logic, GDB discards this as useless and the program continues
> its execution properly. Here random_signal is non-zero and
> target_stopped_by_sw_breakpoint () returns 1 (because gdbserver told GDB so).
>
> The problem of forcing gdbserver to recognize all traps with
> si_code==SI_KERNEL is that even hardcoded traps will be reported back to GDB
> as a swbreak event, which is not ideal.
>
> But currently there is no easy way to tell a software breakpoint hit and a
> hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
Thanks for the detailed explanation.
FWIW, I maintain it's GDB that should be handling it. What if TRAP_BRKPT
is reported for a breakpoint that has not been set by GDB in the first
place and is still there in code? I take it either GDB or gdbserver, as
applicable, will just sit there looping indefinitely in an attempt to
discard the event while executing the breakpoint instruction over and over
again. There's nothing stopping the user from having a MIPS `BREAK 5'
instruction or say INT3 for the x86 target already present in the
executable being debugged.
What I think GDB ought to be doing here is caching addresses for recently
removed breakpoints and discarding spurious hits in that cache. It may
actually be worth verifying, before discarding such a hit, that there is
no breakpoint instruction there anymore in target memory too -- a clever
user might have set a breakpoint on a breakpoint instruction already there
in code!
It seems to me it'll be enough if the cache is only retained over a
single resumption step, per thread of course, as it does not appear to me
that the kernel might queue more than one software breakpoint signal at
once.
NB conceptually this is similar to handling spurious hardware interrupts
in the kernel, where the kernel interrogates all possible interrupt
sources for the reporting interrupt line, which might be a physical shared
wire, a shared vector, etc., before discarding them as invalid. Such
interrupts are sometimes delivered when the originating device pulls its
request away before it's been handled.
> > Did I miss anything? How is this situation handled in a native debug
> > scenario?
>
> I take it native debugging will display the same sympthoms because the
> definitions of si_code are shared between GDB and gdbserver, from
> nat/linux-ptrace.h. Native also uses a similar function to check for
> breakpoint hits, namely gdb/linux-nat.c:check_stopped_by_breakpoint. I did not
> test this with a native debugger though.
OK, noted. Thanks!
> > > > Perhaps we should make it a part of the ABI and teach MIPS/Linux
> > > > about
> > > > the breakpoint encoding used by GDB, which is `BREAK 5' (aka BRK_SSTEPBP
> > > > in kernel-speak, a misnomer I'm afraid), and make it set `si_code' to
> > > > TRAP_BRKPT, as expected. This won't fix history of course, but at least
> > > > it will make debugging a little bit easier to handle in the future.
> > > > Cc-ing `linux-mips' for further input.
> > >
> > > This is the best solution in my opinion and will definitely make the
> > > debugger's life easier if it can tell the difference between multiple
> > > seemingly equivalent SIGTRAP's.
> > >
> > > Does this involve handling BRK_SSTEPBP inside
> > > arch/mips/kernel/traps.c:do_trap_or_bp?
> >
> > No, as I noted in my reply to David elsewhere in this thread, this would
> > have to be in `do_bp' instead, to exclude trap instructions (e.g. TEQ,
> > etc.) from being treated as breakpoints. I can implement this change
> > myself for you, but we need to agree first what the right solution for GDB
> > is. So far it looks to me we'd be only papering over a problem elsewhere.
>
> Hopefully the above makes what we're facing more clear.
I think we could do this as well, so as to save GDB from the need to
verify truly irrelevant traps against its cache. Though I don't think it
buys us anything short-term as we'll have to continue support kernels
which have no notion of TRAP_BRKPT. So no need to rush doing this IMHO.
Maciej
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
2016-02-15 23:50 ` Maciej W. Rozycki
(?)
@ 2016-02-16 0:30 ` Pedro Alves
2016-02-19 16:21 ` Maciej W. Rozycki
-1 siblings, 1 reply; 16+ messages in thread
From: Pedro Alves @ 2016-02-16 0:30 UTC (permalink / raw)
To: Maciej W. Rozycki, Luis Machado
Cc: Maciej W. Rozycki, gdb-patches, linux-mips
On 02/15/2016 11:50 PM, Maciej W. Rozycki wrote:
> FWIW, I maintain it's GDB that should be handling it. What if TRAP_BRKPT
> is reported for a breakpoint that has not been set by GDB in the first
> place and is still there in code? I take it either GDB or gdbserver, as
> applicable, will just sit there looping indefinitely in an attempt to
> discard the event while executing the breakpoint instruction over and over
> again.
Nope.
> There's nothing stopping the user from having a MIPS `BREAK 5'
> instruction or say INT3 for the x86 target already present in the
> executable being debugged.
GDB only ignores the TRAP_BRKPT event if there's no "BREAK 5" instruction
hardcoded in the binary. If there is, then the program is stopped and
a SIGTRAP is reported to the user.
> What I think GDB ought to be doing here is caching addresses for recently
> removed breakpoints and discarding spurious hits in that cache.
That does not work in general. The most problematic archs are those that
leave the PC pointing after the breakpoint instruction, such as x86.
See more below.
> It may
> actually be worth verifying, before discarding such a hit, that there is
> no breakpoint instruction there anymore in target memory too -- a clever
> user might have set a breakpoint on a breakpoint instruction already there
> in code!
Yep, GDB does that already, and we have tests that cover this.
See gdb.base/bp-permanent.exp.
>
> It seems to me it'll be enough if the cache is only retained over a
> single resumption step, per thread of course, as it does not appear to me
> that the kernel might queue more than one software breakpoint signal at
> once.
That wouldn't work, as a new thread GDB doesn't know about yet may report
a stop for the PC where a breakpoint used to be, and then you don't
know whether you need to adjust its PC.
Remembering whether a breakpoint was inserted was what GDB used to
do, and it was because it was problematic that "swbreak" was
invented. See:
https://sourceware.org/ml/gdb-patches/2015-02/msg00726.html
Particularly:
https://sourceware.org/ml/gdb-patches/2015-02/msg00730.html
This was a previous attempt that tried to preserve moribund
locations, but was still not sufficient:
https://sourceware.org/ml/gdb-patches/2014-10/msg00781.html
I'm really hoping that at some point all archs implement
TRAP_BRKPT and the moribund locations heuristic can be removed
from gdb.
Thanks,
Pedro Alves
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
@ 2016-02-19 16:21 ` Maciej W. Rozycki
0 siblings, 0 replies; 16+ messages in thread
From: Maciej W. Rozycki @ 2016-02-19 16:21 UTC (permalink / raw)
To: Pedro Alves; +Cc: Luis Machado, Maciej W. Rozycki, gdb-patches, linux-mips
On Tue, 16 Feb 2016, Pedro Alves wrote:
> > FWIW, I maintain it's GDB that should be handling it. What if TRAP_BRKPT
> > is reported for a breakpoint that has not been set by GDB in the first
> > place and is still there in code? I take it either GDB or gdbserver, as
> > applicable, will just sit there looping indefinitely in an attempt to
> > discard the event while executing the breakpoint instruction over and over
> > again.
>
> Nope.
>
> > There's nothing stopping the user from having a MIPS `BREAK 5'
> > instruction or say INT3 for the x86 target already present in the
> > executable being debugged.
>
> GDB only ignores the TRAP_BRKPT event if there's no "BREAK 5" instruction
> hardcoded in the binary. If there is, then the program is stopped and
> a SIGTRAP is reported to the user.
Ah, that makes a fundamental difference to me! I missed this detail from
Luis's description and concluded that GDB/gdbserver never passes along
TRAP_BRKPT events it cannot associate with a breakpoint set by itself.
> > What I think GDB ought to be doing here is caching addresses for recently
> > removed breakpoints and discarding spurious hits in that cache.
>
> That does not work in general. The most problematic archs are those that
> leave the PC pointing after the breakpoint instruction, such as x86.
> See more below.
[etc.] Fair enough. Thanks for the extra explanation.
> Remembering whether a breakpoint was inserted was what GDB used to
> do, and it was because it was problematic that "swbreak" was
> invented. See:
>
> https://sourceware.org/ml/gdb-patches/2015-02/msg00726.html
>
> Particularly:
>
> https://sourceware.org/ml/gdb-patches/2015-02/msg00730.html
OK, I can see you peek at target text in `program_breakpoint_here_p' to
see if there's a breakpoint there. With this in mind I think Luis's
change is conceptually good as it stands, although I think we should move
the MIPS case along PowerPC right away, because TRAP_BRKPT is the right
code to use here and the MIPS port has never produced it so far and
therefore there's no backwards compatibility concern if we accept both
SI_KERNEL and TRAP_BRKPT.
Luis, such an updated change is preapproved as far as I am concerned,
feel free to commit it without another review round.
As to the kernel side with the observations made in this discussion I
think we should set the trap code for SIGTRAP signals issued with BREAK
instructions to TRAP_BRKPT unconditionally, regardless of the code used.
This won't of course affect encodings which send a different signal such
as SIGFPE.
We're lacking a code suitable for (conditional) trap instructions. I
think TRAP_TRAP or suchlike needs to be added.
> > But currently there is no easy way to tell a software breakpoint hit and
> > a hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
>
> Software breakpoint hits or hardcoded traps are handled the same. Even if GDB
> plants the breakpoint instruction itself with direct memory pokes (instead of
> z0 packets), the target should report "swbreak" stops, so that gdb can do
> the right thing.
Manual planting is historical AFAIK, gdbserver and other stubs used not
to support `z' packets. Now we have the right infrastructure in place and
GDB passes MIPS breakpoint encoding requirements along (although I note
that the encoding for microMIPS BREAK16 has changed with R6, so this will
have to be updated) so we could have software breakpoint support added to
MIPS gdbserver as well.
> Hardware breakpoint hits are distinguished from software breakpoint hits,
> because they're reported with "hwbreak", not "swbreak":
>
> @item hwbreak
> The packet indicates the target stopped for a hardware breakpoint.
> The @var{r} part must be left empty.
Umm, any requirements for this? We have MIPS data hardware breakpoint
support in the Linux kernel (regrettably not for instructions, but that
could be added sometime), but I can't see TRAP_HWBKPT being set for them,
they just use generic SI_KERNEL as everything else right now.
Thanks for your patience in explaining this all to me.
Maciej
^ permalink raw reply [flat|nested] 16+ messages in thread* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
@ 2016-02-19 16:21 ` Maciej W. Rozycki
0 siblings, 0 replies; 16+ messages in thread
From: Maciej W. Rozycki @ 2016-02-19 16:21 UTC (permalink / raw)
To: Pedro Alves; +Cc: Luis Machado, Maciej W. Rozycki, gdb-patches, linux-mips
On Tue, 16 Feb 2016, Pedro Alves wrote:
> > FWIW, I maintain it's GDB that should be handling it. What if TRAP_BRKPT
> > is reported for a breakpoint that has not been set by GDB in the first
> > place and is still there in code? I take it either GDB or gdbserver, as
> > applicable, will just sit there looping indefinitely in an attempt to
> > discard the event while executing the breakpoint instruction over and over
> > again.
>
> Nope.
>
> > There's nothing stopping the user from having a MIPS `BREAK 5'
> > instruction or say INT3 for the x86 target already present in the
> > executable being debugged.
>
> GDB only ignores the TRAP_BRKPT event if there's no "BREAK 5" instruction
> hardcoded in the binary. If there is, then the program is stopped and
> a SIGTRAP is reported to the user.
Ah, that makes a fundamental difference to me! I missed this detail from
Luis's description and concluded that GDB/gdbserver never passes along
TRAP_BRKPT events it cannot associate with a breakpoint set by itself.
> > What I think GDB ought to be doing here is caching addresses for recently
> > removed breakpoints and discarding spurious hits in that cache.
>
> That does not work in general. The most problematic archs are those that
> leave the PC pointing after the breakpoint instruction, such as x86.
> See more below.
[etc.] Fair enough. Thanks for the extra explanation.
> Remembering whether a breakpoint was inserted was what GDB used to
> do, and it was because it was problematic that "swbreak" was
> invented. See:
>
> https://sourceware.org/ml/gdb-patches/2015-02/msg00726.html
>
> Particularly:
>
> https://sourceware.org/ml/gdb-patches/2015-02/msg00730.html
OK, I can see you peek at target text in `program_breakpoint_here_p' to
see if there's a breakpoint there. With this in mind I think Luis's
change is conceptually good as it stands, although I think we should move
the MIPS case along PowerPC right away, because TRAP_BRKPT is the right
code to use here and the MIPS port has never produced it so far and
therefore there's no backwards compatibility concern if we accept both
SI_KERNEL and TRAP_BRKPT.
Luis, such an updated change is preapproved as far as I am concerned,
feel free to commit it without another review round.
As to the kernel side with the observations made in this discussion I
think we should set the trap code for SIGTRAP signals issued with BREAK
instructions to TRAP_BRKPT unconditionally, regardless of the code used.
This won't of course affect encodings which send a different signal such
as SIGFPE.
We're lacking a code suitable for (conditional) trap instructions. I
think TRAP_TRAP or suchlike needs to be added.
> > But currently there is no easy way to tell a software breakpoint hit and
> > a hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
>
> Software breakpoint hits or hardcoded traps are handled the same. Even if GDB
> plants the breakpoint instruction itself with direct memory pokes (instead of
> z0 packets), the target should report "swbreak" stops, so that gdb can do
> the right thing.
Manual planting is historical AFAIK, gdbserver and other stubs used not
to support `z' packets. Now we have the right infrastructure in place and
GDB passes MIPS breakpoint encoding requirements along (although I note
that the encoding for microMIPS BREAK16 has changed with R6, so this will
have to be updated) so we could have software breakpoint support added to
MIPS gdbserver as well.
> Hardware breakpoint hits are distinguished from software breakpoint hits,
> because they're reported with "hwbreak", not "swbreak":
>
> @item hwbreak
> The packet indicates the target stopped for a hardware breakpoint.
> The @var{r} part must be left empty.
Umm, any requirements for this? We have MIPS data hardware breakpoint
support in the Linux kernel (regrettably not for instructions, but that
could be added sometime), but I can't see TRAP_HWBKPT being set for them,
they just use generic SI_KERNEL as everything else right now.
Thanks for your patience in explaining this all to me.
Maciej
^ permalink raw reply [flat|nested] 16+ messages in thread* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
2016-02-19 16:21 ` Maciej W. Rozycki
(?)
@ 2016-02-24 11:52 ` Pedro Alves
-1 siblings, 0 replies; 16+ messages in thread
From: Pedro Alves @ 2016-02-24 11:52 UTC (permalink / raw)
To: Maciej W. Rozycki
Cc: Luis Machado, Maciej W. Rozycki, gdb-patches, linux-mips
Hi Maciej,
> As to the kernel side with the observations made in this discussion I
> think we should set the trap code for SIGTRAP signals issued with BREAK
> instructions to TRAP_BRKPT unconditionally, regardless of the code used.
> This won't of course affect encodings which send a different signal such
> as SIGFPE.
>
> We're lacking a code suitable for (conditional) trap instructions. I
> think TRAP_TRAP or suchlike needs to be added.
Yeah, looks like it.
>> Hardware breakpoint hits are distinguished from software breakpoint hits,
>> because they're reported with "hwbreak", not "swbreak":
>>
>> @item hwbreak
>> The packet indicates the target stopped for a hardware breakpoint.
>> The @var{r} part must be left empty.
>
> Umm, any requirements for this? We have MIPS data hardware breakpoint
> support in the Linux kernel (regrettably not for instructions, but that
> could be added sometime), but I can't see TRAP_HWBKPT being set for them,
> they just use generic SI_KERNEL as everything else right now.
Can userspace/ptrace still tell whether a hardware breakpoint triggered by
consulting debug registers, similarly to how it can for watchpoints,
with PTRACE_GET_WATCH_REGS, and looking at watchhi ?
(assuming this comment is correct):
/* Target to_stopped_by_watchpoint implementation. Return 1 if
stopped by watchpoint. The watchhi R and W bits indicate the watch
register triggered. */
static int
mips_linux_stopped_by_watchpoint (struct target_ops *ops)
{
This is not reachable today, due to lack of TRAP_* in si_code, but I
think that can be fixed.
The only use for hwbreak currently is to know whether to ignore hardware
breakpoint traps gdb can't explain (gdb assumes they're a delayed event for
a hw breakpoint that has since been removed):
/* Maybe this was a trap for a hardware breakpoint/watchpoint that
has since been removed. */
if (random_signal && target_stopped_by_hw_breakpoint ())
{
/* A delayed hardware breakpoint event. Ignore the trap. */
if (debug_infrun)
fprintf_unfiltered (gdb_stdlog,
"infrun: delayed hardware breakpoint/watchpoint "
"trap, ignoring\n");
random_signal = 0;
}
So if the server claims it supports this stop reason, but then doesn't
send it for hw breakpoint trap, users will see their programs
occasionally stop for random spurious SIGTRAPs (if they use hardware
breapoints).
If the server does _not_ claim support for the swbreak/hwbreak stop
reason, then the old moribund breakpoints heuristic kicks in:
/* Check if a moribund breakpoint explains the stop. */
if (!target_supports_stopped_by_sw_breakpoint ()
|| !target_supports_stopped_by_hw_breakpoint ())
{
for (ix = 0; VEC_iterate (bp_location_p, moribund_locations, ix, loc); ++ix)
{
if (breakpoint_location_address_match (loc, aspace, bp_addr)
&& need_moribund_for_location_type (loc))
{
bs = bpstat_alloc (loc, &bs_link);
/* For hits of moribund locations, we should just proceed. */
bs->stop = 0;
bs->print = 0;
bs->print_it = print_it_noop;
}
}
}
Thanks,
Pedro Alves
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] Expect SI_KERNEL si_code for a MIPS software breakpoint trap
2016-02-10 12:52 ` Luis Machado
(?)
(?)
@ 2016-02-16 0:57 ` Pedro Alves
-1 siblings, 0 replies; 16+ messages in thread
From: Pedro Alves @ 2016-02-16 0:57 UTC (permalink / raw)
To: Luis Machado, Maciej W. Rozycki
Cc: Maciej W. Rozycki, gdb-patches, linux-mips
On 02/10/2016 12:52 PM, Luis Machado wrote:
> The problem of forcing gdbserver to recognize all traps with
> si_code==SI_KERNEL is that even hardcoded traps will be reported back to
> GDB as a swbreak event, which is not ideal.
That's how swbreak is defined:
@item swbreak
@anchor{swbreak stop reason}
The packet indicates a memory breakpoint instruction was executed,
irrespective of whether it was @value{GDBN} that planted the
breakpoint or the breakpoint is hardcoded in the program.
>
> But currently there is no easy way to tell a software breakpoint hit and
> a hardcoded trap (and maybe even a hardware breakpoint hit?) apart.
Software breakpoint hits or hardcoded traps are handled the same. Even if GDB
plants the breakpoint instruction itself with direct memory pokes (instead of
z0 packets), the target should report "swbreak" stops, so that gdb can do
the right thing.
GDB knows whether to discard the hit as a delayed breakpoint hit event by
checking whether the thread's PC points at an hardcoded trap. If it does,
the event is not discarded, but instead reported to the user as a SIGTRAP.
Hardware breakpoint hits are distinguished from software breakpoint hits,
because they're reported with "hwbreak", not "swbreak":
@item hwbreak
The packet indicates the target stopped for a hardware breakpoint.
The @var{r} part must be left empty.
Thanks,
Pedro Alves
^ permalink raw reply [flat|nested] 16+ messages in thread