Re: [PATCH 01/31] displaced step: pass down target_waitstatus instead of gdb_signal

[Andrew Burgess <aburgess@redhat.com>]

Pedro Alves <pedro@palves.net> writes:

> On 2023-02-03 10:44 a.m., Andrew Burgess wrote:
>> Pedro Alves <pedro@palves.net> writes:
>> 
>>> This commit tweaks displaced_step_finish & friends to pass down a
>>> target_waitstatus instead of a gdb_signal.  This needed because a
>> 
>> missing word: "This IS needed".
>
> Fixed.
>
>>> @@ -5699,7 +5696,7 @@ handle_inferior_event (struct execution_control_state *ecs)
>>>  	       has been done.  Perform cleanup for parent process here.  Note
>>>  	       that this operation also cleans up the child process for vfork,
>>>  	       because their pages are shared.  */
>>> -	    displaced_step_finish (ecs->event_thread, GDB_SIGNAL_TRAP);
>>> +	    displaced_step_finish (ecs->event_thread, ecs->ws);
>> 
>> This change is interesting.
>> 
>> If I understand the code correctly, this call will eventually end up in
>> displaced_step_buffers::finish (displaced-stepping.c), which in turn
>> calls displaced_step_instruction_executed_successfully.
>> 
>> Previously, we always passed GDB_SIGNAL_TRAP here, which (if we ignore
>> the watchpoint check in
>> displaced_step_instruction_executed_successfully) means that
>> displaced_step_instruction_executed_successfully would always return
>> true, and then displaced_step_buffers::finish would call
>> gdbarch_displaced_step_fixup.
>> 
>> After this change, we know that esc->ws.kind is either
>> TARGET_WAITKIND_FORKED or  TARGET_WAITKIND_VFORKED, so we know that
>> displaced_step_instruction_executed_successfully will always return
>> false, and displaced_step_buffers::finish will no longer call
>> gdbarch_displaced_step_fixup.
>> 
>
> Good catch!
>
> I was tweaking the change to address your comment, and was coming to the
> conclusion that what I really wanted was this:
>
> static bool
> displaced_step_instruction_executed_successfully
>   (gdbarch *arch, const target_waitstatus &status)
> {
>   if (status.kind () == TARGET_WAITKIND_STOPPED
>       && status.sig () != GDB_SIGNAL_TRAP)
>     return false;
>
>   /* All other waitkinds can only happen if the instruction fully
>      executed.  For example, a fork, or a syscall entry can only
>      happen if the syscall instruction actually executed.  */
>
> (the comment is new)
>
> And then, I remembered, I actually wrote that early if like that originally,
> and I changed it in response to this review:
>
>  https://inbox.sourceware.org/gdb-patches/44f74af8-248b-1af8-3612-980c08607bf4@simark.ca/
>
> The review was totally right, it was my response that was misguided.
>
> But I'm confused because I am pretty sure that I wrote a reply to that
> message, saying that I did not intend to change the behavior, so I'd "fix" it.
> I can't find it in my outbox either, I guess I erroneously canceled my
> email window instead of sending the message...
>
> Anyhow, looks like I made it worse while trying to address Simon's comment.  :-P
>
> So I think I should go back to what I had, like before, and my response
> to Simon should have been instead:
>
>  - yes, the change is intended.  If we stopped for an event other than
> TARGET_WAITKIND_STOPPED, like for instance, TARGET_WAITKIND_FORKED,
> TARGET_WAITKIND_SYSCALL_ENTRY, then it must be that the instruction
> executed successfully, otherwise the syscall wouldn't have triggered.
>
>> What I don't understand well enough is what this actually means for a
>> running inferior.
>> 
>> It's odd because the comment in infrun.c (just above your change)
>> indicates that to get to this point the displaced step must have
>> completed successfully, while after this change, the new code path in
>> displaced_step_buffers::finish indicates we believe the displaced step
>> didn't complete successfully:
>> 
>>   /* Since the instruction didn't complete, all we can do is relocate the
>>      PC.  */
>> 
>> Do you know if any of our test cases hit this path?
>> 
>
> I know that you posted a series for this, which I plan to take a good look
> at (I actually planned on doing that earlier this week, but I had a couple
> major distractions, sorry).
>
> WDYT of the version below?

I took a look through and I'm happy with it.  But I would like you to
consider holding off until my displaced step patch has some feedback.

I'm currently rebasing the patch.  My patch deletes
displaced_step_instruction_executed_successfully, which I realised makes
passing the signal (or now target_waitstatus) redundant.  As such I'm
just testing an additional patch in the series which touches every place
you're touching - but removes the signal instead of changing it.

I hope to post my updated series later today (once testing completes).

Thanks,
Andrew

>
> From 0ccfd6822ef83eddb48c6f7bf21533858f177bc9 Mon Sep 17 00:00:00 2001
> From: Pedro Alves <pedro@palves.net>
> Date: Tue, 22 Jun 2021 15:42:51 +0100
> Subject: [PATCH] displaced step: pass down target_waitstatus instead of
>  gdb_signal
>
> This commit tweaks displaced_step_finish & friends to pass down a
> target_waitstatus instead of a gdb_signal.  This is needed because a
> patch later in the series will want to make
> displaced_step_buffers::finish handle TARGET_WAITKIND_THREAD_EXITED.
> It also helps with the TARGET_WAITKIND_THREAD_CLONED patch later in
> the series.
>
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27338
> Change-Id: I4c5d338647b028071bc498c4e47063795a2db4c0
> ---
>  gdb/displaced-stepping.c  | 16 +++++++++++-----
>  gdb/displaced-stepping.h  |  2 +-
>  gdb/gdbarch-gen.h         |  4 ++--
>  gdb/gdbarch.c             |  4 ++--
>  gdb/gdbarch_components.py |  2 +-
>  gdb/infrun.c              | 17 +++++++----------
>  gdb/linux-tdep.c          |  5 +++--
>  gdb/linux-tdep.h          |  2 +-
>  gdb/rs6000-tdep.c         |  4 ++--
>  9 files changed, 30 insertions(+), 26 deletions(-)
>
> diff --git a/gdb/displaced-stepping.c b/gdb/displaced-stepping.c
> index 06b32a80f6a..08ae1ce4397 100644
> --- a/gdb/displaced-stepping.c
> +++ b/gdb/displaced-stepping.c
> @@ -192,12 +192,18 @@ write_memory_ptid (ptid_t ptid, CORE_ADDR memaddr,
>  }
>  
>  static bool
> -displaced_step_instruction_executed_successfully (gdbarch *arch,
> -						  gdb_signal signal)
> +displaced_step_instruction_executed_successfully
> +  (gdbarch *arch, const target_waitstatus &status)
>  {
> -  if (signal != GDB_SIGNAL_TRAP)
> +  if (status.kind () == TARGET_WAITKIND_STOPPED
> +      && status.sig () != GDB_SIGNAL_TRAP)
>      return false;
>  
> +  /* All other (thread event) waitkinds can only happen if the
> +     instruction fully executed.  For example, a fork, or a syscall
> +     entry can only happen if the syscall instruction actually
> +     executed.  */
> +
>    if (target_stopped_by_watchpoint ())
>      {
>        if (gdbarch_have_nonsteppable_watchpoint (arch)
> @@ -210,7 +216,7 @@ displaced_step_instruction_executed_successfully (gdbarch *arch,
>  
>  displaced_step_finish_status
>  displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,
> -				gdb_signal sig)
> +				const target_waitstatus &status)
>  {
>    gdb_assert (thread->displaced_step_state.in_progress ());
>  
> @@ -256,7 +262,7 @@ displaced_step_buffers::finish (gdbarch *arch, thread_info *thread,
>    regcache *rc = get_thread_regcache (thread);
>  
>    bool instruction_executed_successfully
> -    = displaced_step_instruction_executed_successfully (arch, sig);
> +    = displaced_step_instruction_executed_successfully (arch, status);
>  
>    if (instruction_executed_successfully)
>      {
> diff --git a/gdb/displaced-stepping.h b/gdb/displaced-stepping.h
> index e154927ad92..d2c3fc1b2b4 100644
> --- a/gdb/displaced-stepping.h
> +++ b/gdb/displaced-stepping.h
> @@ -168,7 +168,7 @@ struct displaced_step_buffers
>  					 CORE_ADDR &displaced_pc);
>  
>    displaced_step_finish_status finish (gdbarch *arch, thread_info *thread,
> -				       gdb_signal sig);
> +				       const target_waitstatus &status);
>  
>    const displaced_step_copy_insn_closure *
>      copy_insn_closure_by_addr (CORE_ADDR addr);
> diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
> index ddb97f60315..5805dcb75ef 100644
> --- a/gdb/gdbarch-gen.h
> +++ b/gdb/gdbarch-gen.h
> @@ -1103,8 +1103,8 @@ extern void set_gdbarch_displaced_step_prepare (struct gdbarch *gdbarch, gdbarch
>  
>  /* Clean up after a displaced step of THREAD. */
>  
> -typedef displaced_step_finish_status (gdbarch_displaced_step_finish_ftype) (struct gdbarch *gdbarch, thread_info *thread, gdb_signal sig);
> -extern displaced_step_finish_status gdbarch_displaced_step_finish (struct gdbarch *gdbarch, thread_info *thread, gdb_signal sig);
> +typedef displaced_step_finish_status (gdbarch_displaced_step_finish_ftype) (struct gdbarch *gdbarch, thread_info *thread, const target_waitstatus &ws);
> +extern displaced_step_finish_status gdbarch_displaced_step_finish (struct gdbarch *gdbarch, thread_info *thread, const target_waitstatus &ws);
>  extern void set_gdbarch_displaced_step_finish (struct gdbarch *gdbarch, gdbarch_displaced_step_finish_ftype *displaced_step_finish);
>  
>  /* Return the closure associated to the displaced step buffer that is at ADDR. */
> diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
> index 7e4e34d5aca..0b122a86055 100644
> --- a/gdb/gdbarch.c
> +++ b/gdb/gdbarch.c
> @@ -4096,13 +4096,13 @@ set_gdbarch_displaced_step_prepare (struct gdbarch *gdbarch,
>  }
>  
>  displaced_step_finish_status
> -gdbarch_displaced_step_finish (struct gdbarch *gdbarch, thread_info *thread, gdb_signal sig)
> +gdbarch_displaced_step_finish (struct gdbarch *gdbarch, thread_info *thread, const target_waitstatus &ws)
>  {
>    gdb_assert (gdbarch != NULL);
>    gdb_assert (gdbarch->displaced_step_finish != NULL);
>    if (gdbarch_debug >= 2)
>      gdb_printf (gdb_stdlog, "gdbarch_displaced_step_finish called\n");
> -  return gdbarch->displaced_step_finish (gdbarch, thread, sig);
> +  return gdbarch->displaced_step_finish (gdbarch, thread, ws);
>  }
>  
>  void
> diff --git a/gdb/gdbarch_components.py b/gdb/gdbarch_components.py
> index caa65c334ec..8d72bff347e 100644
> --- a/gdb/gdbarch_components.py
> +++ b/gdb/gdbarch_components.py
> @@ -1878,7 +1878,7 @@ Clean up after a displaced step of THREAD.
>  """,
>      type="displaced_step_finish_status",
>      name="displaced_step_finish",
> -    params=[("thread_info *", "thread"), ("gdb_signal", "sig")],
> +    params=[("thread_info *", "thread"), ("const target_waitstatus &", "ws")],
>      predefault="NULL",
>      invalid="(! gdbarch->displaced_step_finish) != (! gdbarch->displaced_step_prepare)",
>  )
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index ab77300f1ff..99f2a8e039d 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -1895,7 +1895,8 @@ displaced_step_prepare (thread_info *thread)
>     DISPLACED_STEP_FINISH_STATUS_OK as well.  */
>  
>  static displaced_step_finish_status
> -displaced_step_finish (thread_info *event_thread, enum gdb_signal signal)
> +displaced_step_finish (thread_info *event_thread,
> +		       const target_waitstatus &event_status)
>  {
>    displaced_step_thread_state *displaced = &event_thread->displaced_step_state;
>  
> @@ -1917,7 +1918,7 @@ displaced_step_finish (thread_info *event_thread, enum gdb_signal signal)
>    /* Do the fixup, and release the resources acquired to do the displaced
>       step. */
>    return gdbarch_displaced_step_finish (displaced->get_original_gdbarch (),
> -					event_thread, signal);
> +					event_thread, event_status);
>  }
>  
>  /* Data to be passed around while handling an event.  This data is
> @@ -5122,7 +5123,7 @@ handle_one (const wait_one_event &event)
>  	  /* We caught the event that we intended to catch, so
>  	     there's no event to save as pending.  */
>  
> -	  if (displaced_step_finish (t, GDB_SIGNAL_0)
> +	  if (displaced_step_finish (t, event.ws)
>  	      == DISPLACED_STEP_FINISH_STATUS_NOT_EXECUTED)
>  	    {
>  	      /* Add it back to the step-over queue.  */
> @@ -5137,7 +5138,6 @@ handle_one (const wait_one_event &event)
>  	}
>        else
>  	{
> -	  enum gdb_signal sig;
>  	  struct regcache *regcache;
>  
>  	  infrun_debug_printf
> @@ -5148,10 +5148,7 @@ handle_one (const wait_one_event &event)
>  	  /* Record for later.  */
>  	  save_waitstatus (t, event.ws);
>  
> -	  sig = (event.ws.kind () == TARGET_WAITKIND_STOPPED
> -		 ? event.ws.sig () : GDB_SIGNAL_0);
> -
> -	  if (displaced_step_finish (t, sig)
> +	  if (displaced_step_finish (t, event.ws)
>  	      == DISPLACED_STEP_FINISH_STATUS_NOT_EXECUTED)
>  	    {
>  	      /* Add it back to the step-over queue.  */
> @@ -5753,7 +5750,7 @@ handle_inferior_event (struct execution_control_state *ecs)
>  	       has been done.  Perform cleanup for parent process here.  Note
>  	       that this operation also cleans up the child process for vfork,
>  	       because their pages are shared.  */
> -	    displaced_step_finish (ecs->event_thread, GDB_SIGNAL_TRAP);
> +	    displaced_step_finish (ecs->event_thread, ecs->ws);
>  	    /* Start a new step-over in another thread if there's one
>  	       that needs it.  */
>  	    start_step_over ();
> @@ -6118,7 +6115,7 @@ resumed_thread_with_pending_status (struct thread_info *tp,
>  static int
>  finish_step_over (struct execution_control_state *ecs)
>  {
> -  displaced_step_finish (ecs->event_thread, ecs->event_thread->stop_signal ());
> +  displaced_step_finish (ecs->event_thread, ecs->ws);
>  
>    bool had_step_over_info = step_over_info_valid_p ();
>  
> diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
> index e6ce13a1c67..036ea9f931d 100644
> --- a/gdb/linux-tdep.c
> +++ b/gdb/linux-tdep.c
> @@ -2621,13 +2621,14 @@ linux_displaced_step_prepare (gdbarch *arch, thread_info *thread,
>  /* See linux-tdep.h.  */
>  
>  displaced_step_finish_status
> -linux_displaced_step_finish (gdbarch *arch, thread_info *thread, gdb_signal sig)
> +linux_displaced_step_finish (gdbarch *arch, thread_info *thread,
> +			     const target_waitstatus &status)
>  {
>    linux_info *per_inferior = get_linux_inferior_data (thread->inf);
>  
>    gdb_assert (per_inferior->disp_step_bufs.has_value ());
>  
> -  return per_inferior->disp_step_bufs->finish (arch, thread, sig);
> +  return per_inferior->disp_step_bufs->finish (arch, thread, status);
>  }
>  
>  /* See linux-tdep.h.  */
> diff --git a/gdb/linux-tdep.h b/gdb/linux-tdep.h
> index 16e1b806b26..e09a6ef32b1 100644
> --- a/gdb/linux-tdep.h
> +++ b/gdb/linux-tdep.h
> @@ -72,7 +72,7 @@ extern displaced_step_prepare_status linux_displaced_step_prepare
>  /* Implementation of gdbarch_displaced_step_finish.  */
>  
>  extern displaced_step_finish_status linux_displaced_step_finish
> -  (gdbarch *arch, thread_info *thread, gdb_signal sig);
> +  (gdbarch *arch, thread_info *thread, const target_waitstatus &status);
>  
>  /* Implementation of gdbarch_displaced_step_copy_insn_closure_by_addr.  */
>  
> diff --git a/gdb/rs6000-tdep.c b/gdb/rs6000-tdep.c
> index 104515de030..f9bbc435184 100644
> --- a/gdb/rs6000-tdep.c
> +++ b/gdb/rs6000-tdep.c
> @@ -1088,13 +1088,13 @@ ppc_displaced_step_prepare  (gdbarch *arch, thread_info *thread,
>  
>  static displaced_step_finish_status
>  ppc_displaced_step_finish (gdbarch *arch, thread_info *thread,
> -			   gdb_signal sig)
> +			   const target_waitstatus &status)
>  {
>    ppc_inferior_data *per_inferior = get_ppc_per_inferior (thread->inf);
>  
>    gdb_assert (per_inferior->disp_step_buf.has_value ());
>  
> -  return per_inferior->disp_step_buf->finish (arch, thread, sig);
> +  return per_inferior->disp_step_buf->finish (arch, thread, status);
>  }
>  
>  /* Implementation of gdbarch_displaced_step_restore_all_in_ptid.  */
>
> base-commit: 2562954ede66f32bff7d985e752b8052c2ae5775
> -- 
> 2.36.0