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

[Pedro Alves <pedro@palves.net>]

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?

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