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

Pedro Alves
2023-03-10T17:15:27Z

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