Re: [PATCH] stepi/nexti: skip signal handler if "handle nostop" signal arrives

[Pedro Alves <palves at redhat dot com>]

On 10/27/2014 07:58 PM, Eli Zaretskii wrote:

>>>> +@cindex stepping into signal handlers
>>>> +@anchor{stepping into signal handlers}
>>>
>>> I would remove this @cindex entry: it doesn't add anything useful to
>>> the previous one, and will likely point to the same page.
>>
>> I'd prefer to keep it, if you don't mind.  The queue-signal reference wants
>> to point here directly, and I can imagine the text above expanding in
>> directions not relevant for that cross reference.
> 
> Both entries use the same words, so why do we need both?  There's one
> paragraph of only a dozen of text lines between them.

I was thinking that "stepping into signal handler" was a concept
on its own.  But I can certainly live without it; removed in the
final version.

>>>> +thread resumes (@pxref{Signaling, ,Giving your Program a Signal}),
>>>> +then a stepping command steps into the signal's handler.
>>>
>>> Not sure I understand the sequence here.  First, I queue-signal, then
>>> the signal is delivered and the thread stops, and _then_ I issue si?
>>> I guess the "when execution of the thread resumes" confused me.
>>
>> Sounds like you're thinking of "queue-signal" like "kill" from the shell,
>> but that's not how "queue-signal" works.  "queue-signal" instead
>> passes the signal to the program immediately as if the thread had
>> _already_ stopped for the signal.  GDB doesn't intercept
>> the signal anymore.
> 
> How is this different from what I wrote?  The program behaves as if
> the signal was delivered to it, right?

Yes, but the "then the signal is delivered and the thread stops" part
doesn't exist.  The thread is (and must be) already stopped.

>> I've added a queue-signal example.  Hopefully that makes things clearer.
> 
> It does, thanks.
> 
>> Let me know how this version looks.
> 
> LGTM.

Great, I've pushed it in then, as below.

Thanks!

>From e5f8a7cc2d376c81749b6e4a4efc034201cf683c Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Mon, 27 Oct 2014 20:24:59 +0000
Subject: [PATCH] stepi/nexti: skip signal handler if "handle nostop" signal
 arrives

I noticed that "si" behaves differently when a "handle nostop" signal
arrives while the step is in progress, depending on whether the
program was stopped at a breakpoint when "si" was entered.
Specifically, in case GDB needs to step off a breakpoint, the handler
is skipped and the program stops in the next "mainline" instruction.
Otherwise, the "si" stops in the first instruction of the signal
handler.

I was surprised the testsuite doesn't catch this difference.  Turns
out gdb.base/sigstep.exp covers a bunch of cases related to stepping
and signal handlers, but does not test stepi nor nexti, only
step/next/continue.

My first reaction was that stopping in the signal handler was the
correct thing to do, as it's where the next user-visible instruction
that is executed is.  I considered then "nexti" -- a signal handler
could be reasonably considered a subroutine call to step over, it'd
seem intuitive to me that "nexti" would skip it.

But then, I realized that signals that arrive while a plain/line
"step" is in progress _also_ have their handler skipped.  A user might
well be excused for being confused by this, given:

  (gdb) help step
  Step program until it reaches a different source line.

And the signal handler's sources will be in different source lines,
after all.

I think that having to explain that "stepi" steps into handlers, (and
that "nexti" wouldn't according to my reasoning above), while "step"
does not, is a sign of an awkward interface.

E.g., if a user truly is interested in stepping into signal handlers,
then it's odd that she has to either force the signal to "handle
stop", or recall to do "stepi" whenever such a signal might be
delivered.  For that use case, it'd seem nicer to me if "step" also
stepped into handlers.

This suggests to me that we either need a global "step-into-handlers"
setting, or perhaps better, make "handle pass/nopass stop/nostop
print/noprint" have have an additional axis - "handle
stepinto/nostepinto", so that the user could configure whether
handlers for specific signals should be stepped into.

In any case, I think it's simpler (and thus better) for all step
commands to behave the same.  This commit thus makes "si/ni" skip
handlers for "handle nostop" signals that arrive while the command was
already in progress, like step/next do.

To be clear, nothing changes if the program was stopped for a signal,
and the user enters a stepping command _then_ -- GDB still steps into
the handler.  The change concerns signals that don't cause a stop and
that arrive while the step is in progress.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* infrun.c (handle_signal_stop): Also skip handlers when a random
	signal arrives while handling a "stepi" or a "nexti".  Set the
	thread's 'step_after_step_resume_breakpoint' flag.

gdb/doc/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Continuing and Stepping): Add cross reference to
	info on stepping and signal handlers.
	(Signals): Explain stepping and signal handlers.  Add context
	index entry, and cross references.

gdb/testsuite/
2014-10-27  Pedro Alves  <palves@redhat.com>

	* gdb.base/sigstep.c (dummy): New global.
	(main): Issue a couple writes to the new global.
	* gdb.base/sigstep.exp (get_next_pc, test_skip_handler): New
	procedures.
	(skip_over_handler): Use test_skip_handler.
	(top level): Call skip_over_handler for stepi and nexti too.
	(breakpoint_over_handler): Use test_skip_handler.
	(top level): Call breakpoint_over_handler for stepi and nexti too.
---
 gdb/ChangeLog                      |  6 ++++
 gdb/doc/ChangeLog                  |  7 ++++
 gdb/testsuite/ChangeLog            | 11 +++++++
 gdb/doc/gdb.texinfo                | 67 +++++++++++++++++++++++++++++++++++++-
 gdb/infrun.c                       |  7 ++--
 gdb/testsuite/gdb.base/sigstep.c   |  7 ++--
 gdb/testsuite/gdb.base/sigstep.exp | 55 +++++++++++++++++++++++--------
 7 files changed, 141 insertions(+), 19 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 32f788c..8a34118 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2014-10-27  Pedro Alves  <palves@redhat.com>
+
+	* infrun.c (handle_signal_stop): Also skip handlers when a random
+	signal arrives while handling a "stepi" or a "nexti".  Set the
+	thread's 'step_after_step_resume_breakpoint' flag.
+
 2014-10-27  Luis Machado  <lgustavo@codesourcery.com>

 	* arm-tdep.c (INSN_S_L_BIT_NUM): Document.
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index f1b2329..f60fd8f 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,10 @@
+2014-10-27  Pedro Alves  <palves@redhat.com>
+
+	* gdb.texinfo (Continuing and Stepping): Add cross reference to
+	info on stepping and signal handlers.
+	(Signals): Explain stepping and signal handlers.  Add context
+	index entry, and cross references.
+
 2014-10-20  Simon Marchi  <simon.marchi@ericsson.com>

 	* python.texi (Breakpoints In Python): Add parenthesis after
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 2eea791..5cc7b0b 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,14 @@
+2014-10-27  Pedro Alves  <palves@redhat.com>
+
+	* gdb.base/sigstep.c (dummy): New global.
+	(main): Issue a couple writes to the new global.
+	* gdb.base/sigstep.exp (get_next_pc, test_skip_handler): New
+	procedures.
+	(skip_over_handler): Use test_skip_handler.
+	(top level): Call skip_over_handler for stepi and nexti too.
+	(breakpoint_over_handler): Use test_skip_handler.
+	(top level): Call breakpoint_over_handler for stepi and nexti too.
+
 2014-10-27  Yao Qi  <yao@codesourcery.com>

 	* gdb.trace/tfile.c (adjust_function_address)
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index a1b8ac7..15c2908 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -5079,7 +5079,9 @@ line of source code, or one machine instruction (depending on what
 particular command you use).  Either when continuing or when stepping,
 your program may stop even sooner, due to a breakpoint or a signal.  (If
 it stops due to a signal, you may want to use @code{handle}, or use
-@samp{signal 0} to resume execution.  @xref{Signals, ,Signals}.)
+@samp{signal 0} to resume execution (@pxref{Signals, ,Signals}),
+or you may step into the signal's handler (@pxref{stepping and signal
+handlers}).)

 @table @code
 @kindex continue
@@ -5573,6 +5575,66 @@ a result of the fatal signal once it saw the signal.  To prevent this,
 you can continue with @samp{signal 0}.  @xref{Signaling, ,Giving your
 Program a Signal}.

+@cindex stepping and signal handlers
+@anchor{stepping and signal handlers}
+
+@value{GDBN} optimizes for stepping the mainline code.  If a signal
+that has @code{handle nostop} and @code{handle pass} set arrives while
+a stepping command (e.g., @code{stepi}, @code{step}, @code{next}) is
+in progress, @value{GDBN} lets the signal handler run and then resumes
+stepping the mainline code once the signal handler returns.  In other
+words, @value{GDBN} steps over the signal handler.  This prevents
+signals that you've specified as not interesting (with @code{handle
+nostop}) from changing the focus of debugging unexpectedly.  Note that
+the signal handler itself may still hit a breakpoint, stop for another
+signal that has @code{handle stop} in effect, or for any other event
+that normally results in stopping the stepping command sooner.  Also
+note that @value{GDBN} still informs you that the program received a
+signal if @code{handle print} is set.
+
+@anchor{stepping into signal handlers}
+
+If you set @code{handle pass} for a signal, and your program sets up a
+handler for it, then issuing a stepping command, such as @code{step}
+or @code{stepi}, when your program is stopped due to the signal will
+step @emph{into} the signal handler (if the target supports that).
+
+Likewise, if you use the @code{queue-signal} command to queue a signal
+to be delivered to the current thread when execution of the thread
+resumes (@pxref{Signaling, ,Giving your Program a Signal}), then a
+stepping command will step into the signal handler.
+
+Here's an example, using @code{stepi} to step to the first instruction
+of @code{SIGUSR1}'s handler:
+
+@smallexample
+(@value{GDBP}) handle SIGUSR1
+Signal        Stop      Print   Pass to program Description
+SIGUSR1       Yes       Yes     Yes             User defined signal 1
+(@value{GDBP}) c
+Continuing.
+
+Program received signal SIGUSR1, User defined signal 1.
+main () sigusr1.c:28
+28        p = 0;
+(@value{GDBP}) si
+sigusr1_handler () at sigusr1.c:9
+9       @{
+@end smallexample
+
+The same, but using @code{queue-signal} instead of waiting for the
+program to receive the signal first:
+
+@smallexample
+(@value{GDBP}) n
+28        p = 0;
+(@value{GDBP}) queue-signal SIGUSR1
+(@value{GDBP}) si
+sigusr1_handler () at sigusr1.c:9
+9       @{
+(@value{GDBP})
+@end smallexample
+
 @cindex extra signal information
 @anchor{extra signal information}

@@ -16654,6 +16716,9 @@ be used to pass a signal whose handling state has been set to @code{nopass}
 @end table
 @c @end group

+@xref{stepping into signal handlers}, for information on how stepping
+commands behave when the thread has a signal queued.
+
 @node Returning
 @section Returning from a Function

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 90a3123..df053e2 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -4463,9 +4463,9 @@ handle_signal_stop (struct execution_control_state *ecs)
 	  return;
 	}

-      if (ecs->event_thread->control.step_range_end != 0
-	  && ecs->event_thread->suspend.stop_signal != GDB_SIGNAL_0
-	  && pc_in_thread_step_range (stop_pc, ecs->event_thread)
+      if (ecs->event_thread->suspend.stop_signal != GDB_SIGNAL_0
+	  && (pc_in_thread_step_range (stop_pc, ecs->event_thread)
+	      || ecs->event_thread->control.step_range_end == 1)
 	  && frame_id_eq (get_stack_frame_id (frame),
 			  ecs->event_thread->control.step_stack_frame_id)
 	  && ecs->event_thread->control.step_resume_breakpoint == NULL)
@@ -4485,6 +4485,7 @@ handle_signal_stop (struct execution_control_state *ecs)
                                 "single-step range\n");

 	  insert_hp_step_resume_breakpoint_at_frame (frame);
+	  ecs->event_thread->step_after_step_resume_breakpoint = 1;
 	  /* Reset trap_expected to ensure breakpoints are re-inserted.  */
 	  ecs->event_thread->control.trap_expected = 0;
 	  keep_going (ecs);
diff --git a/gdb/testsuite/gdb.base/sigstep.c b/gdb/testsuite/gdb.base/sigstep.c
index aa2384a..cc69184 100644
--- a/gdb/testsuite/gdb.base/sigstep.c
+++ b/gdb/testsuite/gdb.base/sigstep.c
@@ -24,6 +24,7 @@
 #include <errno.h>

 static volatile int done;
+static volatile int dummy;

 static void
 handler (int sig)
@@ -74,8 +75,10 @@ main ()
 	      return 1;
 	    }
 	}
-      /* Wait.  */
-      while (!done);
+      /* Wait.  Issue a couple writes to a dummy volatile var to be
+	 reasonably sure our simple "get-next-pc" logic doesn't
+	 stumble on branches.  */
+      dummy = 0; dummy = 0; while (!done);
       done = 0;
     }
   return 0;
diff --git a/gdb/testsuite/gdb.base/sigstep.exp b/gdb/testsuite/gdb.base/sigstep.exp
index 184d46e..53152b8 100644
--- a/gdb/testsuite/gdb.base/sigstep.exp
+++ b/gdb/testsuite/gdb.base/sigstep.exp
@@ -269,9 +269,38 @@ proc skip_to_handler_entry { i } {
     gdb_test "clear *handler" ".*" "$prefix; clear handler"
 }

-skip_to_handler_entry step
-skip_to_handler_entry next
-skip_to_handler_entry continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    skip_to_handler_entry $cmd
+}
+
+# Get the address of where a single-step should land.
+
+proc get_next_pc {test} {
+    global gdb_prompt
+    global hex
+
+    set next ""
+    gdb_test_multiple "x/2i \$pc" $test {
+	-re "$hex .*:\[^\r\n\]+\r\n\[ \]+($hex).*\.\r\n$gdb_prompt $" {
+	    set next $expect_out(1,string)
+	    pass $test
+	}
+    }
+
+    return $next
+}
+
+# Test that the command skipped over the handler.
+
+proc test_skip_handler {prefix i} {
+    if {$i == "stepi" || $i == "nexti"} {
+	set next_pc [get_next_pc "$prefix; get next PC"]
+	gdb_test "$i" "dummy = 0.*" "$prefix; performing $i"
+	gdb_test "p /x \$pc" " = $next_pc" "$prefix; advanced"
+    } else {
+	gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+    }
+}

 # Try stepping when there's a signal pending but no breakpoints.
 # Should skip the handler advancing to the next line.
@@ -295,13 +324,13 @@ proc skip_over_handler { i } {

     # Make the signal pending
     sleep 1
-
-    gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+
+    test_skip_handler $prefix $i
 }

-skip_over_handler step
-skip_over_handler next
-skip_over_handler continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    skip_over_handler $cmd
+}

 # Try stepping when there's a signal pending, a pre-existing
 # breakpoint at the current instruction, and a breakpoint in the
@@ -385,7 +414,7 @@ breakpoint_to_handler_entry continue

 # Try stepping when there's a signal pending, and a pre-existing
 # breakpoint at the current instruction, and no breakpoint in the
-# handler.  Should advance to the next line.
+# handler.  Should advance to the next line/instruction.

 proc breakpoint_over_handler { i } {
     global gdb_prompt
@@ -409,10 +438,10 @@ proc breakpoint_over_handler { i } {
     # Make the signal pending
     sleep 1

-    gdb_test "$i" "done = 0.*" "$prefix; performing $i"
+    test_skip_handler $prefix $i
     gdb_test "clear $infinite_loop" ".*" "$prefix; clear infinite loop"
 }

-breakpoint_over_handler step
-breakpoint_over_handler next
-breakpoint_over_handler continue
+foreach cmd {"stepi" "nexti" "step" "next" "continue"} {
+    breakpoint_over_handler $cmd
+}
-- 
1.9.3