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

Pedro Alves
2014-10-25T14:00:48Z

On 10/15/2014 03:30 PM, Eli Zaretskii wrote:
>> From: Pedro Alves <palves@redhat.com>
>> On 10/14/2014 08:22 PM, Eli Zaretskii wrote:
>>> I think this text mixes 2 different things: (1) how the signal
>>> handling affects whether the signal gets to the program or not, and is
>>> it announced or not; and (2) the fine details of when the signal
>>> becomes "known" to the program and how stepping commands affect and
>>> are affected by that.
>>>
>>> It might be the simplest to separate the two issues, and describe each
>>> one on its own.
>>
>> I'll give that a try.
>
> Thanks. Let me know if I can help.

Alright, here's a new, expanded version.

Let me know what you think.

From f95a5005ede386ce026b095ebeb48f238392fffa Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Sat, 25 Oct 2014 14:14:15 +0100
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/
2014-10-25 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/
2014-10-25 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 entries, and cross references.

gdb/testsuite/
2014-10-25 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/doc/gdb.texinfo | 57 +++++++++++++++++++++++++++++++++++++-
gdb/infrun.c | 7 +++--
gdb/testsuite/gdb.base/sigstep.c | 7 +++--
gdb/testsuite/gdb.base/sigstep.exp | 52 +++++++++++++++++++++++++---------
4 files changed, 104 insertions(+), 19 deletions(-)

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index a1b8ac7..af1c1c7 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 +5575,56 @@ 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. If the signal has
+@code{handle noprint} set, then you won't even hear about it. 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.
+
+@cindex stepping into signal handlers
+@anchor{stepping into signal handlers}
+
+If the program was stopped for a signal (that is, stopped before the
+program sees it), due to @code{handle stop} being set, and
+@code{handle pass} is in effect for that signal too, and your program
+handles the signal, a stepping command such as for example
+@code{stepi} or @code{step} steps @emph{into} the signal's handler (if
+the target supports it).
+
+Likewise, if the @code{queue-signal} command was used 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 steps into the signal's 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
+
@cindex extra signal information
@anchor{extra signal information}

@@ -16654,6 +16706,9 @@ 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..25a4647 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 = 1; while (!done);
done = 0;
}
return 0;
diff --git a/gdb/testsuite/gdb.base/sigstep.exp b/gdb/testsuite/gdb.base/sigstep.exp
index 184d46e..b589e12 100644
--- a/gdb/testsuite/gdb.base/sigstep.exp
+++ b/gdb/testsuite/gdb.base/sigstep.exp
@@ -269,9 +269,35 @@ 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 $next
+}
+
+proc test_skip_handler {prefix i} {
+ if {$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 +321,13 @@ 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 +411,7 @@ 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 +435,10 @@ 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