Re: [PATCH 2/7] Deliver signal in hardware single step

Yao Qi
2016-04-22T10:54:42Z

Pedro Alves <palves@redhat.com> writes:

> making test result diffing unstable. If so, I think it should
> either be written as:
>
> pass "tracepoint $i hit $iterations times (spurious collection)"
>

I prefer this.

> making use of the rule that terminating " (...)" bits don't really
> count as test message, or always:
>
> pass "tracepoint $i hit $iterations times"
>
> Otherwise LGTM.

Patch below is pushed in.

--
Yao (齐尧)
From b3273b50e69dda8ac50278ab9cc57e67e4f45465 Mon Sep 17 00:00:00 2001
From: Yao Qi <yao.qi@linaro.org>
Date: Thu, 3 Mar 2016 08:58:52 +0000
Subject: [PATCH] Deliver signal in hardware single step

GDBserver doesn't deliver signal when stepping over a breakpoint even
hardware single step is used. When GDBserver started to step over
(thread creation) breakpoint for mutlit-threaded debugging in 2002 [1],
GDBserver behaves this way.

This behavior gets trouble on conditional breakpoints on branch to
self instruction like this,

0x00000000004005b6 <+29>: jmp 0x4005b6 <main+29>

and I set breakpoint

$(gdb) break branch-to-self.c:43 if counter > 3

and the variable counter will be set to 5 in SIGALRM signal handler.
Since GDBserver keeps stepping over breakpoint, the SIGALRM can never
be dequeued and delivered to the inferior, so the program can't stop.
The test can be found in gdb.base/branch-to-self.exp.

GDBserver didn't deliver signal when stepping over a breakpoint because
a tracepoint is collected twice if GDBserver does so in the following
scenario, which can be reproduced by gdb.trace/signal.exp.

- program stops at tracepoint, and tracepoint is collected,
- gdbserver starts a step-over,
- a signal arrives, step-over is canceled, and signal should be passed,
- gdbserver starts a new step-over again, pass the signal as well,
- program stops at the entry of signal handler, step-over finished,
- gdbserver proceeds,
- program returns from the signal handler, again to the tracepoint,
and thus is collected again.

The spurious collection isn't that harmful, IMO, so it should be OK
to let GDBserver deliver signal when stepping over a breakpoint.

gdb/gdbserver:

2016-04-22 Yao Qi <yao.qi@linaro.org>

* linux-low.c (lwp_signal_can_be_delivered): Don't deliver
signal when stepping over breakpoint with software single
step.

gdb/testsuite:

2016-04-22 Yao Qi <yao.qi@linaro.org>

* gdb.trace/signal.exp: Also pass if
$tracepoint_hits($i) > $iterations.

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 2cf192e..7e55349 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -4118,13 +4118,17 @@ single_step (struct lwp_info* lwp)
}

/* The signal can be delivered to the inferior if we are not trying to
- reinsert a breakpoint and not trying to finish a fast tracepoint
- collect. */
+ reinsert a breakpoint for software single step and not trying to
+ finish a fast tracepoint collect. Since signal can be delivered in
+ the step-over, the program may go to signal handler and trap again
+ after return from the signal handler. We can live with the spurious
+ double traps. */

static int
lwp_signal_can_be_delivered (struct lwp_info *lwp)
{
- return (lwp->bp_reinsert == 0 && !lwp->collecting_fast_tracepoint);
+ return (!(lwp->bp_reinsert != 0 && can_software_single_step ())
+ && !lwp->collecting_fast_tracepoint);
}

/* Resume execution of LWP. If STEP is nonzero, single-step it. If
diff --git a/gdb/testsuite/gdb.trace/signal.exp b/gdb/testsuite/gdb.trace/signal.exp
index 7118a9f..48e495e 100644
--- a/gdb/testsuite/gdb.trace/signal.exp
+++ b/gdb/testsuite/gdb.trace/signal.exp
@@ -174,6 +174,14 @@ while { 1 } {
# Step 3, check the number of collections on each tracepoint.

for { set i $tpnum } { $i < [expr $tpnum + 2] } { incr i } {
- gdb_assert { $tracepoint_hits($i) == $iterations } \
- "tracepoint $i hit $iterations times"
+
+ if { $tracepoint_hits($i) == $iterations } {
+ pass "tracepoint $i hit $iterations times"
+ } elseif { $tracepoint_hits($i) > $iterations } {
+ # GDBserver deliver the signal while stepping over tracepoint,
+ # so it is possible that a tracepoint is collected twice.
+ pass "tracepoint $i hit $iterations times (spurious collection)"
+ } else {
+ fail "tracepoint $i hit $iterations times"
+ }
}