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/ChangeLog b/gdb/gdbserver/ChangeLog
index db76e1a0ccb831371b435a9c445855e8e141ce79..e0ed616aba959b87548fe773a31a785eaeba1770 100644 (file)
--- a/gdb/gdbserver/ChangeLog
+++ b/gdb/gdbserver/ChangeLog
@@ -1,3 +1,9 @@
+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.
+
 2016-04-21  Pedro Alves  <palves@redhat.com>
 
        * linux-s390-low.c (s390_collect_ptrace_register)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 7630f9f81a47b04d7484520781c444298902028d..5cbd3088ef14bf46d684ce479ed4cb37ef0d9b2c 100644 (file)
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -4119,13 +4119,17 @@ 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/ChangeLog b/gdb/testsuite/ChangeLog
index 29447afc6bf6291cf25590f44830531c7d740ee5..249cbc0b09576813f4f49f39e645832a5da4439a 100644 (file)
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-04-22  Yao Qi  <yao.qi@linaro.org>
+
+       * gdb.trace/signal.exp: Also pass if
+       $tracepoint_hits($i) > $iterations.
+
 2016-04-22  Yao Qi  <yao.qi@linaro.org>
 
        * gdb.trace/signal.c: New file.

diff --git a/gdb/testsuite/gdb.trace/signal.exp b/gdb/testsuite/gdb.trace/signal.exp
index 7118a9fea75ca39ada7e8522a69fa0348de13f3b..48e495ee78e5a1106e7271ee3aad206f40e3938a 100644 (file)
--- 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"
+    }
 }