[Bug gdb/17511] New: Program received signal SIGTRAP, after step to signal handler -> step inside handler -> continue

["palves at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17511

            Bug ID: 17511
           Summary: Program received signal SIGTRAP, after step to signal
                    handler -> step inside handler -> continue
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: palves at redhat dot com

While writing a GDB test I noticed that when I have a signal pending/queued,
and I single-step into signal handler, and then issue another step inside the
handler, the following continues will result in spurious SIGTRAPS.

The problem is that $eflags.TF ends stuck/set.

I'm on Fedora 20 (Linux 3.16.4-200.fc20.x86_64).

Vis:

 (gdb) start
 Temporary breakpoint 1, main () at si-handler.c:48
 48        setup ();
 (gdb) next
 50        global = 0; /* set break here */

Let's queue a signal, so we can step into the handler:

 (gdb) handle SIGUSR1
 Signal        Stop      Print   Pass to program Description
 SIGUSR1       Yes       Yes     Yes             User defined signal 1
 (gdb) info inferiors
   Num  Description       Executable
 * 1    process 29953     si-handler
 (gdb) shell kill -SIGUSR1 29953
 (gdb) c
 Continuing.

 Program received signal SIGUSR1, User defined signal 1.
 main () at si-handler.c:50
 50        global = 0; /* set break here */
 (gdb) display $eflags
 1: $eflags = [ PF ZF IF ]

(With mainline GDB, you can instead just do "queue-signal SIGUSR1".)

Now step into the handler -- "si" does PTRACE_SINGLESTEP+SIGUSR1:

 (gdb) si
 sigusr1_handler (sig=0) at si-handler.c:31
 31      {
 1: $eflags = [ PF ZF IF ]

Looks fine so far.  But another single-step...

 (gdb) si
 0x0000000000400621      31      {
 1: $eflags = [ PF ZF TF IF ]

... ends up with TF left set.  This results in PTRACE_CONTINUE trapping
after each instruction is executed:

 (gdb) c
 Continuing.

 Program received signal SIGTRAP, Trace/breakpoint trap.
 0x0000000000400624 in sigusr1_handler (sig=0) at si-handler.c:31
 31      {
 1: $eflags = [ PF ZF TF IF ]

 (gdb) c
 Continuing.

 Program received signal SIGTRAP, Trace/breakpoint trap.
 sigusr1_handler (sig=10) at si-handler.c:32
 32        global = 0;
 1: $eflags = [ PF ZF TF IF ]
 (gdb)

Note that even another PTRACE_SINGLESTEP does not fix it:

 (gdb) si
 33      }
 1: $eflags = [ PF ZF TF IF ]
 (gdb)

Eventually, it gets "fixed" by the rt_sigreturn syscall, when returning
out of the handler:

 (gdb) bt
 #0  sigusr1_handler (sig=10) at si-handler.c:33
 #1  <signal handler called>
 #2  main () at si-handler.c:50
 (gdb) set disassemble-next-line on
 (gdb) si
 0x0000000000400632      33      }
    0x0000000000400631 <sigusr1_handler+17>:     5d      pop    %rbp
 => 0x0000000000400632 <sigusr1_handler+18>:     c3      retq
 1: $eflags = [ PF ZF TF IF ]
 (gdb)
 <signal handler called>
 => 0x0000003b36a358f0 <__restore_rt+0>: 48 c7 c0 0f 00 00 00    mov   
$0xf,%rax
 1: $eflags = [ PF ZF TF IF ]
 (gdb) si
 <signal handler called>
 => 0x0000003b36a358f7 <__restore_rt+7>: 0f 05   syscall
 1: $eflags = [ PF ZF TF IF ]
 (gdb)
 main () at si-handler.c:50
 50        global = 0; /* set break here */
 => 0x000000000040066b <main+9>: c7 05 cb 09 20 00 00 00 00 00   movl  
$0x0,0x2009cb(%rip)        # 0x601040 <global>
 1: $eflags = [ PF ZF IF ]
 (gdb)

I don't get the bug if I instead PTRACE_CONTINUE into the signal
handler -- e.g., set a breakpoint in the handler, queue a signal,
and "continue".

Below's the code I was using to test this.

~~~~
/* This testcase is part of GDB, the GNU debugger.

   Copyright 2014 Free Software Foundation, Inc.

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

#include <signal.h>

volatile int global;

static void
signal_handler (int sig)
{
  global = 0;
  global = 0;
  global = 0;
  global = 0;
  global = 0;
}

void
setup (void)
{
  /* Set up the signal handler.  */
  signal (SIGUSR1, signal_handler);
}

void
begin (void)
{
}

void
end (void)
{
}

int
main (void)
{
  setup ();
  begin ();
  end ();
  return 0;
}
~~~~

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