Test for PR gdb/17511, spurious SIGTRAP after stepping into+in signal handler

Test for PR gdb/17511, spurious SIGTRAP after stepping into+in signal handler

I noticed that when I single-step into a signal handler with a
pending/queued signal, the following single-steps while the program is
in the signal handler leave $eflags.TF set.  That means subsequent
continues will trap after one instruction, resulting in a spurious
SIGTRAP being reported to the user.

This is a kernel bug; I've reported it to kernel devs (turned out to
be a known bug).  I'm seeing it on x86_64 Fedora 20 (Linux
3.16.4-200.fc20.x86_64), and I was told it's still not fixed upstream.

This commit extends gdb.base/sigstep.exp to cover this use case,
xfailed.

Here's what the bug looks like:

 (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) queue-signal SIGUSR1

TF is not set:

 (gdb) display $eflags
 1: $eflags = [ PF ZF IF ]

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 ]

No TF yet.  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)

The bug doesn't happen if we instead PTRACE_CONTINUE into the signal
handler -- e.g., set a breakpoint in the handler, queue a signal, and
"continue".

gdb/testsuite/
2014-10-28  Pedro Alves  <palves@redhat.com>

	PR gdb/17511
	* gdb.base/sigstep.c (handler): Add a few more writes to 'done'.
	* gdb.base/sigstep.exp (other_handler_location): New global.
	(advance): Support stepping into the signal handler, and running
	commands while in the handler.
	(in_handler_map): New global.
	(top level): In the advance test, add combinations for getting
	into the handler with stepping commands, and for running commands
	in the handler.  Add comment descripting the advancei tests.