abbdbd03db
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.
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