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.
+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.
+
2014-10-28 Pedro Alves <palves@redhat.com>
* gdb.base/sigstep.exp: Use build_executable instead of
static void
handler (int sig)
{
+ /* This is more than one write so that the breakpoint location below
+ is more than one instruction away. */
done = 1;
+ done = 1;
+ done = 1;
+ done = 1; /* other handler location */
} /* handler */
struct itimerval itime;
set clear_done [gdb_get_line_number {done = 0}]
set infinite_loop [gdb_get_line_number {while (!done)}]
+set other_handler_location [gdb_get_line_number "other handler location"]
# Restart GDB, set a display showing $PC, and run to main.
validate_backtrace
-proc advance { cmd } {
+# Goes to handler using ENTER_CMD, runs IN_HANDLER while in the signal
+# hander, and then steps out of the signal handler using EXIT_CMD.
+
+proc advance { enter_cmd in_handler_prefix in_handler exit_cmd } {
global gdb_prompt inferior_exited_re
+ global clear_done other_handler_location
- with_test_prefix "$cmd from handler" {
- restart
+ set prefix "$enter_cmd to handler, $in_handler_prefix in handler, $exit_cmd from handler"
- gdb_test "break handler"
+ with_test_prefix $prefix {
+ restart
# Get us into the handler
- gdb_test "continue" ".* handler .*" "continue to handler"
+ if { $enter_cmd == "continue" } {
+ gdb_test "break handler"
+ } else {
+ gdb_test "handle SIGALRM print pass stop"
+ gdb_test "handle SIGVTALRM print pass stop"
+ gdb_test "continue" "Program received signal.*" "continue to signal"
+ }
+ gdb_test "$enter_cmd" ".*handler .*" "$enter_cmd to handler"
+
+ delete_breakpoints
+
+ uplevel 1 $in_handler
+
+ if { $exit_cmd == "continue" } {
+ gdb_test "break $clear_done" ".*" "break clear done"
+ }
set test "leave handler"
- gdb_test_multiple "$cmd" "${test}" {
+ gdb_test_multiple "$exit_cmd" "${test}" {
-re "Could not insert single-step breakpoint.*$gdb_prompt $" {
setup_kfail gdb/8841 "sparc*-*-openbsd*"
fail "$test (could not insert single-step breakpoint)"
}
-re "done = 1;.*${gdb_prompt} $" {
- send_gdb "$cmd\n"
+ send_gdb "$exit_cmd\n"
exp_continue -continue_timer
}
-re "\} .. handler .*${gdb_prompt} $" {
- send_gdb "$cmd\n"
+ send_gdb "$exit_cmd\n"
exp_continue -continue_timer
}
-re "$inferior_exited_re normally.*${gdb_prompt} $" {
}
}
+# Map of PREFIX => "things to do within the signal handler", for the
+# advance tests.
+
+set in_handler_map {
+ "nothing" {
+ }
+ "si+advance" {
+ # Advance to the second location in handler.
+ gdb_test "si" "handler.*" "si in handler"
+
+ set test "advance in handler"
+ gdb_test_multiple "advance $other_handler_location" $test {
+ -re "Program received signal SIGTRAP.*$gdb_prompt $" {
+ # On some versions of Linux (observed on
+ # 3.16.4-200.fc20.x86_64), using PTRACE_SINGLESTEP+sig
+ # to step into a signal handler, and then issuing
+ # another PTRACE_SINGLESTEP within the handler ends up
+ # with $eflags.TF mistakenly set, which results in
+ # subsequent PTRACE_CONTINUEs trapping after each
+ # insn.
+ if {$enter_cmd != "continue"} {
+ setup_xfail "x86_64-*-linux*" gdb/17511
+ }
+ fail "$test (spurious SIGTRAP)"
+ return
+ }
+ -re "other handler location.*$gdb_prompt $" {
+ pass $test
+ }
+ }
+ }
+}
+
+# Check that we can step/next/continue, etc. our way in and out of a
+# signal handler. Also test that we can step, and run to a breakpoint
+# within the handler.
+
+foreach enter_cmd { "stepi" "nexti" "step" "next" "continue" } {
+ if { $enter_cmd != "continue" && ![can_single_step_to_signal_handler] } {
+ continue
+ }
+
+ foreach exit_cmd { "step" "next" "continue" } {
+ foreach {in_handler_prefix in_handler} $in_handler_map {
+ advance $enter_cmd $in_handler_prefix $in_handler $exit_cmd
+ }
+ }
+}
+
proc advancei { cmd } {
global gdb_prompt inferior_exited_re
}
}
-# Check that we can step/next our way out of a signal handler.
-
-foreach cmd {"step" "next"} {
- advance $cmd
-}
+# Check that we can step our way out of a signal handler, using
+# commands that first step out to the signal trampoline, and then out
+# to the mainline code.
foreach cmd {"stepi" "nexti" "finish" "return"} {
advancei $cmd
projects / binutils-gdb.git / commitdiff