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.

1 files changed, 80 insertions, 13 deletions

diff --git a/gdb/testsuite/gdb.base/sigstep.exp b/gdb/testsuite/gdb.base/sigstep.exp
index ef98b3d..08ad828 100644
--- a/gdb/testsuite/gdb.base/sigstep.exp
+++ b/gdb/testsuite/gdb.base/sigstep.exp
@@ -36,6 +36,7 @@ if {[build_executable $testfile.exp $testfile $srcfile debug]} {
 
 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.
 
@@ -72,29 +73,48 @@ proc validate_backtrace {} {
 
 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} $" {
@@ -114,6 +134,55 @@ proc advance { cmd } {
     }
 }
 
+# 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
 
@@ -199,11 +268,9 @@ proc advancei { cmd } {
     }
 }
 
-# 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