btrace, frame: fix crash in get_frame_type

In skip_artificial_frames we repeatedly call get_prev_frame_always until we get
a non-inline and non-tailcall frame assuming that there must be such a frame
eventually.

For record targets, however, we may have a frame chain that consists only of
artificial frames.  This leads to a crash in get_frame_type when dereferencing a
NULL frame pointer.

Change skip_artificial_frames and skip_tailcall_frames to return NULL in such a
case and modify each caller to cope with a NULL return.

In frame_unwind_caller_pc and frame_unwind_caller_arch, we simply assert that
the returned value is not NULL.  Their caller was supposed to check
frame_unwind_caller_id before calling those functions.

In other cases, we thrown an error.

In infcmd further move the skip_tailcall_frames call to the forward-stepping
case since we don't need a frame for reverse execution and we don't want to fail
because of that.  Reverse-finish does make sense for a tailcall frame.

gdb/
	* frame.h (skip_tailcall_frames): Update comment.
	* frame.c (skip_artificial_frames, skip_tailcall_frames): Return NULL
	if only	artificial frames are found.  Update comment.
	(frame_unwind_caller_id): Handle NULL return.
	(frame_unwind_caller_pc, frame_unwind_caller_arch): Assert that
	skip_artificial_frames does not return NULL.
	(frame_pop): Add an error if only tailcall frames are found.
	* infcmd.c (finish_command): Move skip_tailcall_frames call into forward-
	execution case.  Add an error if only tailcall frames are found.

testsuite/
	* gdb.btrace/tailcall-only.exp: New.
	* gdb.btrace/tailcall-only.c: New.
	* gdb.btrace/x86_64-tailcall-only.S: New.
	* gdb.btrace/i686-tailcall-only.S: New.

1 files changed, 53 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.btrace/tailcall-only.c b/gdb/testsuite/gdb.btrace/tailcall-only.c
new file mode 100644
index 0000000..5913ddc
--- /dev/null
+++ b/gdb/testsuite/gdb.btrace/tailcall-only.c
@@ -0,0 +1,53 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   Contributed by Intel Corp. <markus.t.metzger@intel.com>
+
+   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/>.  */
+
+static __attribute__ ((noinline)) int
+bar_1 (void)
+{
+  return 42;
+}
+
+static __attribute__ ((noinline)) int
+bar (void)
+{
+  return bar_1 ();
+}
+
+static __attribute__ ((noinline)) int
+foo_1 (void)
+{
+  return bar ();
+}
+
+static __attribute__ ((noinline)) int
+foo (void)
+{
+  return foo_1 ();
+}
+
+int
+main (void)
+{
+  int answer;
+
+  answer = foo ();
+  answer += 1;
+
+  return answer;
+}