* scripttempl/avr.sc: Set .data section's LMA to next available
	address in text region.
Don't let two frames with the same id end up in the frame chain.

The UNWIND_SAME_ID check is done between THIS_FRAME and the next frame
when we go try to unwind the previous frame.  But at this point, it's
already too late -- we ended up with two frames with the same ID in
the frame chain.  Each frame having its own ID is an invariant assumed
throughout GDB.  This patch applies the UNWIND_SAME_ID detection
earlier, right after the previous frame is unwound, discarding the dup
frame if a cycle is detected.

The patch includes a new test that fails before the change.  Before
the patch, the test causes an infinite loop in GDB, after the patch,
the UNWIND_SAME_ID logic kicks in and makes the backtrace stop with:

  Backtrace stopped: previous frame identical to this frame (corrupt stack?)

The test uses dwarf CFI to emulate a corrupted stack with a cycle.  It
has a function with registers marked DW_CFA_same_value (most
importantly RSP/RIP), so that GDB computes the same ID for that frame
and its caller.  IOW, something like this:

 #0 - frame_id_1
 #1 - frame_id_2
 #2 - frame_id_3
 #3 - frame_id_4
 #4 - frame_id_4  <<<< outermost (UNWIND_SAME_ID).

(The test's code is just a copy of dw2-reg-undefined.S /
dw2-reg-undefined.c, adjusted to use DW_CFA_same_value instead of
DW_CFA_undefined, and to mark a different set of registers.)

The infinite loop is here, in value_fetch_lazy:

      while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
	{
	  frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
...
	  new_val = get_frame_register_value (frame, regnum);
	}

get_frame_register_value can return a lazy register value pointing to
the next frame.  This means that the register wasn't clobbered by
FRAME; the debugger should therefore retrieve its value from the next
frame.

To be clear, get_frame_register_value unwinds the value in question
from the next frame:

 struct value *
 get_frame_register_value (struct frame_info *frame, int regnum)
 {
   return frame_unwind_register_value (frame->next, regnum);
                                       ^^^^^^^^^^^
 }

In other words, if we get a lazy lval_register, it should have the
frame ID of the _next_ frame, never of FRAME.

At this point in value_fetch_lazy, the whole relevant chunk of the
stack up to frame #4 has already been unwound.  The loop always
"unlazies" lval_registers in the "next/innermost" direction, not in
the "prev/unwind further/outermost" direction.

So say we're looking at frame #4.  get_frame_register_value in frame
#4 can return a lazy register value of frame #3.  So the next
iteration, frame_find_by_id tries to read the register from frame #3.
But, since frame #4 happens to have same id as frame #3,
frame_find_by_id returns frame #4 instead.  Rinse, repeat, and we have
an infinite loop.

This is an old latent problem, exposed by the recent addition of the
frame stash.  Before we had a stash, frame_find_by_id(frame_id_4)
would walk over all frames starting at the current frame, and would
always find #3 first.  The stash happens to return #4 instead:

struct frame_info *
frame_find_by_id (struct frame_id id)
{
  struct frame_info *frame, *prev_frame;

...
  /* Try using the frame stash first.  Finding it there removes the need
     to perform the search by looping over all frames, which can be very
     CPU-intensive if the number of frames is very high (the loop is O(n)
     and get_prev_frame performs a series of checks that are relatively
     expensive).  This optimization is particularly useful when this function
     is called from another function (such as value_fetch_lazy, case
     VALUE_LVAL (val) == lval_register) which already loops over all frames,
     making the overall behavior O(n^2).  */
  frame = frame_stash_find (id);
  if (frame)
    return frame;

  for (frame = get_current_frame (); ; frame = prev_frame)
    {

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* frame.c (get_prev_frame_1): Do the UNWIND_SAME_ID check between
	this frame and the new previous frame, not between this frame and
	the next frame.

gdb/testsuite/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* gdb.dwarf2/dw2-dup-frame.S: New file.
 	* gdb.dwarf2/dw2-dup-frame.c: New file.
 	* gdb.dwarf2/dw2-dup-frame.exp: New file.
Make use of the frame stash to detect wider stack cycles.

Tested on x86_64 Fedora 17.

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>
	    Tom Tromey  <tromey@redhat.com>

	* frame.c (frame_stash_add): Now returns whether a frame with the
	same ID was already known.
	(compute_frame_id): New function, factored out from get_frame_id.
	(get_frame_id): No longer lazilly compute the frame id here.
	(get_prev_frame_if_no_cycle): New function.  Detects wider stack
	cycles.
	(get_prev_frame_1): Use it instead of get_prev_frame_raw directly,
	and checking for stack cycles here.
Eliminate dwarf2_frame_cache recursion, don't unwind from the dwarf2 sniffer (move dwarf2_tailcall_sniffer_first elsewhere).

Two rationales, same patch.

TL;DR 1:

 dwarf2_frame_cache recursion is evil.  dwarf2_frame_cache calls
 dwarf2_tailcall_sniffer_first which then recurses into
 dwarf2_frame_cache.

TL;DR 2:

 An unwinder trying to unwind is evil.  dwarf2_frame_sniffer calls
 dwarf2_frame_cache which calls dwarf2_tailcall_sniffer_first which
 then tries to unwind the PC of the previous frame.

Avoid all that by deferring dwarf2_tailcall_sniffer_first until it's
really necessary.

Rationale 1
===========

A frame sniffer should not try to unwind, because that bypasses all
the validation checks done by get_prev_frame.  The UNWIND_SAME_ID
scenario is one such case where GDB is currently broken because (in
part) of this (the next patch adds a test that would fail without
this).

GDB goes into an infinite loop in value_fetch_lazy, here:

      while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
	{
	  frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
...
	  new_val = get_frame_register_value (frame, regnum);
	}

(top-gdb) bt
#0  value_fetch_lazy (val=0x11516d0) at ../../src/gdb/value.c:3510
#1  0x0000000000584bd8 in value_optimized_out (value=0x11516d0) at ../../src/gdb/value.c:1096
#2  0x00000000006fe7a1 in frame_register_unwind (frame=0x1492600, regnum=16, optimizedp=0x7fffffffcdec, unavailablep=0x7fffffffcde8, lvalp=0x7fffffffcdd8, addrp=
    0x7fffffffcde0, realnump=0x7fffffffcddc, bufferp=0x7fffffffce10 "@\316\377\377\377\177") at ../../src/gdb/frame.c:940
#3  0x00000000006fea3a in frame_unwind_register (frame=0x1492600, regnum=16, buf=0x7fffffffce10 "@\316\377\377\377\177") at ../../src/gdb/frame.c:990
#4  0x0000000000473b9b in i386_unwind_pc (gdbarch=0xf54660, next_frame=0x1492600) at ../../src/gdb/i386-tdep.c:1771
#5  0x0000000000601dfa in gdbarch_unwind_pc (gdbarch=0xf54660, next_frame=0x1492600) at ../../src/gdb/gdbarch.c:2870
#6  0x0000000000693db5 in dwarf2_tailcall_sniffer_first (this_frame=0x1492600, tailcall_cachep=0x14926f0, entry_cfa_sp_offsetp=0x7fffffffcf00)
    at ../../src/gdb/dwarf2-frame-tailcall.c:389
#7  0x0000000000690928 in dwarf2_frame_cache (this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/dwarf2-frame.c:1245
#8  0x0000000000690f46 in dwarf2_frame_sniffer (self=0x8e4980, this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/dwarf2-frame.c:1423
#9  0x000000000070203b in frame_unwind_find_by_frame (this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/frame-unwind.c:112
#10 0x00000000006fd681 in get_frame_id (fi=0x1492600) at ../../src/gdb/frame.c:408
#11 0x00000000007006c2 in get_prev_frame_1 (this_frame=0xdc1860) at ../../src/gdb/frame.c:1826
#12 0x0000000000700b7a in get_prev_frame (this_frame=0xdc1860) at ../../src/gdb/frame.c:2056
#13 0x0000000000514588 in frame_info_to_frame_object (frame=0xdc1860) at ../../src/gdb/python/py-frame.c:322
#14 0x000000000051784c in bootstrap_python_frame_filters (frame=0xdc1860, frame_low=0, frame_high=-1) at ../../src/gdb/python/py-framefilter.c:1396
#15 0x0000000000517a6f in apply_frame_filter (frame=0xdc1860, flags=7, args_type=CLI_SCALAR_VALUES, out=0xed7a90, frame_low=0, frame_high=-1)
    at ../../src/gdb/python/py-framefilter.c:1492
#16 0x00000000005e77b0 in backtrace_command_1 (count_exp=0x0, show_locals=0, no_filters=0, from_tty=1) at ../../src/gdb/stack.c:1777
#17 0x00000000005e7c0f in backtrace_command (arg=0x0, from_tty=1) at ../../src/gdb/stack.c:1891
#18 0x00000000004e37a7 in do_cfunc (c=0xda4fa0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:107
#19 0x00000000004e683c in cmd_func (cmd=0xda4fa0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:1882
#20 0x00000000006f35ed in execute_command (p=0xcc66c2 "", from_tty=1) at ../../src/gdb/top.c:468
#21 0x00000000005f8853 in command_handler (command=0xcc66c0 "bt") at ../../src/gdb/event-top.c:435
#22 0x00000000005f8e12 in command_line_handler (rl=0xfe05f0 "@") at ../../src/gdb/event-top.c:632
#23 0x000000000074d2c6 in rl_callback_read_char () at ../../src/readline/callback.c:220
#24 0x00000000005f8375 in rl_callback_read_char_wrapper (client_data=0x0) at ../../src/gdb/event-top.c:164
#25 0x00000000005f876a in stdin_event_handler (error=0, client_data=0x0) at ../../src/gdb/event-top.c:375
#26 0x00000000005f72fa in handle_file_event (data=...) at ../../src/gdb/event-loop.c:768
#27 0x00000000005f67a3 in process_event () at ../../src/gdb/event-loop.c:342
#28 0x00000000005f686a in gdb_do_one_event () at ../../src/gdb/event-loop.c:406
#29 0x00000000005f68bb in start_event_loop () at ../../src/gdb/event-loop.c:431
#30 0x00000000005f83a7 in cli_command_loop (data=0x0) at ../../src/gdb/event-top.c:179
#31 0x00000000005eeed3 in current_interp_command_loop () at ../../src/gdb/interps.c:327
#32 0x00000000005ef8ff in captured_command_loop (data=0x0) at ../../src/gdb/main.c:267
#33 0x00000000005ed2f6 in catch_errors (func=0x5ef8e4 <captured_command_loop>, func_args=0x0, errstring=0x8b6554 "", mask=RETURN_MASK_ALL)
    at ../../src/gdb/exceptions.c:524
#34 0x00000000005f0d21 in captured_main (data=0x7fffffffd9e0) at ../../src/gdb/main.c:1067
#35 0x00000000005ed2f6 in catch_errors (func=0x5efb9b <captured_main>, func_args=0x7fffffffd9e0, errstring=0x8b6554 "", mask=RETURN_MASK_ALL)
    at ../../src/gdb/exceptions.c:524
#36 0x00000000005f0d57 in gdb_main (args=0x7fffffffd9e0) at ../../src/gdb/main.c:1076
#37 0x000000000045bb6a in main (argc=4, argv=0x7fffffffdae8) at ../../src/gdb/gdb.c:34
(top-gdb)

GDB is trying to unwind the PC register of the previous frame (frame
#5 above), starting from the frame being sniffed (the THIS frame).
But the THIS frame's unwinder says the PC of the previous frame is
actually the same as the previous's frame's next frame (which is the
same frame we started with, the THIS frame), therefore it returns an
lval_register lazy value with frame set to THIS frame.  And so the
value_fetch_lazy loop never ends.


Rationale 2
===========

As an experiment, I tried making dwarf2-frame.c:read_addr_from_reg use
address_from_register.  That caused a bunch of regressions, but it
actually took me a long while to figure out what was going on.  Turns
out dwarf2-frame.c:read_addr_from_reg is called while computing the
frame's CFA, from within dwarf2_frame_cache.  address_from_register
wants to create a register with frame_id set to the frame being
constructed.  To create the frame id, we again call dwarf2_frame_cache,
which given:

static struct dwarf2_frame_cache *
dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
{
...
  if (*this_cache)
    return *this_cache;

returns an incomplete object to the caller:
static void
dwarf2_frame_this_id (struct frame_info *this_frame, void **this_cache,
		      struct frame_id *this_id)
{
  struct dwarf2_frame_cache *cache =
    dwarf2_frame_cache (this_frame, this_cache);
...
 (*this_id) = frame_id_build (cache->cfa, get_frame_func (this_frame));
}

As cache->cfa is still 0 (we were trying to compute it!), and
get_frame_id recalls this id from here on, we end up with a broken
frame id in recorded for this frame.  Later, when inspecting locals,
the dwarf machinery needs to know the selected frame's base, which
calls get_frame_base:

CORE_ADDR
get_frame_base (struct frame_info *fi)
{
  return get_frame_id (fi).stack_addr;
}

which as seen above then returns 0 ...

So I gave up using address_from_register.

But, the pain of investigating this made me want to have GDB itself
assert that recursion never happens here.  So I wrote a patch to do
that.  But, it triggers on current mainline, because
dwarf2_tailcall_sniffer_first, called from dwarf2_frame_cache, unwinds
the this_frame.

A sniffer shouldn't be trying to unwind, exactly because of this sort
of tricky issue.  The patch defers calling
dwarf2_tailcall_sniffer_first until it's really necessary, in
dwarf2_frame_prev_register (thus actually outside the sniffer path).
As this makes the call to dwarf2_frame_sniffer in dwarf2_frame_cache
unnecessary again, the patch removes that too.

Tested on x86_64 Fedora 17.

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* dwarf2-frame.c (struct dwarf2_frame_cache)
	<checked_tailcall_bottom, entry_cfa_sp_offset,
	entry_cfa_sp_offset_p>: New fields.
	(dwarf2_frame_cache): Adjust to use the new cache fields instead
	of locals.  Don't call dwarf2_tailcall_sniffer_first here.
	(dwarf2_frame_prev_register): Call it here, but only once.
Revert "Eliminate dwarf2_frame_cache recursion, don't unwind from the dwarf2 sniffer (move dwarf2_tailcall_sniffer_first elsewhere)."

This reverts commit 1dc8686c48e72fc02723d44ee0fecde0d233c74e.
Revert "Make use of the frame stash to detect wider stack cycles."

This reverts commit f5b0ed3c8ce42b0dd6b6caa0b3d7b7e734311afe.
Revert "Don't let two frames with the same id end up in the frame chain."

This reverts commit be2c48b4d50b992ba83bc51f086e316621a03a14.
Eliminate dwarf2_frame_cache recursion, don't unwind from the dwarf2 sniffer (move dwarf2_tailcall_sniffer_first elsewhere).

Two rationales, same patch.

TL;DR 1:

 dwarf2_frame_cache recursion is evil.  dwarf2_frame_cache calls
 dwarf2_tailcall_sniffer_first which then recurses into
 dwarf2_frame_cache.

TL;DR 2:

 An unwinder trying to unwind is evil.  dwarf2_frame_sniffer calls
 dwarf2_frame_cache which calls dwarf2_tailcall_sniffer_first which
 then tries to unwind the PC of the previous frame.

Avoid all that by deferring dwarf2_tailcall_sniffer_first until it's
really necessary.

Rationale 1
===========

A frame sniffer should not try to unwind, because that bypasses all
the validation checks done by get_prev_frame.  The UNWIND_SAME_ID
scenario is one such case where GDB is currently broken because (in
part) of this (the next patch adds a test that would fail without
this).

GDB goes into an infinite loop in value_fetch_lazy, here:

      while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
	{
	  frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
...
	  new_val = get_frame_register_value (frame, regnum);
	}

(top-gdb) bt
#0  value_fetch_lazy (val=0x11516d0) at ../../src/gdb/value.c:3510
#1  0x0000000000584bd8 in value_optimized_out (value=0x11516d0) at ../../src/gdb/value.c:1096
#2  0x00000000006fe7a1 in frame_register_unwind (frame=0x1492600, regnum=16, optimizedp=0x7fffffffcdec, unavailablep=0x7fffffffcde8, lvalp=0x7fffffffcdd8, addrp=
    0x7fffffffcde0, realnump=0x7fffffffcddc, bufferp=0x7fffffffce10 "@\316\377\377\377\177") at ../../src/gdb/frame.c:940
#3  0x00000000006fea3a in frame_unwind_register (frame=0x1492600, regnum=16, buf=0x7fffffffce10 "@\316\377\377\377\177") at ../../src/gdb/frame.c:990
#4  0x0000000000473b9b in i386_unwind_pc (gdbarch=0xf54660, next_frame=0x1492600) at ../../src/gdb/i386-tdep.c:1771
#5  0x0000000000601dfa in gdbarch_unwind_pc (gdbarch=0xf54660, next_frame=0x1492600) at ../../src/gdb/gdbarch.c:2870
#6  0x0000000000693db5 in dwarf2_tailcall_sniffer_first (this_frame=0x1492600, tailcall_cachep=0x14926f0, entry_cfa_sp_offsetp=0x7fffffffcf00)
    at ../../src/gdb/dwarf2-frame-tailcall.c:389
#7  0x0000000000690928 in dwarf2_frame_cache (this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/dwarf2-frame.c:1245
#8  0x0000000000690f46 in dwarf2_frame_sniffer (self=0x8e4980, this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/dwarf2-frame.c:1423
#9  0x000000000070203b in frame_unwind_find_by_frame (this_frame=0x1492600, this_cache=0x1492618) at ../../src/gdb/frame-unwind.c:112
#10 0x00000000006fd681 in get_frame_id (fi=0x1492600) at ../../src/gdb/frame.c:408
#11 0x00000000007006c2 in get_prev_frame_1 (this_frame=0xdc1860) at ../../src/gdb/frame.c:1826
#12 0x0000000000700b7a in get_prev_frame (this_frame=0xdc1860) at ../../src/gdb/frame.c:2056
#13 0x0000000000514588 in frame_info_to_frame_object (frame=0xdc1860) at ../../src/gdb/python/py-frame.c:322
#14 0x000000000051784c in bootstrap_python_frame_filters (frame=0xdc1860, frame_low=0, frame_high=-1) at ../../src/gdb/python/py-framefilter.c:1396
#15 0x0000000000517a6f in apply_frame_filter (frame=0xdc1860, flags=7, args_type=CLI_SCALAR_VALUES, out=0xed7a90, frame_low=0, frame_high=-1)
    at ../../src/gdb/python/py-framefilter.c:1492
#16 0x00000000005e77b0 in backtrace_command_1 (count_exp=0x0, show_locals=0, no_filters=0, from_tty=1) at ../../src/gdb/stack.c:1777
#17 0x00000000005e7c0f in backtrace_command (arg=0x0, from_tty=1) at ../../src/gdb/stack.c:1891
#18 0x00000000004e37a7 in do_cfunc (c=0xda4fa0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:107
#19 0x00000000004e683c in cmd_func (cmd=0xda4fa0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:1882
#20 0x00000000006f35ed in execute_command (p=0xcc66c2 "", from_tty=1) at ../../src/gdb/top.c:468
#21 0x00000000005f8853 in command_handler (command=0xcc66c0 "bt") at ../../src/gdb/event-top.c:435
#22 0x00000000005f8e12 in command_line_handler (rl=0xfe05f0 "@") at ../../src/gdb/event-top.c:632
#23 0x000000000074d2c6 in rl_callback_read_char () at ../../src/readline/callback.c:220
#24 0x00000000005f8375 in rl_callback_read_char_wrapper (client_data=0x0) at ../../src/gdb/event-top.c:164
#25 0x00000000005f876a in stdin_event_handler (error=0, client_data=0x0) at ../../src/gdb/event-top.c:375
#26 0x00000000005f72fa in handle_file_event (data=...) at ../../src/gdb/event-loop.c:768
#27 0x00000000005f67a3 in process_event () at ../../src/gdb/event-loop.c:342
#28 0x00000000005f686a in gdb_do_one_event () at ../../src/gdb/event-loop.c:406
#29 0x00000000005f68bb in start_event_loop () at ../../src/gdb/event-loop.c:431
#30 0x00000000005f83a7 in cli_command_loop (data=0x0) at ../../src/gdb/event-top.c:179
#31 0x00000000005eeed3 in current_interp_command_loop () at ../../src/gdb/interps.c:327
#32 0x00000000005ef8ff in captured_command_loop (data=0x0) at ../../src/gdb/main.c:267
#33 0x00000000005ed2f6 in catch_errors (func=0x5ef8e4 <captured_command_loop>, func_args=0x0, errstring=0x8b6554 "", mask=RETURN_MASK_ALL)
    at ../../src/gdb/exceptions.c:524
#34 0x00000000005f0d21 in captured_main (data=0x7fffffffd9e0) at ../../src/gdb/main.c:1067
#35 0x00000000005ed2f6 in catch_errors (func=0x5efb9b <captured_main>, func_args=0x7fffffffd9e0, errstring=0x8b6554 "", mask=RETURN_MASK_ALL)
    at ../../src/gdb/exceptions.c:524
#36 0x00000000005f0d57 in gdb_main (args=0x7fffffffd9e0) at ../../src/gdb/main.c:1076
#37 0x000000000045bb6a in main (argc=4, argv=0x7fffffffdae8) at ../../src/gdb/gdb.c:34
(top-gdb)

GDB is trying to unwind the PC register of the previous frame (frame
#5 above), starting from the frame being sniffed (the THIS frame).
But the THIS frame's unwinder says the PC of the previous frame is
actually the same as the previous's frame's next frame (which is the
same frame we started with, the THIS frame), therefore it returns an
lval_register lazy value with frame set to THIS frame.  And so the
value_fetch_lazy loop never ends.


Rationale 2
===========

As an experiment, I tried making dwarf2-frame.c:read_addr_from_reg use
address_from_register.  That caused a bunch of regressions, but it
actually took me a long while to figure out what was going on.  Turns
out dwarf2-frame.c:read_addr_from_reg is called while computing the
frame's CFA, from within dwarf2_frame_cache.  address_from_register
wants to create a register with frame_id set to the frame being
constructed.  To create the frame id, we again call dwarf2_frame_cache,
which given:

static struct dwarf2_frame_cache *
dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
{
...
  if (*this_cache)
    return *this_cache;

returns an incomplete object to the caller:
static void
dwarf2_frame_this_id (struct frame_info *this_frame, void **this_cache,
		      struct frame_id *this_id)
{
  struct dwarf2_frame_cache *cache =
    dwarf2_frame_cache (this_frame, this_cache);
...
 (*this_id) = frame_id_build (cache->cfa, get_frame_func (this_frame));
}

As cache->cfa is still 0 (we were trying to compute it!), and
get_frame_id recalls this id from here on, we end up with a broken
frame id in recorded for this frame.  Later, when inspecting locals,
the dwarf machinery needs to know the selected frame's base, which
calls get_frame_base:

CORE_ADDR
get_frame_base (struct frame_info *fi)
{
  return get_frame_id (fi).stack_addr;
}

which as seen above then returns 0 ...

So I gave up using address_from_register.

But, the pain of investigating this made me want to have GDB itself
assert that recursion never happens here.  So I wrote a patch to do
that.  But, it triggers on current mainline, because
dwarf2_tailcall_sniffer_first, called from dwarf2_frame_cache, unwinds
the this_frame.

A sniffer shouldn't be trying to unwind, exactly because of this sort
of tricky issue.  The patch defers calling
dwarf2_tailcall_sniffer_first until it's really necessary, in
dwarf2_frame_prev_register (thus actually outside the sniffer path).
As this makes the call to dwarf2_frame_sniffer in dwarf2_frame_cache
unnecessary again, the patch removes that too.

Tested on x86_64 Fedora 17.

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* dwarf2-frame.c (struct dwarf2_frame_cache)
	<checked_tailcall_bottom, entry_cfa_sp_offset,
	entry_cfa_sp_offset_p>: New fields.
	(dwarf2_frame_cache): Adjust to use the new cache fields instead
	of locals.  Don't call dwarf2_tailcall_sniffer_first here.
	(dwarf2_frame_prev_register): Call it here, but only once.
Don't let two frames with the same id end up in the frame chain.

The UNWIND_SAME_ID check is done between THIS_FRAME and the next frame
when we go try to unwind the previous frame.  But at this point, it's
already too late -- we ended up with two frames with the same ID in
the frame chain.  Each frame having its own ID is an invariant assumed
throughout GDB.  This patch applies the UNWIND_SAME_ID detection
earlier, right after the previous frame is unwound, discarding the dup
frame if a cycle is detected.

The patch includes a new test that fails before the change.  Before
the patch, the test causes an infinite loop in GDB, after the patch,
the UNWIND_SAME_ID logic kicks in and makes the backtrace stop with:

  Backtrace stopped: previous frame identical to this frame (corrupt stack?)

The test uses dwarf CFI to emulate a corrupted stack with a cycle.  It
has a function with registers marked DW_CFA_same_value (most
importantly RSP/RIP), so that GDB computes the same ID for that frame
and its caller.  IOW, something like this:

 #0 - frame_id_1
 #1 - frame_id_2
 #2 - frame_id_3
 #3 - frame_id_4
 #4 - frame_id_4  <<<< outermost (UNWIND_SAME_ID).

(The test's code is just a copy of dw2-reg-undefined.S /
dw2-reg-undefined.c, adjusted to use DW_CFA_same_value instead of
DW_CFA_undefined, and to mark a different set of registers.)

The infinite loop is here, in value_fetch_lazy:

      while (VALUE_LVAL (new_val) == lval_register && value_lazy (new_val))
	{
	  frame = frame_find_by_id (VALUE_FRAME_ID (new_val));
...
	  new_val = get_frame_register_value (frame, regnum);
	}

get_frame_register_value can return a lazy register value pointing to
the next frame.  This means that the register wasn't clobbered by
FRAME; the debugger should therefore retrieve its value from the next
frame.

To be clear, get_frame_register_value unwinds the value in question
from the next frame:

 struct value *
 get_frame_register_value (struct frame_info *frame, int regnum)
 {
   return frame_unwind_register_value (frame->next, regnum);
                                       ^^^^^^^^^^^
 }

In other words, if we get a lazy lval_register, it should have the
frame ID of the _next_ frame, never of FRAME.

At this point in value_fetch_lazy, the whole relevant chunk of the
stack up to frame #4 has already been unwound.  The loop always
"unlazies" lval_registers in the "next/innermost" direction, not in
the "prev/unwind further/outermost" direction.

So say we're looking at frame #4.  get_frame_register_value in frame
#4 can return a lazy register value of frame #3.  So the next
iteration, frame_find_by_id tries to read the register from frame #3.
But, since frame #4 happens to have same id as frame #3,
frame_find_by_id returns frame #4 instead.  Rinse, repeat, and we have
an infinite loop.

This is an old latent problem, exposed by the recent addition of the
frame stash.  Before we had a stash, frame_find_by_id(frame_id_4)
would walk over all frames starting at the current frame, and would
always find #3 first.  The stash happens to return #4 instead:

struct frame_info *
frame_find_by_id (struct frame_id id)
{
  struct frame_info *frame, *prev_frame;

...
  /* Try using the frame stash first.  Finding it there removes the need
     to perform the search by looping over all frames, which can be very
     CPU-intensive if the number of frames is very high (the loop is O(n)
     and get_prev_frame performs a series of checks that are relatively
     expensive).  This optimization is particularly useful when this function
     is called from another function (such as value_fetch_lazy, case
     VALUE_LVAL (val) == lval_register) which already loops over all frames,
     making the overall behavior O(n^2).  */
  frame = frame_stash_find (id);
  if (frame)
    return frame;

  for (frame = get_current_frame (); ; frame = prev_frame)
    {

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* frame.c (get_prev_frame_1): Do the UNWIND_SAME_ID check between
	this frame and the new previous frame, not between this frame and
	the next frame.

gdb/testsuite/
2013-11-22  Pedro Alves  <palves@redhat.com>

	PR 16155
	* gdb.dwarf2/dw2-dup-frame.S: New file.
	* gdb.dwarf2/dw2-dup-frame.c: New file.
	* gdb.dwarf2/dw2-dup-frame.exp: New file.
Make use of the frame stash to detect wider stack cycles.

Given we already have the frame id stash, which holds the ids of all
frames in the chain, detecting corrupted stacks with wide stack cycles
with non-consecutive dup frame ids is just as cheap as just detecting
cycles in consecutive frames:

 #0 frame_id1
 #1 frame_id2
 #2 frame_id3
 #3 frame_id1
 #4 frame_id2
 #5 frame_id3
 #6 frame_id1
 ... forever ...

We just need to check whether the stash already knows about a given
frame id instead of comparing the ids of the previous/this frames.

Tested on x86_64 Fedora 17.

gdb/
2013-11-22  Pedro Alves  <palves@redhat.com>
	    Tom Tromey  <tromey@redhat.com>

	* frame.c (frame_stash_add): Now returns whether a frame with the
	same ID was already known.
	(compute_frame_id): New function, factored out from get_frame_id.
	(get_frame_id): No longer lazilly compute the frame id here.
	(get_prev_frame_if_no_cycle): New function.  Detects wider stack
	cycles.
	(get_prev_frame_1): Use it instead of get_prev_frame_raw directly,
	and checking for stack cycles here.
	* windres.c (define_resource): Use zero for timestamp, making
	output deterministic. time.h include is no longer needed.
	* resres.c (res_append_resource): Likewise.

	* pe-dll.c (fill_edata): Only use a real timestamp if
	--insert-timestamp was used.
	* emultempl/pe.em: Add the --insert-timestamp option.
	* emultempl/pep.em: Likewise for 64bit.
	* ld.texinfo: Document the --insert-timestamp option.

	* libcoff-in.h: Add insert_timestamp flag to the pe_data struct.
	* libcoff.h: Regenerate.
	* peXXigen.c (_bfd_XXi_only_swap_filehdr_out): Only use a real
	timestamp if --insert-timestamp was used.
Detect infinite loop in value_fetch_lazy's lval_register handling.

If value_fetch_lazy loops infinitely while unwrapping lval_register
values, it means we either somehow ended up with two frames with the
same ID in the frame chain, or some code is trying to unwind behind
get_prev_frame's back (e.g., a frame unwind sniffer trying to unwind).
In any case, it should always be an internal error to end up in this
situation.

This patch adds a check and throws an internal error if the same frame
is returned.

2013-11-22  Tom Tromey  <tromey@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR backtrace/16155
	* value.c (value_fetch_lazy): Internal error if
	get_frame_register_value returns the same register.
handle an unspecified return address column

Debugging PR 16155 further, I found that the DWARF unwinder found the
function in question, but thought it had no registers saved
(fs->regs.num_regs == 0).

It seems to me that if a frame does not specify the return address
column, or if the return address column is explicitly marked as
DWARF2_FRAME_REG_UNSPECIFIED, then we should set the
"undefined_retaddr" flag and let the DWARF unwinder gracefully stop.

This patch implements that idea.

With this patch the backtrace works properly:

    (gdb) bt
    #0  0x0000007fb7ed485c in nanosleep () from /lib64/libc.so.6
    #1  0x0000007fb7ed4508 in sleep () from /lib64/libc.so.6
    #2  0x00000000004008bc in thread_function (arg=0x4) at threadapply.c:73
    #3  0x0000007fb7fad950 in start_thread () from /lib64/libpthread.so.0
    #4  0x0000007fb7f0956c in clone () from /lib64/libc.so.6

2013-11-22  Tom Tromey  <tromey@redhat.com>

	PR backtrace/16155:
	* dwarf2-frame.c (dwarf2_frame_cache): Set undefined_retaddr if
	the return address column is unspecified.

2013-11-22  Tom Tromey  <tromey@redhat.com>

	* gdb.dwarf2/dw2-bad-cfi.c: New file.
	* gdb.dwarf2/dw2-bad-cfi.exp: New file.
	* gdb.dwarf2/dw2-bad-cfi.S: New file.
update comment in dw2-bad-cfi.S.

Pedro asked me to add a comment to dw2-bad-cfi.S explaining the nature
of the badness.

I'm checking this in.

2013-11-22  Tom Tromey  <tromey@redhat.com>

	* gdb.dwarf2/dw2-bad-cfi.S: Update comment.
Rename gdb.dwarf2/dw2-bad-cfi.* to gdb.dwarf2/dw2-unspecified-ret-addr.*.

gdb/testsuite/
2013-11-22  Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/dw2-bad-cfi.S: Rename to ...
	* gdb.dwarf2/dw2-unspecified-ret-addr.S: ... this.  Adjust.
	* gdb.dwarf2/dw2-bad-cfi.c: Rename to ...
	* gdb.dwarf2/dw2-unspecified-ret-addr.c: ... this.
	* gdb.dwarf2/dw2-bad-cfi.exp: Rename to ...
	* gdb.dwarf2/dw2-unspecified-ret-addr.exp: ... this.
2013-11-22  Sterling Augustine  <saugustine@google.com>

     PR gdb/16196:
     * valprint.c (read_string): Set new variable fetchlen based on
     fetchlimit and size.  Use it in call to partial_memory_read.
     Update comment.
2013-11-22  Sterling Augustine  <saugustine@google.com>

     PR gdb/16196:
     * valprint.c (read_string): Set new variable fetchlen based on
     fetchlimit and size.  Use it in call to partial_memory_read.
     Update comment.
Import config.sub and config.guess from upstream.
daily update
Use in-tree assembler for exception_x86_64_bnd_test.

gold/
	* testsuite/Makefile.am (exception_x86_64_bnd_test): Use in-tree
	assembler.
	* testsuite/Makefile.in: Regenerate.