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.
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