Fix remaining inline/tailcall unwinding breakage for x86_64

Commit 5939967b35 fixed inline
frame unwinding breakage for some targets (aarch64, riscv, s390...)
but regressed a few amd64 testcases related to tailcalls.

Given the following example situation...

Frame #-1 - sentinel frame
Frame # 0 - inline frame
Frame # 1 - normal frame

... suppose we're at level #1 and call into dwarf2_tailcall_sniffer_first.

We'll attempt to fetch PC, which used to be done via the gdbarch_unwind_pc call
(before 5939967b35), but now it is being handled
by the get_frame_register function.

gdbarch_unwind_pc will attempt to use frame #1's cache to retrieve information
about the PC. Here's where different architectures behave differently.

x86_64 will find a dwarf rule to retrieve PC from memory, at a CFA + offset
location. So the PC value is readily available and there is no need to
create a lazy value.

For aarch64 (and others), GCC doesn't emit an explicit location for PC, so we
eventually will find that PC is DWARF2_FRAME_REG_UNSPECIFIED. This is known
and is handled by GDB by assuming GCC really meant DWARF2_FRAME_REG_SAME_VALUE.

This means we'll attempt to fetch the register value from frame #0, via a call
to frame_unwind_got_register, which will trigger the creation of a lazy value
that requires a valid frame id for frame #0.

We don't have a valid id for frame #0 yet, so we assert.

Given the above, the following patch attempts to handle the situation without
being too hacky. We verify if the next frame is an inline frame and if its
frame id has been computed already. If it hasn't been computed yet, then we
use the safer get_frame_register function, otherwise we use the regular
gdbarch_unwind_pc hook.

gdb/ChangeLog:

2020-04-27  Luis Machado  <luis.machado@linaro.org>

	* dwarf2/frame-tailcall.c (dwarf2_tailcall_sniffer_first): Handle
	problematic inline frame unwinding situation.
	* frame.c (frame_id_computed_p): New function.
	* frame.h (frame_id_computed_p): New prototype.
This commit is contained in:
Luis Machado 2020-04-25 00:32:44 -03:00
parent 714e6c969f
commit 991a3e2e99
4 changed files with 55 additions and 3 deletions

View File

@ -1,3 +1,10 @@
2020-04-27 Luis Machado <luis.machado@linaro.org>
* dwarf2/frame-tailcall.c (dwarf2_tailcall_sniffer_first): Handle
problematic inline frame unwinding situation.
* frame.c (frame_id_computed_p): New function.
* frame.h (frame_id_computed_p): New prototype.
2020-04-26 Tom Tromey <tom@tromey.com>
* command.h (enum command_class) <class_pseudo>: Remove.

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@ -384,10 +384,43 @@ dwarf2_tailcall_sniffer_first (struct frame_info *this_frame,
prev_gdbarch = frame_unwind_arch (this_frame);
/* The dwarf2 tailcall sniffer runs early, at the end of populating the
dwarf2 frame cache for the current frame. If there exists inline
frames inner (next) to the current frame, there is a good possibility
of that inline frame not having a computed frame id yet.
This is because computing such a frame id requires us to walk through
the frame chain until we find the first normal frame after the inline
frame and then compute the normal frame's id first.
Some architectures' compilers generate enough register location
information for a dwarf unwinder to fetch PC without relying on inner
frames (x86_64 for example). In this case the PC is retrieved
according to dwarf rules.
But others generate less strict dwarf data for which assumptions are
made (like interpreting DWARF2_FRAME_REG_UNSPECIFIED as
DWARF2_FRAME_REG_SAME_VALUE). For such cases, GDB may attempt to
create lazy values for registers, and those lazy values must be
created with a valid frame id, but we potentially have no valid id.
So, to avoid breakage, if we see a dangerous situation with inline
frames without a computed id, use safer functions to retrieve the
current frame's PC. Otherwise use the provided dwarf rules. */
frame_info *next_frame = get_next_frame (this_frame);
/* Simulate frame_unwind_pc without setting this_frame->prev_pc.p. */
get_frame_register (this_frame, gdbarch_pc_regnum (prev_gdbarch),
(gdb_byte *) &prev_pc);
prev_pc = gdbarch_addr_bits_remove (prev_gdbarch, prev_pc);
if (next_frame != nullptr && get_frame_type (next_frame) == INLINE_FRAME
&& !frame_id_computed_p (next_frame))
{
/* The next frame is an inline frame and its frame id has not been
computed yet. */
get_frame_register (this_frame, gdbarch_pc_regnum (prev_gdbarch),
(gdb_byte *) &prev_pc);
prev_pc = gdbarch_addr_bits_remove (prev_gdbarch, prev_pc);
}
else
prev_pc = gdbarch_unwind_pc (prev_gdbarch, this_frame);
/* call_site_find_chain can throw an exception. */
chain = call_site_find_chain (prev_gdbarch, prev_pc, this_pc);

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@ -687,6 +687,14 @@ frame_id_build_wild (CORE_ADDR stack_addr)
return id;
}
bool
frame_id_computed_p (struct frame_info *frame)
{
gdb_assert (frame != nullptr);
return frame->this_id.p != 0;
}
int
frame_id_p (struct frame_id l)
{

View File

@ -236,6 +236,10 @@ extern struct frame_id
as the special identifier address are set to indicate wild cards. */
extern struct frame_id frame_id_build_wild (CORE_ADDR stack_addr);
/* Returns true if FRAME's id has been computed.
Returns false otherwise. */
extern bool frame_id_computed_p (struct frame_info *frame);
/* Returns non-zero when L is a valid frame (a valid frame has a
non-zero .base). The outermost frame is valid even without an
ID. */