2023-01-01 16:49:04 +04:00
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# Copyright 2013-2023 Free Software Foundation, Inc.
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2013-07-04 11:11:25 +00:00
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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load_lib dwarf.exp
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# This test can only be run on targets which support DWARF-2 and use gas.
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2022-12-13 15:21:57 -07:00
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require dwarf2_support
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2013-07-04 11:11:25 +00:00
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# This test can only be run on x86_64 targets.
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2023-01-26 10:09:44 +01:00
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require is_x86_64_m64_target
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2013-07-04 11:11:25 +00:00
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standard_testfile .S
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2020-08-05 17:38:28 -04:00
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if { [prepare_for_testing "failed to prepare" $testfile $srcfile {nodebug nopie}] } {
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2013-07-04 11:11:25 +00:00
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return -1
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}
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if ![runto stop_frame] {
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perror "Failed to stop in stop_frame"
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return -1
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}
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gdb_test "bt" "#0 (0x\[0-9a-f\]+ in )?stop_frame \[^\r\n\]*\r\n#1 \[^\r\n\]*first_frame \[^\r\n\]*\r\n#2 \[^\r\n\]*main\[^\r\n\]*" \
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"backtrace from stop_frame"
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for {set f 0} {$f < 3} {incr f} {
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if {${f} == 0} {
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2013-09-18 14:02:31 +00:00
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set pattern_rax_rbx_rcx_print "$hex"
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set pattern_rax_rbx_rcx_info "$hex\\s+$decimal"
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set pattern_r8_r9_print "$hex"
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set pattern_r8_r9_info "$hex\\s+$decimal"
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2013-07-04 11:11:25 +00:00
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} else {
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Print registers not saved in the frame as "<not saved>" instead of "<optimized out>".
Currently, in some scenarios, GDB prints <optimized out> when printing
outer frame registers. An <optimized out> register is a confusing
concept. What this really means is that the register is
call-clobbered, or IOW, not saved by the callee. This patch makes GDB
say that instead.
Before patch:
(gdb) p/x $rax $1 = <optimized out>
(gdb) info registers rax
rax <optimized out>
After patch:
(gdb) p/x $rax
$1 = <not saved>
(gdb) info registers rax
rax <not saved>
However, if for some reason the debug info describes a variable as
being in such a register (**), we still want to print <optimized out>
when printing the variable. IOW, <not saved> is reserved for
inspecting registers at the machine level. The patch uses
lval_register+optimized_out to encode the not saved registers, and
makes it so that optimized out variables always end up in
!lval_register values.
** See <https://sourceware.org/ml/gdb-patches/2012-08/msg00787.html>.
Current/recent enough GCC doesn't mark variables/arguments as being in
call-clobbered registers in the ranges corresponding to function
calls, while older GCCs did. Newer GCCs will just not say where the
variable is, so GDB will end up realizing the variable is optimized
out.
frame_unwind_got_optimized creates not_lval optimized out registers,
so by default, in most cases, we'll see <optimized out>.
value_of_register is the function eval.c uses for evaluating
OP_REGISTER (again, $pc, etc.), and related bits. It isn't used for
anything else. This function makes sure to return lval_register
values. The patch makes "info registers" and the MI equivalent use it
too. I think it just makes a lot of sense, as this makes it so that
when printing machine registers ($pc, etc.), we go through a central
function.
We're likely to need a different encoding at some point, if/when we
support partially saved registers. Even then, I think
value_of_register will still be the spot to tag the intention to print
machine register values differently.
value_from_register however may also return optimized out
lval_register values, so at a couple places where we're computing a
variable's location from a dwarf expression, we convert the resulting
value away from lval_register to a regular optimized out value.
Tested on x86_64 Fedora 17
gdb/
2013-10-02 Pedro Alves <palves@redhat.com>
* cp-valprint.c (cp_print_value_fields): Adjust calls to
val_print_optimized_out.
* jv-valprint.c (java_print_value_fields): Likewise.
* p-valprint.c (pascal_object_print_value_fields): Likewise.
* dwarf2loc.c (dwarf2_evaluate_loc_desc_full)
<DWARF_VALUE_REGISTER>: If the register was not saved, return a
new optimized out value.
* findvar.c (address_from_register): Likewise.
* frame.c (put_frame_register): Tweak error string to say the
register was not saved, rather than optimized out.
* infcmd.c (default_print_one_register_info): Adjust call to
val_print_optimized_out. Use value_of_register instead of
get_frame_register_value.
* mi/mi-main.c (output_register): Use value_of_register instead of
get_frame_register_value.
* valprint.c (valprint_check_validity): Likewise.
(val_print_optimized_out): New value parameter. If the value is
lval_register, print <not saved> instead.
(value_check_printable, val_print_scalar_formatted): Adjust calls
to val_print_optimized_out.
* valprint.h (val_print_optimized_out): New value parameter.
* value.c (struct value) <optimized_out>: Extend comment.
(error_value_optimized_out): New function.
(require_not_optimized_out): Use it. Use a different string for
lval_register values.
* value.h (error_value_optimized_out): New declaration.
* NEWS: Mention <not saved>.
gdb/testsuite/
2013-10-02 Pedro Alves <palves@redhat.com>
* gdb.dwarf2/dw2-reg-undefined.exp <pattern_rax_rbx_rcx_print,
pattern_rax_rbx_rcx_info>: Set to "<not saved>".
* gdb.mi/mi-reg-undefined.exp (opt_out_pattern): Delete.
(not_saved_pattern): New.
Replace use of the former with the latter.
gdb/doc/
2013-10-02 Pedro Alves <palves@redhat.com>
* gdb.texinfo (Registers): Expand description of saved registers
in frames. Explain <not saved>.
2013-10-02 16:15:46 +00:00
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set pattern_rax_rbx_rcx_print "<not saved>"
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set pattern_rax_rbx_rcx_info "<not saved>"
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2013-09-18 14:02:31 +00:00
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set pattern_r8_r9_print "$hex"
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set pattern_r8_r9_info "$hex\\s+$decimal"
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2013-07-04 11:11:25 +00:00
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}
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# Select frame.
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2016-12-01 14:41:23 -06:00
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gdb_test "frame ${f}" "#${f}.*" "switch to frame ${f}"
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2013-07-04 11:11:25 +00:00
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2013-09-18 14:02:31 +00:00
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gdb_test "p/x \$rax" ".*$pattern_rax_rbx_rcx_print.*" \
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"print \$rax in frame ${f}"
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gdb_test "p/x \$rbx" "$pattern_rax_rbx_rcx_print" \
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"print \$rbx in frame ${f}"
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gdb_test "p/x \$rcx" "$pattern_rax_rbx_rcx_print" \
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"print \$rcx in frame ${f}"
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gdb_test "p/x \$r8" "$pattern_r8_r9_print" "print \$r8 in frame ${f}"
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gdb_test "p/x \$r9" "$pattern_r8_r9_print" "print \$r9 in frame ${f}"
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2013-07-04 11:11:25 +00:00
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# Display register values.
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2020-08-27 14:42:12 -04:00
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gdb_test "info registers rax rbx rcx r8 r9" \
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[multi_line "rax\\s+${pattern_rax_rbx_rcx_info}\\s*" \
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"rbx\\s+${pattern_rax_rbx_rcx_info}\\s*" \
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"rcx\\s+${pattern_rax_rbx_rcx_info}\\s*" \
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"r8\\s+${pattern_r8_r9_info}\\s*" \
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"r9\\s+${pattern_r8_r9_info}\\s*"] \
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2013-07-04 11:11:25 +00:00
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"Check values of rax, rbx, rcx, r8, r9 in frame ${f}"
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}
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gdb: make frame_unwind_got_optimized return a not_lval value
TLDR: frame_unwind_got_optimized uses wrong frame id value, trying to
fix it makes GDB sad, return not_lval value and don't use frame id value
instead.
Longer version:
The `prev_register` method of the `frame_unwind` interface corresponds
to asking the question: "where did this frame - passed as a parameter -
save the value this register had in its caller frame?". When "this
frame" did not save that register value (DW_CFA_undefined in DWARF), the
implementation can use the `frame_unwind_got_optimized` function to
create a struct value that represents the optimized out / not saved
register.
`frame_unwind_got_optimized` marks the value as fully optimized out,
sets the lval field to lval_register and assigns the required data for
lval_register: the next frame id and the register number. The problem
is that it uses the frame id from the wrong frame (see below for in
depth explanation). In practice, this is not problematic because the
frame id is never used: the value is already not lazy (and is marked as
optimized out), so the value is never fetched from the target.
When trying to change it to put the right next frame id in the value, we
bump into problems: computing the frame id for some frame requires
unwinding some register, if that register is not saved / optimized out,
we try to get the frame id that we are currently computing.
This patch addresses the problem by changing
`frame_unwind_got_optimized` to return a not_lval value instead. Doing
so, we don't need to put a frame id, so we don't hit that problem. It
may seem like an unnecessary change today, because it looks like we're
fixing something that is not broken (from the user point of view).
However, the bug becomes user visible with the following patches, where
inline frames are involved. I put this change in its own patch to keep
it logically separate.
Let's now illustrate how we are putting the wrong frame id in the value
returned by `frame_unwind_got_optimized`. Let's assume this stack:
frame #0
frame #1
frame #2
frame #3
Let's suppose that we are calling `frame_unwind_register_value` with
frame #2 as the "next_frame" parameter and some register number X as the
regnum parameter. That is like asking the question "where did frame #2
save frame #3's value for register X".
`frame_unwind_register_value` calls the frame unwinder's `prev_register`
method, which in our case is `dwarf2_frame_prev_register`. Note that in
`dwarf2_frame_prev_register`, the parameter is now called `this_frame`,
but its value is still frame #2, and we are still looking for where
frame #2 saved frame #3's value of register X.
Let's now suppose that frame #2's CFI explicitly indicates that the
register X is was not saved (DW_CFA_undefined). We go into
`frame_unwind_got_optimized`.
In `frame_unwind_got_optimized`, the intent is to create a value that
represents register X in frame #3. An lval_register value requires that
we specify the id of the _next_ frame, that is the frame from which we
would need to unwind in order to get the value. Therefore, we would
want to put the id of frame #2 in there.
However, `frame_unwind_got_optimized` does:
VALUE_NEXT_FRAME_ID (val)
= get_frame_id (get_next_frame_sentinel_okay (frame));
where `frame` is frame #2. The get_next_frame_sentinel_okay call
returns frame #1, so we end up putting frame #1's id in the value.
Let's now pretend that we try to "fix" it by placing the right frame id,
in other words doing this change:
--- a/gdb/frame-unwind.c
+++ b/gdb/frame-unwind.c
@@ -260,8 +260,7 @@ frame_unwind_got_optimized (struct frame_info *frame, int regnum)
mark_value_bytes_optimized_out (val, 0, TYPE_LENGTH (type));
VALUE_LVAL (val) = lval_register;
VALUE_REGNUM (val) = regnum;
- VALUE_NEXT_FRAME_ID (val)
- = get_frame_id (get_next_frame_sentinel_okay (frame));
+ VALUE_NEXT_FRAME_ID (val) = get_frame_id (frame);
return val;
}
This makes some tests fails, such as gdb.dwarf2/dw2-undefined-ret-addr.exp,
like so:
...
#9 0x0000557a8ab15a5d in internal_error (file=0x557a8b31ef80 "/home/simark/src/binutils-gdb/gdb/frame.c", line=623, fmt=0x557a8b31efe0 "%s: Assertion `%s' failed.") at /home/simark/src/binutils-gdb/gdbsupport/errors.cc:55
#10 0x0000557a87f816d6 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:623
#11 0x0000557a87f7cac7 in frame_unwind_got_optimized (frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame-unwind.c:264
#12 0x0000557a87a71a76 in dwarf2_frame_prev_register (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, regnum=16) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1267
#13 0x0000557a87f86621 in frame_unwind_register_value (next_frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame.c:1288
#14 0x0000557a87f855d5 in frame_register_unwind (next_frame=0x62100034bde0, regnum=16, optimizedp=0x7fff5f459070, unavailablep=0x7fff5f459080, lvalp=0x7fff5f4590a0, addrp=0x7fff5f4590b0, realnump=0x7fff5f459090, bufferp=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1191
#15 0x0000557a87f860ef in frame_unwind_register (next_frame=0x62100034bde0, regnum=16, buf=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1247
#16 0x0000557a881875f9 in i386_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/i386-tdep.c:1971
#17 0x0000557a87fe58a5 in gdbarch_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/gdbarch.c:3062
#18 0x0000557a87a6267b in dwarf2_tailcall_sniffer_first (this_frame=0x62100034bde0, tailcall_cachep=0x62100034bee0, entry_cfa_sp_offsetp=0x7fff5f4593f0) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame-tailcall.c:387
#19 0x0000557a87a70cdf in dwarf2_frame_cache (this_frame=0x62100034bde0, this_cache=0x62100034bdf8) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1198
#20 0x0000557a87a711c2 in dwarf2_frame_this_id (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, this_id=0x62100034be40) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1226
#21 0x0000557a87f81167 in compute_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:587
#22 0x0000557a87f81803 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:635
#23 0x0000557a87f7efef in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7fff5f459920) at /home/simark/src/binutils-gdb/gdb/frame.c:320
#24 0x0000557a891488ae in print_frame_args (fp_opts=..., func=0x621000183b90, frame=0x62100034bde0, num=-1, stream=0x6030000caa20) at /home/simark/src/binutils-gdb/gdb/stack.c:750
#25 0x0000557a8914e87a in print_frame (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/simark/src/binutils-gdb/gdb/stack.c:1394
#26 0x0000557a8914c2ae in print_frame_info (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:1119
...
We end up calling get_frame_id (in the hunk above, frame #10) while we are
computing it (frame #21), and that's not good.
Now, the question is how do we fix this. I suggest making the unwinder
return a not_lval value in this case.
The reason why we return an lval_register here is to make sure that this
is printed as "not saved" and not "optimized out" down the line. See
these two commits:
1. 901461f8eb40 ("Print registers not saved in the frame as "<not saved>"
instead of "<optimized out>".").
2. 6bd273ae450b ("Make "set debug frame 1" output print <not saved> instead of
<optimized out>.")
The current design (introduced by the first commit) is to check the
value's lval to choose which one to print (see val_print_optimized_out).
Making the unwinder return not_lval instead of lval_register doesn't
break "not saved" when doing "print $rax" or "info registers", because
value_fetch_lazy_register only consumes the contents and optimized-out
property from the value the unwinder returned. The value being
un-lazified stays an lval_register.
I believe that this is a correct technical solution (and not just
papering over the problem), because what we expect of unwinders is to
tell us where a given register's value is saved. If the value is saved
in memory, -> lval_memory. If the value is saved in some other register
of the next frame, -> lval_register. If the value is not saved, it
doesn't really make sense to return an lval_register value. not_lval
would be more appropriate. If the code then wants to represent an
optimized out register value (like value_fetch_lazy_register does), then
it's a separate concern which shouldn't involve the unwinder.
This change breaks the output of "set debug frame 1" though (introduced
by the second commit), since that logging statement consumes the return
value of the unwinder directly. To keep the correct behavior, just make
`frame_unwind_register_value` call `val_print_not_saved` directly,
instead of `val_print_optimized_out`. This is fine because we know in
this context that we are always talking about a register value, and that
we want to show "not saved" for those.
I augmented the gdb.dwarf2/dw2-reg-undefined.exp test case to test some
cases I stumbled on while working on this, which I think are not tested
anywhere:
- the "set debug frame 1" debug output mentioned above. It's just debug
output, but if we want to make sure it doesn't change, it should be
tested
- printing not-saved register values from the history (should print not
saved)
- copying a not-saved register value in a convenience variable. In this
case, we expect that printing the convenience variable shows
"optimized out", because we copied the value, not the property of
where the value came from.
gdb/ChangeLog:
* frame-unwind.c (frame_unwind_got_optimized): Don't set
regnum/frame in value. Call allocate_value_lazy.
* frame.c (frame_unwind_register_value): Use
val_print_not_saved.
gdb/testsuite/ChangeLog:
* gdb.dwarf2/dw2-reg-undefined.exp: Test "set debug frame 1"
output, printing a "not saved" value from history and printing a
convenience variable created from a "not saved" value.
Change-Id: If451739a3ef7a5b453b1f50707e21ce16d74807e
2020-08-31 13:22:54 -04:00
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# Test that the debug log statement in frame_unwind_register_value produces
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2020-08-31 13:31:01 -04:00
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# "not saved" and not "optimized out".
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gdb: make frame_unwind_got_optimized return a not_lval value
TLDR: frame_unwind_got_optimized uses wrong frame id value, trying to
fix it makes GDB sad, return not_lval value and don't use frame id value
instead.
Longer version:
The `prev_register` method of the `frame_unwind` interface corresponds
to asking the question: "where did this frame - passed as a parameter -
save the value this register had in its caller frame?". When "this
frame" did not save that register value (DW_CFA_undefined in DWARF), the
implementation can use the `frame_unwind_got_optimized` function to
create a struct value that represents the optimized out / not saved
register.
`frame_unwind_got_optimized` marks the value as fully optimized out,
sets the lval field to lval_register and assigns the required data for
lval_register: the next frame id and the register number. The problem
is that it uses the frame id from the wrong frame (see below for in
depth explanation). In practice, this is not problematic because the
frame id is never used: the value is already not lazy (and is marked as
optimized out), so the value is never fetched from the target.
When trying to change it to put the right next frame id in the value, we
bump into problems: computing the frame id for some frame requires
unwinding some register, if that register is not saved / optimized out,
we try to get the frame id that we are currently computing.
This patch addresses the problem by changing
`frame_unwind_got_optimized` to return a not_lval value instead. Doing
so, we don't need to put a frame id, so we don't hit that problem. It
may seem like an unnecessary change today, because it looks like we're
fixing something that is not broken (from the user point of view).
However, the bug becomes user visible with the following patches, where
inline frames are involved. I put this change in its own patch to keep
it logically separate.
Let's now illustrate how we are putting the wrong frame id in the value
returned by `frame_unwind_got_optimized`. Let's assume this stack:
frame #0
frame #1
frame #2
frame #3
Let's suppose that we are calling `frame_unwind_register_value` with
frame #2 as the "next_frame" parameter and some register number X as the
regnum parameter. That is like asking the question "where did frame #2
save frame #3's value for register X".
`frame_unwind_register_value` calls the frame unwinder's `prev_register`
method, which in our case is `dwarf2_frame_prev_register`. Note that in
`dwarf2_frame_prev_register`, the parameter is now called `this_frame`,
but its value is still frame #2, and we are still looking for where
frame #2 saved frame #3's value of register X.
Let's now suppose that frame #2's CFI explicitly indicates that the
register X is was not saved (DW_CFA_undefined). We go into
`frame_unwind_got_optimized`.
In `frame_unwind_got_optimized`, the intent is to create a value that
represents register X in frame #3. An lval_register value requires that
we specify the id of the _next_ frame, that is the frame from which we
would need to unwind in order to get the value. Therefore, we would
want to put the id of frame #2 in there.
However, `frame_unwind_got_optimized` does:
VALUE_NEXT_FRAME_ID (val)
= get_frame_id (get_next_frame_sentinel_okay (frame));
where `frame` is frame #2. The get_next_frame_sentinel_okay call
returns frame #1, so we end up putting frame #1's id in the value.
Let's now pretend that we try to "fix" it by placing the right frame id,
in other words doing this change:
--- a/gdb/frame-unwind.c
+++ b/gdb/frame-unwind.c
@@ -260,8 +260,7 @@ frame_unwind_got_optimized (struct frame_info *frame, int regnum)
mark_value_bytes_optimized_out (val, 0, TYPE_LENGTH (type));
VALUE_LVAL (val) = lval_register;
VALUE_REGNUM (val) = regnum;
- VALUE_NEXT_FRAME_ID (val)
- = get_frame_id (get_next_frame_sentinel_okay (frame));
+ VALUE_NEXT_FRAME_ID (val) = get_frame_id (frame);
return val;
}
This makes some tests fails, such as gdb.dwarf2/dw2-undefined-ret-addr.exp,
like so:
...
#9 0x0000557a8ab15a5d in internal_error (file=0x557a8b31ef80 "/home/simark/src/binutils-gdb/gdb/frame.c", line=623, fmt=0x557a8b31efe0 "%s: Assertion `%s' failed.") at /home/simark/src/binutils-gdb/gdbsupport/errors.cc:55
#10 0x0000557a87f816d6 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:623
#11 0x0000557a87f7cac7 in frame_unwind_got_optimized (frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame-unwind.c:264
#12 0x0000557a87a71a76 in dwarf2_frame_prev_register (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, regnum=16) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1267
#13 0x0000557a87f86621 in frame_unwind_register_value (next_frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame.c:1288
#14 0x0000557a87f855d5 in frame_register_unwind (next_frame=0x62100034bde0, regnum=16, optimizedp=0x7fff5f459070, unavailablep=0x7fff5f459080, lvalp=0x7fff5f4590a0, addrp=0x7fff5f4590b0, realnump=0x7fff5f459090, bufferp=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1191
#15 0x0000557a87f860ef in frame_unwind_register (next_frame=0x62100034bde0, regnum=16, buf=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1247
#16 0x0000557a881875f9 in i386_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/i386-tdep.c:1971
#17 0x0000557a87fe58a5 in gdbarch_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/gdbarch.c:3062
#18 0x0000557a87a6267b in dwarf2_tailcall_sniffer_first (this_frame=0x62100034bde0, tailcall_cachep=0x62100034bee0, entry_cfa_sp_offsetp=0x7fff5f4593f0) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame-tailcall.c:387
#19 0x0000557a87a70cdf in dwarf2_frame_cache (this_frame=0x62100034bde0, this_cache=0x62100034bdf8) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1198
#20 0x0000557a87a711c2 in dwarf2_frame_this_id (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, this_id=0x62100034be40) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1226
#21 0x0000557a87f81167 in compute_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:587
#22 0x0000557a87f81803 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:635
#23 0x0000557a87f7efef in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7fff5f459920) at /home/simark/src/binutils-gdb/gdb/frame.c:320
#24 0x0000557a891488ae in print_frame_args (fp_opts=..., func=0x621000183b90, frame=0x62100034bde0, num=-1, stream=0x6030000caa20) at /home/simark/src/binutils-gdb/gdb/stack.c:750
#25 0x0000557a8914e87a in print_frame (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/simark/src/binutils-gdb/gdb/stack.c:1394
#26 0x0000557a8914c2ae in print_frame_info (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:1119
...
We end up calling get_frame_id (in the hunk above, frame #10) while we are
computing it (frame #21), and that's not good.
Now, the question is how do we fix this. I suggest making the unwinder
return a not_lval value in this case.
The reason why we return an lval_register here is to make sure that this
is printed as "not saved" and not "optimized out" down the line. See
these two commits:
1. 901461f8eb40 ("Print registers not saved in the frame as "<not saved>"
instead of "<optimized out>".").
2. 6bd273ae450b ("Make "set debug frame 1" output print <not saved> instead of
<optimized out>.")
The current design (introduced by the first commit) is to check the
value's lval to choose which one to print (see val_print_optimized_out).
Making the unwinder return not_lval instead of lval_register doesn't
break "not saved" when doing "print $rax" or "info registers", because
value_fetch_lazy_register only consumes the contents and optimized-out
property from the value the unwinder returned. The value being
un-lazified stays an lval_register.
I believe that this is a correct technical solution (and not just
papering over the problem), because what we expect of unwinders is to
tell us where a given register's value is saved. If the value is saved
in memory, -> lval_memory. If the value is saved in some other register
of the next frame, -> lval_register. If the value is not saved, it
doesn't really make sense to return an lval_register value. not_lval
would be more appropriate. If the code then wants to represent an
optimized out register value (like value_fetch_lazy_register does), then
it's a separate concern which shouldn't involve the unwinder.
This change breaks the output of "set debug frame 1" though (introduced
by the second commit), since that logging statement consumes the return
value of the unwinder directly. To keep the correct behavior, just make
`frame_unwind_register_value` call `val_print_not_saved` directly,
instead of `val_print_optimized_out`. This is fine because we know in
this context that we are always talking about a register value, and that
we want to show "not saved" for those.
I augmented the gdb.dwarf2/dw2-reg-undefined.exp test case to test some
cases I stumbled on while working on this, which I think are not tested
anywhere:
- the "set debug frame 1" debug output mentioned above. It's just debug
output, but if we want to make sure it doesn't change, it should be
tested
- printing not-saved register values from the history (should print not
saved)
- copying a not-saved register value in a convenience variable. In this
case, we expect that printing the convenience variable shows
"optimized out", because we copied the value, not the property of
where the value came from.
gdb/ChangeLog:
* frame-unwind.c (frame_unwind_got_optimized): Don't set
regnum/frame in value. Call allocate_value_lazy.
* frame.c (frame_unwind_register_value): Use
val_print_not_saved.
gdb/testsuite/ChangeLog:
* gdb.dwarf2/dw2-reg-undefined.exp: Test "set debug frame 1"
output, printing a "not saved" value from history and printing a
convenience variable created from a "not saved" value.
Change-Id: If451739a3ef7a5b453b1f50707e21ce16d74807e
2020-08-31 13:22:54 -04:00
|
|
|
gdb_test "set debug frame 1"
|
2021-06-29 12:03:50 -04:00
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gdb_test {print $rax} [multi_line \
|
gdb: introduce FRAME_SCOPED_DEBUG_ENTER_EXIT
Introduce FRAME_SCOPED_DEBUG_ENTER_EXIT and use it to print enter/exit
messages in important frame-related functions. I think this helps
understand which lower-level operations are done as part of which
higher-level operation. And it helps visually skip over a higher-level
operation you are not interested in.
Here's an example, combined with some py-unwind messages:
[frame] frame_unwind_find_by_frame: enter
[frame] frame_unwind_find_by_frame: this_frame=0
[frame] frame_unwind_try_unwinder: trying unwinder "dummy"
[frame] frame_unwind_try_unwinder: no
[frame] frame_unwind_try_unwinder: trying unwinder "dwarf2 tailcall"
[frame] frame_unwind_try_unwinder: no
[frame] frame_unwind_try_unwinder: trying unwinder "inline"
[frame] frame_unwind_try_unwinder: no
[frame] frame_unwind_try_unwinder: trying unwinder "jit"
[frame] frame_unwind_try_unwinder: no
[frame] frame_unwind_try_unwinder: trying unwinder "python"
[py-unwind] pyuw_sniffer: enter
[frame] frame_unwind_register_value: enter
[frame] frame_unwind_register_value: frame=-1, regnum=7(rsp)
[frame] frame_unwind_register_value: -> register=7 bytes=[40ddffffff7f0000]
[frame] frame_unwind_register_value: exit
[py-unwind] pyuw_sniffer: frame=0, sp=0x7fffffffdd40, pc=0x5555555551ec
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_unwind_register_value: enter
[frame] frame_unwind_register_value: frame=-1, regnum=6(rbp)
[frame] frame_unwind_register_value: -> register=6 bytes=[50ddffffff7f0000]
[frame] frame_unwind_register_value: exit
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] get_prev_frame: enter
[frame] get_prev_frame_always_1: enter
[frame] get_prev_frame_always_1: this_frame=-1
[frame] get_prev_frame_always_1: -> {level=0,type=NORMAL_FRAME,unwind=0x5588ee3d17c0,pc=0x5555555551ec,id=<not computed>,func=<unknown>} // cached
[frame] get_prev_frame_always_1: exit
[frame] get_prev_frame: exit
[frame] value_fetch_lazy_register: (frame=0, regnum=6(rbp), ...) -> register=6 bytes=[50ddffffff7f0000]
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_unwind_register_value: enter
[frame] frame_unwind_register_value: frame=-1, regnum=7(rsp)
[frame] frame_unwind_register_value: -> register=7 bytes=[40ddffffff7f0000]
[frame] frame_unwind_register_value: exit
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] get_prev_frame: enter
[frame] get_prev_frame_always_1: enter
[frame] get_prev_frame_always_1: this_frame=-1
[frame] get_prev_frame_always_1: -> {level=0,type=NORMAL_FRAME,unwind=0x5588ee3d1824,pc=0x5555555551ec,id=<not computed>,func=<unknown>} // cached
[frame] get_prev_frame_always_1: exit
[frame] get_prev_frame: exit
[frame] value_fetch_lazy_register: (frame=0, regnum=7(rsp), ...) -> register=7 bytes=[40ddffffff7f0000]
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_unwind_register_value: enter
[frame] frame_unwind_register_value: frame=-1, regnum=16(rip)
[frame] frame_unwind_register_value: -> register=16 bytes=[ec51555555550000]
[frame] frame_unwind_register_value: exit
[frame] frame_id_p: l={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] frame_id_eq: l={stack=<sentinel>,!code,special=0x0000000000000000}, r={stack=<sentinel>,!code,special=0x0000000000000000} -> 1
[frame] get_prev_frame: enter
[frame] get_prev_frame_always_1: enter
[frame] get_prev_frame_always_1: this_frame=-1
[frame] get_prev_frame_always_1: -> {level=0,type=NORMAL_FRAME,unwind=0x5588ee3d1888,pc=0x5555555551ec,id=<not computed>,func=<unknown>} // cached
[frame] get_prev_frame_always_1: exit
[frame] get_prev_frame: exit
[frame] value_fetch_lazy_register: (frame=0, regnum=16(rip), ...) -> register=16 bytes=[ec51555555550000]
[py-unwind] pyuw_sniffer: frame claimed by unwinder test unwinder
[py-unwind] pyuw_sniffer: exit
[frame] frame_unwind_try_unwinder: yes
[frame] frame_unwind_find_by_frame: exit
gdb/ChangeLog:
* frame.h (FRAME_SCOPED_DEBUG_ENTER_EXIT): New.
* frame.c (compute_frame_id, get_prev_frame_always_1,
get_prev_frame): Use FRAME_SCOPED_DEBUG_ENTER_EXIT.
* frame-unwind.c (frame_unwind_find_by_frame): Likewise.
(frame_unwind_register_value): Likewise.
Change-Id: I45b69b4ed962e70572bc55b8adfb211483c1eeed
2021-06-29 12:05:14 -04:00
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{ \[frame\] frame_unwind_register_value: frame=0, regnum=0\(rax\)} \
|
|
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{ \[frame\] frame_unwind_register_value: -> <not saved>} \
|
2021-06-29 12:03:50 -04:00
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{.*}]
|
gdb: make frame_unwind_got_optimized return a not_lval value
TLDR: frame_unwind_got_optimized uses wrong frame id value, trying to
fix it makes GDB sad, return not_lval value and don't use frame id value
instead.
Longer version:
The `prev_register` method of the `frame_unwind` interface corresponds
to asking the question: "where did this frame - passed as a parameter -
save the value this register had in its caller frame?". When "this
frame" did not save that register value (DW_CFA_undefined in DWARF), the
implementation can use the `frame_unwind_got_optimized` function to
create a struct value that represents the optimized out / not saved
register.
`frame_unwind_got_optimized` marks the value as fully optimized out,
sets the lval field to lval_register and assigns the required data for
lval_register: the next frame id and the register number. The problem
is that it uses the frame id from the wrong frame (see below for in
depth explanation). In practice, this is not problematic because the
frame id is never used: the value is already not lazy (and is marked as
optimized out), so the value is never fetched from the target.
When trying to change it to put the right next frame id in the value, we
bump into problems: computing the frame id for some frame requires
unwinding some register, if that register is not saved / optimized out,
we try to get the frame id that we are currently computing.
This patch addresses the problem by changing
`frame_unwind_got_optimized` to return a not_lval value instead. Doing
so, we don't need to put a frame id, so we don't hit that problem. It
may seem like an unnecessary change today, because it looks like we're
fixing something that is not broken (from the user point of view).
However, the bug becomes user visible with the following patches, where
inline frames are involved. I put this change in its own patch to keep
it logically separate.
Let's now illustrate how we are putting the wrong frame id in the value
returned by `frame_unwind_got_optimized`. Let's assume this stack:
frame #0
frame #1
frame #2
frame #3
Let's suppose that we are calling `frame_unwind_register_value` with
frame #2 as the "next_frame" parameter and some register number X as the
regnum parameter. That is like asking the question "where did frame #2
save frame #3's value for register X".
`frame_unwind_register_value` calls the frame unwinder's `prev_register`
method, which in our case is `dwarf2_frame_prev_register`. Note that in
`dwarf2_frame_prev_register`, the parameter is now called `this_frame`,
but its value is still frame #2, and we are still looking for where
frame #2 saved frame #3's value of register X.
Let's now suppose that frame #2's CFI explicitly indicates that the
register X is was not saved (DW_CFA_undefined). We go into
`frame_unwind_got_optimized`.
In `frame_unwind_got_optimized`, the intent is to create a value that
represents register X in frame #3. An lval_register value requires that
we specify the id of the _next_ frame, that is the frame from which we
would need to unwind in order to get the value. Therefore, we would
want to put the id of frame #2 in there.
However, `frame_unwind_got_optimized` does:
VALUE_NEXT_FRAME_ID (val)
= get_frame_id (get_next_frame_sentinel_okay (frame));
where `frame` is frame #2. The get_next_frame_sentinel_okay call
returns frame #1, so we end up putting frame #1's id in the value.
Let's now pretend that we try to "fix" it by placing the right frame id,
in other words doing this change:
--- a/gdb/frame-unwind.c
+++ b/gdb/frame-unwind.c
@@ -260,8 +260,7 @@ frame_unwind_got_optimized (struct frame_info *frame, int regnum)
mark_value_bytes_optimized_out (val, 0, TYPE_LENGTH (type));
VALUE_LVAL (val) = lval_register;
VALUE_REGNUM (val) = regnum;
- VALUE_NEXT_FRAME_ID (val)
- = get_frame_id (get_next_frame_sentinel_okay (frame));
+ VALUE_NEXT_FRAME_ID (val) = get_frame_id (frame);
return val;
}
This makes some tests fails, such as gdb.dwarf2/dw2-undefined-ret-addr.exp,
like so:
...
#9 0x0000557a8ab15a5d in internal_error (file=0x557a8b31ef80 "/home/simark/src/binutils-gdb/gdb/frame.c", line=623, fmt=0x557a8b31efe0 "%s: Assertion `%s' failed.") at /home/simark/src/binutils-gdb/gdbsupport/errors.cc:55
#10 0x0000557a87f816d6 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:623
#11 0x0000557a87f7cac7 in frame_unwind_got_optimized (frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame-unwind.c:264
#12 0x0000557a87a71a76 in dwarf2_frame_prev_register (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, regnum=16) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1267
#13 0x0000557a87f86621 in frame_unwind_register_value (next_frame=0x62100034bde0, regnum=16) at /home/simark/src/binutils-gdb/gdb/frame.c:1288
#14 0x0000557a87f855d5 in frame_register_unwind (next_frame=0x62100034bde0, regnum=16, optimizedp=0x7fff5f459070, unavailablep=0x7fff5f459080, lvalp=0x7fff5f4590a0, addrp=0x7fff5f4590b0, realnump=0x7fff5f459090, bufferp=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1191
#15 0x0000557a87f860ef in frame_unwind_register (next_frame=0x62100034bde0, regnum=16, buf=0x7fff5f459150 "") at /home/simark/src/binutils-gdb/gdb/frame.c:1247
#16 0x0000557a881875f9 in i386_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/i386-tdep.c:1971
#17 0x0000557a87fe58a5 in gdbarch_unwind_pc (gdbarch=0x621000190110, next_frame=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/gdbarch.c:3062
#18 0x0000557a87a6267b in dwarf2_tailcall_sniffer_first (this_frame=0x62100034bde0, tailcall_cachep=0x62100034bee0, entry_cfa_sp_offsetp=0x7fff5f4593f0) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame-tailcall.c:387
#19 0x0000557a87a70cdf in dwarf2_frame_cache (this_frame=0x62100034bde0, this_cache=0x62100034bdf8) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1198
#20 0x0000557a87a711c2 in dwarf2_frame_this_id (this_frame=0x62100034bde0, this_cache=0x62100034bdf8, this_id=0x62100034be40) at /home/simark/src/binutils-gdb/gdb/dwarf2/frame.c:1226
#21 0x0000557a87f81167 in compute_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:587
#22 0x0000557a87f81803 in get_frame_id (fi=0x62100034bde0) at /home/simark/src/binutils-gdb/gdb/frame.c:635
#23 0x0000557a87f7efef in scoped_restore_selected_frame::scoped_restore_selected_frame (this=0x7fff5f459920) at /home/simark/src/binutils-gdb/gdb/frame.c:320
#24 0x0000557a891488ae in print_frame_args (fp_opts=..., func=0x621000183b90, frame=0x62100034bde0, num=-1, stream=0x6030000caa20) at /home/simark/src/binutils-gdb/gdb/stack.c:750
#25 0x0000557a8914e87a in print_frame (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, sal=...) at /home/simark/src/binutils-gdb/gdb/stack.c:1394
#26 0x0000557a8914c2ae in print_frame_info (fp_opts=..., frame=0x62100034bde0, print_level=0, print_what=SRC_AND_LOC, print_args=1, set_current_sal=1) at /home/simark/src/binutils-gdb/gdb/stack.c:1119
...
We end up calling get_frame_id (in the hunk above, frame #10) while we are
computing it (frame #21), and that's not good.
Now, the question is how do we fix this. I suggest making the unwinder
return a not_lval value in this case.
The reason why we return an lval_register here is to make sure that this
is printed as "not saved" and not "optimized out" down the line. See
these two commits:
1. 901461f8eb40 ("Print registers not saved in the frame as "<not saved>"
instead of "<optimized out>".").
2. 6bd273ae450b ("Make "set debug frame 1" output print <not saved> instead of
<optimized out>.")
The current design (introduced by the first commit) is to check the
value's lval to choose which one to print (see val_print_optimized_out).
Making the unwinder return not_lval instead of lval_register doesn't
break "not saved" when doing "print $rax" or "info registers", because
value_fetch_lazy_register only consumes the contents and optimized-out
property from the value the unwinder returned. The value being
un-lazified stays an lval_register.
I believe that this is a correct technical solution (and not just
papering over the problem), because what we expect of unwinders is to
tell us where a given register's value is saved. If the value is saved
in memory, -> lval_memory. If the value is saved in some other register
of the next frame, -> lval_register. If the value is not saved, it
doesn't really make sense to return an lval_register value. not_lval
would be more appropriate. If the code then wants to represent an
optimized out register value (like value_fetch_lazy_register does), then
it's a separate concern which shouldn't involve the unwinder.
This change breaks the output of "set debug frame 1" though (introduced
by the second commit), since that logging statement consumes the return
value of the unwinder directly. To keep the correct behavior, just make
`frame_unwind_register_value` call `val_print_not_saved` directly,
instead of `val_print_optimized_out`. This is fine because we know in
this context that we are always talking about a register value, and that
we want to show "not saved" for those.
I augmented the gdb.dwarf2/dw2-reg-undefined.exp test case to test some
cases I stumbled on while working on this, which I think are not tested
anywhere:
- the "set debug frame 1" debug output mentioned above. It's just debug
output, but if we want to make sure it doesn't change, it should be
tested
- printing not-saved register values from the history (should print not
saved)
- copying a not-saved register value in a convenience variable. In this
case, we expect that printing the convenience variable shows
"optimized out", because we copied the value, not the property of
where the value came from.
gdb/ChangeLog:
* frame-unwind.c (frame_unwind_got_optimized): Don't set
regnum/frame in value. Call allocate_value_lazy.
* frame.c (frame_unwind_register_value): Use
val_print_not_saved.
gdb/testsuite/ChangeLog:
* gdb.dwarf2/dw2-reg-undefined.exp: Test "set debug frame 1"
output, printing a "not saved" value from history and printing a
convenience variable created from a "not saved" value.
Change-Id: If451739a3ef7a5b453b1f50707e21ce16d74807e
2020-08-31 13:22:54 -04:00
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gdb_test "set debug frame 0"
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# Test that history values show "not saved" and not "optimized out".
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gdb_test "print" " = <not saved>"
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# Test that convenience variables _don't_ show "not saved".
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gdb_test {print $foo = $rax} " = <optimized out>"
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