[Bug gdb/31436] New: <error reading variable: value has been optimized out> with .cfi_undefined

[Bug gdb/31436] New: <error reading variable: value has been optimized out> with .cfi_undefined

[lukas.graetz@tu-darmstadt.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=31436

            Bug ID: 31436
           Summary: <error reading variable: value has been optimized out>
                    with .cfi_undefined
           Product: gdb
           Version: 15.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: lukas.graetz@tu-darmstadt.de
  Target Milestone: ---

Created attachment 15384
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15384&action=edit
bar-artificial.zip

Backstory:

Currently, the gcc trunk (14.0.1 20240228 (experimental)) has an optimization
to not save callee-saved registers for noreturn functions [1]. It was observed
in [2] that gdb printed out incorrect variable values due to clobbered
registers. A fix adds ".cfi_undefined" annotations [3].

---

The problem for gdb:

The backtrace is still incorrect (also observed in [2]):

$ gdb bar-artificial-fix-undefined
GNU gdb (GDB) 15.0.50.20240229-git

(gdb) run
Starting program: bar-artificial-fix-undefined

Program received signal SIGABRT, Aborted.
0x00007ffff7dee00b in raise () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007ffff7dee00b in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7dcd859 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00000000004011b0 in bar () at bar-artificial.c:30
#3  0x00000000004011d1 in baz (
    a=<error reading variable: value has been optimised out>, 
    b=<optimised out>, c=<optimised out>, d=<optimised out>, 
    e=<optimised out>, f=<optimised out>, g=48, h=49) at bar-artificial.c:38


Here, the backtrace did not reach the functions qux() and main(). In this case,
the stack base pointer rbp was clobbered and marked with .cfi_undefined. But it
still should be possible without rbp to reconstruct the backtrace.

---

There is also another patch [4] to partially revert [1] by saving rbp only.
When applying this patch (and manually adding .cfi_undefined to the assembler
output, because both patches [3],[4] are incompatible), we get the following
result:

$ gdb bar-artificial-save-rbp

(gdb) run
Program received signal SIGABRT, Aborted.
(gdb) bt
#0  0x00007ffff7dee00b in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7dcd859 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00000000004011b1 in bar () at bar-artificial.c:30
#3  0x00000000004011d2 in baz (a=a@entry=42, b=b@entry=43, c=c@entry=44, 
    d=<error reading variable: value has been optimised out>, 
    e=<error reading variable: value has been optimised out>, 
    f=<error reading variable: value has been optimised out>, g=48, h=49)
    at bar-artificial.c:38
#4  0x00000000004012aa in qux () at bar-artificial.c:55
#5  0x00000000004012e4 in main () at bar-artificial.c:62


Were it just <optimized out> it would be as expected, but the wording "error
reading" still leaves the question whether there went something wrong.


---

How to reproduce:

See the attached .zip archive. To run it yourself, you don't need to build the
gcc trunk, you can just use the .S files on x86-64.

$ gcc bar-artificial-fix-undefined.S -o bar-artificial-fix-undefined

$ gcc bar-artificial-save-rbp.S -o bar-artificial-save-rbp

[[ Was produced by
$ cc1 -Og -g bar-artificial.c -o bar-artificial-fix-undefined.S
Manually edited: bar-artificial-save-rbp.S
]]

---

Credit:

The observation and test file come from Jakub Jelinek, see [1].

---

Additional info:

The gcc optimization is disabled for -Og and could be disabled by default,
because it could add more <optimized out>, even indirectly for parts that were
compiled with gcc -Og. There seems to be a discussion about it in some meailing
list. Nevertheless, gdb should not "error" or give up on a .cfi_undefined.


[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38534
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=38534#c28
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114116#c6
and https://patchwork.sourceware.org/project/gcc/list/?series=30327
[4] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114116#c2
and https://gcc.gnu.org/bugzilla/attachment.cgi?id=57545&action=diff

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[Bug gdb/31436] <error reading variable: value has been optimized out> with .cfi_undefined

[lukas.graetz@tu-darmstadt.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=31436

Lukas Grätz <lukas.graetz@tu-darmstadt.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
               Host|                            |x86_64-pc-linux-gnu
             Target|                            |x86_64-pc-linux-gnu

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[Bug gdb/31436] <error reading variable: value has been optimized out> with .cfi_undefined

[lukas.graetz@tu-darmstadt.de]

https://sourceware.org/bugzilla/show_bug.cgi?id=31436

--- Comment #1 from Lukas Grätz <lukas.graetz@tu-darmstadt.de> ---
(In reply to Lukas Grätz from comment #0) 
> The backtrace is still incorrect (also observed in [2]):
> 
> $ gdb bar-artificial-fix-undefined
> GNU gdb (GDB) 15.0.50.20240229-git
> 
> (gdb) run
> Starting program: bar-artificial-fix-undefined
> 
> Program received signal SIGABRT, Aborted.
> 0x00007ffff7dee00b in raise () from /lib/x86_64-linux-gnu/libc.so.6
> (gdb) bt
> #0  0x00007ffff7dee00b in raise () from /lib/x86_64-linux-gnu/libc.so.6
> #1  0x00007ffff7dcd859 in abort () from /lib/x86_64-linux-gnu/libc.so.6
> #2  0x00000000004011b0 in bar () at bar-artificial.c:30
> #3  0x00000000004011d1 in baz (
>     a=<error reading variable: value has been optimised out>, 
>     b=<optimised out>, c=<optimised out>, d=<optimised out>, 
>     e=<optimised out>, f=<optimised out>, g=48, h=49) at bar-artificial.c:38
> 
> 
> Here, the backtrace did not reach the functions qux() and main(). In this
> case, the stack base pointer rbp was clobbered and marked with
> .cfi_undefined. But it still should be possible without rbp to reconstruct
> the backtrace.


Edit: The last sentence was partly incorrect, I re-checked the assembler code.
In qux(), there is a:

        movq    %rsp, %rbp
        .cfi_def_cfa_register 6

And then rsp is clobbered in qux(). And rbp is clobbert in bar(). So the return
address from qux() is lost and main() cannot be reached by the backtrace. But
the backtrace should still have reached qux(). And the parameters a,b,c of
baz() are also reconstructable.

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[Bug gdb/31436] <error reading variable: value has been optimized out> with .cfi_undefined

[sam at gentoo dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=31436

Sam James <sam at gentoo dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=38534,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=114116
                 CC|                            |sam at gentoo dot org

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