[PATCH v2] arm: backtrace-clang: fix wrong sp usage for unwinding

[Nick Desaulniers]

On Tue, Aug 4, 2026 at 9:55 PM Maninder Singh <maninder1.s at samsung.com> wrote:
>
> show_stack() can be called for any task, but c_backtrace is always
> unwinding frames based on "sp" register.
>
> Which leads to printing backtrace of current task instead of
> required task.
>
> Try with normal TC:
> ==================
>        for_each_process(p){
>                sched_show_task(p);
>        }
>
> [7.433271] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
> [7.433633] Call trace:
> [7.433640] [<80113418>] (dump_backtrace) from [<80113510>] (show_stack+0x14/0x18)
> ..
> [7.433676] [<8016749c>] (sched_show_task) from [<803cb324>] (meminfo_proc_show+0x6c/0x930)
> [7.434019]  r5:8158e300 r4:8b4882d0
> [7.434024] [<803cb324>] (meminfo_proc_show) from [<80365788>] (seq_read_iter+0x148/0x4bc)
> [7.434045] [<80365788>] (seq_read_iter) from [<803c044c>] (proc_reg_read_iter+0xb8/0xc4)
> [7.434060] [<803c044c>] (proc_reg_read_iter) from [<80377104>] (copy_splice_read+0x228/0x308)
>
> It should unwind frame based on passed "fp".
> (CONFIG_UNWINDER_FRAME_POINTER=y)
>
> Extra sp manipulation is needed only when c_backtrace()
> is called from dump_backtrace() with "frame == fp" i.e.
> NULL regs and not current task. (Suggested by Ard Biesheuvel)
>
> Otherwise passed "fp" should be used to unwind backtraces.
>
> With fix:
> =========
> [13.933732] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
> [13.934165] Call trace:
> [13.934604] [<80afa1a8>] (schedule) from [<8015426c>] (kthreadd+0x124/0x208)
> [13.934654]  r10:8100bbf0 r4:8116f440
> [13.934664] [<8015426c>] (kthreadd) from [<8010010c>] (ret_from_fork+0x14/0x28)
> [13.934691] Exception stack(0xf081df9c to 0xf081dfe4)
>
> Signed-off-by: Onkarnath <onkarnath.1 at samsung.com>
> Signed-off-by: Maninder Singh <maninder1.s at samsung.com>

Suggested-by:  Ard Biesheuvel <ardb at kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>

Thanks for reporting the issue and sending a patch. Thanks for Ard for
suggesting a better fix.
I assume you all know how to submit patches to
https://www.arm.linux.org.uk/developer/patches/ ?

> ---
> v1 -> v2: don't ignore sp manipulation completely, instead use it for NULL regs.
>
>  arch/arm/lib/backtrace-clang.S | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
> index 290c52a60fc6..bf9738070af7 100644
> --- a/arch/arm/lib/backtrace-clang.S
> +++ b/arch/arm/lib/backtrace-clang.S
> @@ -107,8 +107,10 @@ ENDPROC(c_backtrace)
>
>  /*
>   * Switches the current frame to be the frame for dump_stack.
> + * (Only in case of current task using current registers)
>   */
> -               add     frame, sp, #24          @ switch to false frame
> +               cmp     frame, fp
> +               addeq   frame, sp, #24          @ switch to false frame
>  for_each_frame:        tst     frame, mask             @ Check for address exceptions
>                 bne     no_frame
>
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers