Re: [PATCH 1/1] arm: backtrace-clang: fix wrong sp usage for unwinding

[Nathan Chancellor]

Hi Maninder,

On Wed, Jul 29, 2026 at 09:11:01AM +0530, Maninder Singh wrote:
> Ping! Any comments?

I am not that familiar with this code but maybe Nick (whose address I
have now updated to point to a better one) or Ard could help take a
look? Original patch is at

  https://lore.kernel.org/20260624054916.1571701-1-maninder1.s@xxxxxxxxxxx/

but I have left it inline as well.

> > show_stack() can be called for any task, however c_backtrace always unwinds
> > frames based on the "sp" register. This results in printing the backtrace of
> > the current task instead of the target 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)
> > 
> > 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)
> > 
> > Additionally, the extra manipulation of "sp" register appears unnecessary in the "current"
> > task also since the "fp" register is already provided.
> > 
> > Signed-off-by: Onkarnath 
> > Signed-off-by: Maninder Singh 
> > ---
> >  arch/arm/lib/backtrace-clang.S | 4 ----
> >  1 file changed, 4 deletions(-)
> > 
> > diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
> > index 290c52a60fc6..993410a6afd1 100644
> > --- a/arch/arm/lib/backtrace-clang.S
> > +++ b/arch/arm/lib/backtrace-clang.S
> > @@ -105,10 +105,6 @@ ENDPROC(c_backtrace)
> >  		moveq	mask, #0xfc000003
> >  		movne	mask, #0		@ mask for 32-bit
> >  
> > -/*
> > - * Switches the current frame to be the frame for dump_stack.
> > - */
> > -		add	frame, sp, #24		@ switch to false frame
> >  for_each_frame:	tst	frame, mask		@ Check for address exceptions
> >  		bne	no_frame
> >  
> > -- 
> > 2.34.1
> 
> 
> 

-- 
Cheers,
Nathan