linux-kernel - Re: [PATCH v1] LoongArch: Remove unnecessary checks in bt

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Message-ID: <CAAhV-H5OWewfPDQ92yw-e4UerTkg2KUXL4nyRyJRrxfBLzv63g@mail.gmail.com>
Date: Mon, 15 Dec 2025 11:35:38 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Bibo Mao <maobibo@...ngson.cn>, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] LoongArch: Remove unnecessary checks in bt_address()

On Sun, Dec 14, 2025 at 9:15 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
>
> On 12/13/25 20:24, Huacai Chen wrote:
> > On Wed, Dec 10, 2025 at 4:00 PM Tiezhu Yang <yangtiezhu@...ngson.cn> wrote:
> >>
> >> On 2025/12/10 上午10:25, Bibo Mao wrote:
> >>>
> >>>
> >>> On 2025/12/10 上午9:28, Tiezhu Yang wrote:
> >>>> On 2025/12/9 下午4:30, Huacai Chen wrote:
> >>>>> Hi, Tiezhu,
> >>>>>
> >>>>> On Tue, Dec 9, 2025 at 2:18 PM Tiezhu Yang <yangtiezhu@...ngson.cn>
> >>>>> wrote:
> >>>>>>
> >>>> ...
> >>>>>>           extern unsigned long eentry;
> >>>>>>
> >>>>>> -       if (__kernel_text_address(ra))
> >>>>>> -               return ra;
> >>>>>> -
> >>>>>> -       if (__module_text_address(ra))
> >>>>>> -               return ra;
> >>>>> I think the correct way is to remove the __module_text_address()
> >>>>> condition but keep the __kernel_text_address() condition. Then return
> >>>>> 0 at the end of this function, and remove the __kernel_text_address()
> >>>>> condition out of this function.
> >>>>
> >>>> It can not remove the check of __kernel_text_address() after calling
> >>>> bt_address() because it needs to validate the calculated address for
> >>>> exception, then no need to keep the __kernel_text_address() condition
> >>>> in bt_address() because it will check the PC outside bt_address().
> >>>           state->pc = bt_address(pc);
> >>>           if (!state->pc) {
> >>>                   pr_err("cannot find unwind pc at %p\n", (void *)pc);
> >>>                   goto err;
> >>>           }
> >>>
> >>>           if (!__kernel_text_address(state->pc))
> >>>                   goto err;
> >>> I guess that you both comes from different views :) one treats these
> >>> piece of code into one, one only views function bt_address().
> >>>
> >>> Especially with if (!state->pc), how could this happen?
> >>
> >> IMO, state->pc will be not 0 forever in practice, this check is just an
> >> error path and can be removed too if possible.
> > bt_address() return ra both in "good path" and "bad path" is strange.
> > I still suggest using my method, but move __kernel_text_address()
> > after the "if (ra >= eentry && ra < eentry +  EXCCODE_INT_END *
> > VECSIZE)" block to verify the modified ra.
>
> I am OK if you mean like this:
>
> static inline unsigned long bt_address(unsigned long ra)
> {
>         extern unsigned long eentry;
>
>         if (ra >= eentry && ra < eentry +  EXCCODE_INT_END * VECSIZE) {
>                 ...
>                 ra = func + offset;
>         }
>
>         if (__kernel_text_address(ra))
>                 return ra;
>
>         return 0;
> }
>
> bool unwind_next_frame(struct unwind_state *state)
> {
>         ...
>         state->pc = bt_address(pc);
>         if (!state->pc) {
>                 pr_err("cannot find unwind pc at %p\n", (void *)pc);
>                 goto err;
>         }
>
>         return true;
>         ...
> }
>
> If so, I will send v2 later.
Yes, this is what I want.

Huacai

>
> Thanks,
> Tiezhu
>
>