linux-kernel - Re: [PATCH v3 1/2] arm64: Implement arch_stack_walk

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Message-ID: <CAPhsuW7RAXHRt3c+8DYqirPFRA+Sdw4ZNJ+Ad1uSiAHGOJ9RSA@mail.gmail.com>
Date: Thu, 20 Mar 2025 10:54:32 -0700
From: Song Liu <song@...nel.org>
To: Weinan Liu <wnliu@...gle.com>
Cc: indu.bhagat@...cle.com, irogers@...gle.com, joe.lawrence@...hat.com, 
	jpoimboe@...nel.org, kernel-team@...a.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-toolchains@...r.kernel.org, live-patching@...r.kernel.org, 
	mark.rutland@....com, peterz@...radead.org, puranjay@...nel.org, 
	roman.gushchin@...ux.dev, rostedt@...dmis.org, will@...nel.org
Subject: Re: [PATCH v3 1/2] arm64: Implement arch_stack_walk_reliable

On Thu, Mar 20, 2025 at 10:46 AM Weinan Liu <wnliu@...gle.com> wrote:
>
> On Thu, Mar 20, 2025 at 10:16 AM Song Liu <song@...nel.org> wrote:
> >
> >  static __always_inline void
> > @@ -230,8 +231,26 @@ kunwind_next_frame_record(struct kunwind_state *state)
> >         new_fp = READ_ONCE(record->fp);
> >         new_pc = READ_ONCE(record->lr);
> >
> > -       if (!new_fp && !new_pc)
> > -               return kunwind_next_frame_record_meta(state);
> > +       if (!new_fp && !new_pc) {
> > +               int ret;
> > +
> > +               ret = kunwind_next_frame_record_meta(state);
>
> The exception case kunwind_next_regs_pc() will return 0 when unwind success.
> Should we return a different value for the success case of kunwind_next_regs_pc()?

I am assuming once the unwinder hits an exception boundary, the stack is not
100% reliable. This does mean we will return -EINVAL for some reliable stack
walk, but this is safer and good enough for livepatch. IIUC, SFrame based
unwinder should not have this limitation.

Thanks,
Song

>
> > +               if (ret < 0) {
> > +                       /*
> > +                        * This covers two different conditions:
> > +                        *  1. ret == -ENOENT, unwinding is done.
> > +                        *  2. ret == -EINVAL, unwinding hit error.
> > +                        */
> > +                       return ret;
> > +               }
> > +               /*
> > +                * Searching across exception boundaries. The stack is now
> > +                * unreliable.
> > +                */
> > +               if (state->end_on_unreliable)
> > +                       return -EINVAL;
> > +               return 0;
> > +       }
>