linux-kernel - Re: [PATCH v2 0/6] unwind, arm64: add sframe unwinder for kernel

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Message-ID: <CANk7y0hKH6vvWf3Lyc678uvF9YWStMzO-Sj8yb3sbS4=4dxC6Q@mail.gmail.com>
Date: Tue, 18 Nov 2025 01:49:06 +0100
From: Puranjay Mohan <puranjay12@...il.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, Dylan Hatch <dylanbhatch@...gle.com>, 
	Song Liu <song@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>, 
	Peter Zijlstra <peterz@...radead.org>, Will Deacon <will@...nel.org>, 
	Catalin Marinas <catalin.marinas@....com>, Jiri Kosina <jikos@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Weinan Liu <wnliu@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, Ian Rogers <irogers@...gle.com>, 
	linux-toolchains@...r.kernel.org, linux-kernel@...r.kernel.org, 
	live-patching@...r.kernel.org, joe.lawrence@...hat.com, 
	Puranjay Mohan <puranjay@...nel.org>
Subject: Re: [PATCH v2 0/6] unwind, arm64: add sframe unwinder for kernel

On Tue, Nov 18, 2025 at 1:10 AM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Mon, Nov 17, 2025 at 06:42:23PM -0500, Steven Rostedt wrote:
> > On Mon, 17 Nov 2025 15:06:32 -0800
> > Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> >
> > > The ORC unwinder marks the unwind "unreliable" if it has to fall back to
> > > frame pointers.
> > >
> > > But that's not a problem for livepatch because it only[*] unwinds
> > > blocked/sleeping tasks, which shouldn't have BPF on their stack anyway.
> > >
> > > [*] with one exception: the task calling into livepatch
> >
> > It may be a problem with preempted tasks right? I believe with PREEMPT_LAZY
> > (and definitely with PREEMPT_RT) BPF programs can be preempted.
>
> In that case, then yes, that stack would be marked unreliable and
> livepatch would have to go try and patch the task later.
>
> If it were an isolated case, that would be fine, but if BPF were
> consistently on the same task's stack, it could stall the completion of
> the livepatch indefinitely.
>
> I haven't (yet?) heard of BPF-induced livepatch stalls happening in
> reality, but maybe it's only a matter of time :-/
>
> To fix that, I suppose we would need some kind of dynamic ORC
> registration interface.  Similar to what has been discussed with
> sframe+JIT.

I work with the BPF JITs and would be interested in exploring this further,
can you point me to this discussion if it happened on the list.

>
> If BPF were to always use frame pointers then there would be only a very
> limited set of ORC entries (either "frame pointer" or "undefined") for a
> given BPF function and it shouldn't be too complicated.
>
> --
> Josh