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

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Message-ID: <bcxe5hhenqdodutgp7vd7b7aqn7emrlsezlu7stjjmfxgwc3gw@q3ggnid7ooyd>
Date: Mon, 17 Nov 2025 21:18:41 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Puranjay Mohan <puranjay12@...il.com>
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 01:49:06AM +0100, Puranjay Mohan wrote:
> 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.

Sorry, nothing specific has been discussed that I'm aware of :-)

-- 
Josh