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

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Message-ID: <f24ae67c-b4f1-4c31-992b-d414cb793f7d@oracle.com>
Date: Tue, 18 Nov 2025 10:29:46 -0800
From: Indu Bhagat <indu.bhagat@...cle.com>
To: Puranjay Mohan <puranjay12@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
        Dylan Hatch
 <dylanbhatch@...gle.com>, Song Liu <song@...nel.org>,
        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 11/17/25 4:49 PM, 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.
> 

We discussed SFrame/JIT topic earlier this year in our monthly SFrame 
meetings.  I can point you to the meeting notes in a separate email.  We 
had some discussion around:

   - SFrame specification: Allow efficient addition, removal and update 
of data in SFrame sections.  A part of the challenge is in representing 
the variety of frames a JIT may use.
   - SFrame APIs with JIT: Efficient SFrame stack trace data 
manipulation by JIT.
   - Interface with Linux kernel: Efficient SFrame stack trace data 
registration and update stack trace data.

It will be great to have more collaboration and brainstorming, and to 
include BPF/JIT in the discussions.

>>
>> 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