linux-kernel - Re: [RFC PATCH 0/2] livepatch: Add support for hybrid mode

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Message-ID: <CALOAHbAaCbvr=F6PBJ+gnQa1WNidELzZW-P2_HmBsZ1tJd6FFg@mail.gmail.com>
Date: Sat, 8 Feb 2025 14:41:26 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Song Liu <song@...nel.org>
Cc: bpf <bpf@...r.kernel.org>, Miroslav Benes <mbenes@...e.cz>, Petr Mladek <pmladek@...e.com>, 
	jpoimboe@...nel.org, jikos@...nel.org, joe.lawrence@...hat.com, 
	live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] livepatch: Add support for hybrid mode

On Fri, Feb 7, 2025 at 2:01 AM Song Liu <song@...nel.org> wrote:
>
> On Wed, Feb 5, 2025 at 6:55 PM Yafang Shao <laoar.shao@...il.com> wrote:
> [...]
> > > I think we should first understand why the trampoline is not
> > > freed.
> >
> > IIUC, the fexit works as follows,
> >
> >   bpf_trampoline
> >     + __bpf_tramp_enter
> >        + percpu_ref_get(&tr->pcref);
> >
> >     + call do_exit()
> >
> >     + __bpf_tramp_exit
> >        + percpu_ref_put(&tr->pcref);
> >
> > Since do_exit() never returns, the refcnt of the trampoline image is
> > never decremented, preventing it from being freed.
>
> Thanks for the explanation. In this case, I think it makes sense to
> disallow attaching fexit programs on __noreturn functions. I am not
> sure what is the best solution for it though.

There is a tools/objtool/noreturns.h. Perhaps we could create a
similar noreturns.h under kernel/bpf and add all relevant functions to
the fexit deny list.

--
Regards
Yafang