linux-kernel - Re: [PATCH] random: do not use jump labels before they are initialized

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Message-ID: <CAMj1kXHfs46UKjkVoWwtSGiRACP0v69x0mr8_J_BWmx-WCVMhw@mail.gmail.com>
Date:   Tue, 7 Jun 2022 14:21:13 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Stephen Boyd <swboyd@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Phil Elwell <phil@...pberrypi.com>
Subject: Re: [PATCH] random: do not use jump labels before they are initialized

On Tue, 7 Jun 2022 at 14:16, Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi Ard,
>
> On Tue, Jun 07, 2022 at 02:03:28PM +0200, Ard Biesheuvel wrote:
> > On Tue, 7 Jun 2022 at 13:35, Jason A. Donenfeld <Jason@...c4.com> wrote:
> > >
> > > Hi Ard,
> > >
> > > On Tue, Jun 07, 2022 at 01:10:52PM +0200, Ard Biesheuvel wrote:
> > > > Fair enough. What I would like is to remove the need to play around
> > > > with the placement of jump_label_init() across architectures. Jump
> > > > labels are fundamentally a performance optimization, so unless you can
> > > > explain how setting it as early as possible makes a material
> > > > difference, performance or otherwise, I really think we should pursue
> > > > a solution that does the static key manipulation at some later time.
> > >
> > > Alright. It sounds like Catalin also prefers the same. This seems simple
> > > enough with minimal downsides: https://lore.kernel.org/lkml/20220607113238.769088-1-Jason@zx2c4.com/
> > >
> >
> > That looks simple enough. Do we risk causing any boot stalls due to
> > the crediting being deferred? Or new warnings about randomness being
> > used before CRNG is ready?
>
> We don't risk boot stalls. But there will be warnings for developers who
> have enabled the CONFIG_WARN_ALL_UNSEEDED_RANDOM debug option.
>
>
> > > So maybe we should just go that route.
> > >
> >
> > It is not my preferred approach, but I can live with it.
>
> I'm not sure what your preferred approach is at this point in time
> actually. I'll summarize all the approaches discussed so far:
>
> 1) Fix archs to initialize jump labels earlier:
>    https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=73e2d827a501
>    https://lore.kernel.org/lkml/20220603121543.360283-1-Jason@zx2c4.com/
>
> 2) Defer mixing & crediting until random_init():
>    https://lore.kernel.org/lkml/20220607111514.755009-1-Jason@zx2c4.com/
>
> 3) Defer crediting (but not mixing) until random_init():
>    https://lore.kernel.org/lkml/20220607113238.769088-1-Jason@zx2c4.com/
>
> 4) Defer changing the static branch (but neither mixing nor crediting) until random_init():
>    https://lore.kernel.org/lkml/20220607100210.683136-1-Jason@zx2c4.com/
>
>
> My first choice is (1) if it's feasible.
>
> (2) is not possible without introducing a copy, so that's out.
>
> What's your preferred approach? Or is there a number 5 you have in mind?
>

Seems like we need a mutex instead of going back concurrently on two
different threads :-)

I'll shut up now and wait for your reply on the other thread.