linux-kernel - Re: [PATCH] random: defer use of bootloader randomness to random

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Date:   Tue, 7 Jun 2022 14:21:55 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.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: defer use of bootloader randomness to
 random_init()

Hi Ard,

On Tue, Jun 07, 2022 at 02:15:27PM +0200, Ard Biesheuvel wrote:
> Note that jump labels use asm() blocks, which are opaque to the
> compiler, and so it is not guaranteed that codegen will be better than

I actually spent a lot of time looking at the codegen on a few
platforms.

> > > - Why do we need to enable this static key so early?
> >
> > We don't need to enable it especially early. I've now sent three
> > different approaches for deferring it until later and you suggested one.
> > The first of mine is kind of ugly (checking static_key_initialized and
> > such at different points).  Your suggested one after that did the same
> > but deferred into crng_reseed(), which I'm not a fan of. My second one
> > is this patch, which is flawed for the reason you pointed out. But
> > perhaps my third one is the right amount of simple and okay? That's the
> > one I linked up top, [1]. Let me know what you think of that.
> >
> > My motivation for not wanting to defer it is that if the arch solution
> > winds up being easy and straight forward (as it was for arm64), then it
> > would be nice to not need to clutter up random.c as a result.
> 
> If clutter is a concern, how about getting rid of the
> execute_in_process_context() dance, and just use a late_initcall()
> instead?

As I already explained in [1], this does not work. If the order is
(A)(B), then all this will happen *after* the late init call.

[1] https://lore.kernel.org/lkml/Yp8oOH+9V336LrLk@zx2c4.com/

Jason