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

On Tue, 7 Jun 2022 at 14:16, Jason A. Donenfeld [off-list ref] wrote:

Hi Ard,

On Tue, Jun 07, 2022 at 02:03:28PM +0200, Ard Biesheuvel wrote:

quoted

On Tue, 7 Jun 2022 at 13:35, Jason A. Donenfeld [off-list ref] wrote:

quoted

Hi Ard,

On Tue, Jun 07, 2022 at 01:10:52PM +0200, Ard Biesheuvel wrote:

quoted

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/ (local)

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.

quoted

quoted

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/ (local)

2) Defer mixing & crediting until random_init():
   https://lore.kernel.org/lkml/20220607111514.755009-1-Jason@zx2c4.com/ (local)

3) Defer crediting (but not mixing) until random_init():
   https://lore.kernel.org/lkml/20220607113238.769088-1-Jason@zx2c4.com/ (local)

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/ (local)


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.

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