linux-kernel - Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER

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Message-ID: <CAHk-=wie0Kb-+XOZNasoay7AKCaQ8Ew8=LyvWTBeiPXC3v2GSA@mail.gmail.com>
Date:   Mon, 14 Sep 2020 23:55:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally

On Mon, Sep 14, 2020 at 11:45 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I mean, I did find one case that didn't set it (cb710-mmc.c), but
> pattern-matching to the other mmc cases, that one looks like it
> _should_ have set the atomic flag like everybody else did.

Oh, and immediately after sending that out I notice
nvmet_bdev_execute_rw(), which does seem to make allocations inside
that sg_miter loop.

So those non-atomic cases do clearly exist.

It does make the case for why kmap_atomic() wants to have the
debugging code. It will "just work" on 64-bit to do it wrong, because
the address doesn't become invalid just because you sleep or get
rescheduled. But then the code that every developer tests (since
developers tend to run on good hardware) might be completely broken on
bad old hardware.

Maybe we could hide it behind a debug option, at least.

Or, alterantively, introduce a new "debug_preempt_count" that doesn't
actually disable preemption, but warns about actual sleeping
operations..

             Linus