linux-kernel - Re: [PATCH v7 3/3] x86: vdso: Wire up getrandom() vDSO implementation

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Date:   Sun, 27 Nov 2022 22:39:27 +0000
From:   Samuel Neves <sneves@....uc.pt>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev, linux-crypto@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        "Carlos O'Donell" <carlos@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <brauner@...nel.org>
Subject: Re: [PATCH v7 3/3] x86: vdso: Wire up getrandom() vDSO implementation

On Sun, Nov 27, 2022 at 10:13 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Hi Thomas,
>
> On Sat, Nov 26, 2022 at 12:08:41AM +0100, Thomas Gleixner wrote:
> > Jason!
> >
> > On Thu, Nov 24 2022 at 17:55, Jason A. Donenfeld wrote:
> > > +++ b/arch/x86/entry/vdso/vgetrandom-chacha.S
> > > +/*
> > > + * Very basic SSE2 implementation of ChaCha20. Produces a given positive number
> > > + * of blocks of output with a nonce of 0, taking an input key and 8-byte
> > > + * counter. Importantly does not spill to the stack. Its arguments are:
> >
> > Basic or not.
>
> Heh, FYI I didn't mean "basic" here as in "doesn't need a review", but
> just that it's a straightforward technique and doesn't do any
> complicated multiblock pyrotechnics (which frankly aren't really
> needed).
>
> > This needs a Reviewed-by from someone who understands SSE2
> > and ChaCha20 before this can go anywhere near the x86 tree.
>
> No problem. I'll see to it that somebody qualified gives this a review.
>

I did look at this earlier. It looks fine. I would recommend changing

+ /* copy1,copy2 = key */
+ movdqu 0x00(key),copy1
+ movdqu 0x10(key),copy2

to

+ /* copy1,copy2 = key */
+ movups 0x00(key),copy1
+ movups 0x10(key),copy2

which has the same semantics, but saves a couple of code bytes. Likewise for

+ movdqu state0,0x00(output)
+ movdqu state1,0x10(output)
+ movdqu state2,0x20(output)
+ movdqu state3,0x30(output)

Otherwise,

Reviewed-by: Samuel Neves <sneves@....uc.pt> # for vgetrandom-chacha.S