linux-kernel - Re: [PATCH v8 3/3] x86: vdso: Wire up getrandom() vDSO implementation

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Message-ID: <Y4UKpP7/NOwPIkYe@zx2c4.com>
Date:   Mon, 28 Nov 2022 20:23:16 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-kernel@...r.kernel.org, patches@...ts.linux.dev,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        Christian Brauner <brauner@...nel.org>,
        Samuel Neves <sneves@....uc.pt>
Subject: Re: [PATCH v8 3/3] x86: vdso: Wire up getrandom() vDSO implementation

Hi Arnd,

On Mon, Nov 28, 2022 at 08:18:12PM +0100, Arnd Bergmann wrote:
> On Mon, Nov 28, 2022, at 12:18, Jason A. Donenfeld wrote:
> > Hook up the generic vDSO implementation to the x86 vDSO data page. Since
> > the existing vDSO infrastructure is heavily based on the timekeeping
> > functionality, which works over arrays of bases, a new macro is
> > introduced for vvars that are not arrays.
> >
> > Also enable the vgetrandom_alloc() syscall, which the vDSO
> > implementation relies on.
> >
> > The vDSO function requires a ChaCha20 implementation that does not write
> > to the stack, yet can still do an entire ChaCha20 permutation, so
> > provide this using SSE2, since this is userland code that must work on
> > all x86-64 processors.
> >
> > Reviewed-by: Samuel Neves <sneves@....uc.pt> # for vgetrandom-chacha.S
> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> > ---
> >  arch/x86/Kconfig                        |   1 +
> >  arch/x86/entry/syscalls/syscall_64.tbl  |   1 +
> 
> I see that this enables the syscall in x86-64, while patch 1
> adds it to the eight architecures that use 
> include/uapi/asm-generic/unistd.h (with the __ARCH_WANT_*
> guard at the moment, but you already said that will be removed)
> 
> I think ideally the syscall.tbl and unistd.h changes should be done
> in one patch for all architectures that doesn't mix it with
> any other changes. In particular I think it should be separate
> from the vdso changes, but could be in the patch that implements
> the syscall.

That's more or less how v7 was, but Thomas thought the x86 stuff should
be separate. So for v8, the organization is:

1) generic syscall
2) generic vdso
3) x86 wiring

The primary advantage is that future archs wanting to add this now can
just look at commit (3) only, and make a similar commit for that new
arch.

If you think a different organization outweighs that advantage, can you
spell out what division of patches you want, and I'll do that for v9?
Or maybe this v8 is okay?

Jason