linux-kernel - Re: [PATCH v4 2/3] random: introduce generic vDSO getrandom() implementation

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Message-ID: <Y3gbVorxZyb4SBJa@zx2c4.com>
Date:   Sat, 19 Nov 2022 00:55:02 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Adhemerval Zanella Netto <adhemerval.zanella@...aro.org>,
        Carlos O'Donell <carlos@...hat.com>
Subject: Re: [PATCH v4 2/3] random: introduce generic vDSO getrandom()
 implementation

Hi Christophe,

On Fri, Nov 18, 2022 at 07:34:26PM +0000, Christophe Leroy wrote:
> > +static __always_inline ssize_t
> > +__cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state)
> > +{
> > +	struct vgetrandom_state *state = opaque_state;
> > +	const struct vdso_rng_data *rng_info = __arch_get_vdso_rng_data();
> 
> In order to ease wiring up to powerpc, can it be done the same way as 
> commit e876f0b69dc9 ("lib/vdso: Allow architectures to provide the vdso 
> data pointer")

It is already. At least I think it is, unless I'm missing a subtle
distinction? The call to __arch_get_vdso_rng_data() goes to
arch-specific code, implemented in the 3/3 of this patch set for x86
inside of arch/x86/include/asm/vdso/getrandom.h. On powerpc, you'd make
a powerpc-specific __arch_get_vdso_rng_data() instead of
arch/powerpc/include/asm/vdso/getrandom.h.

Or maybe I'm not reading that commit right? The commit message says
something about __arch_get_vdso_rng_data() being problematic because of
clobbering a register, but then the same commit still seems to call
__arch_get_vdso_rng_data()? Is this one of those things where what you'd
prefer is that I define an inline function, __cvdso_get_vdso_rng_data(),
that by default calls __arch_get_vdso_rng_data(), but can be overridden
on powerpc to do some other type of magic? But
__arch_get_vdso_rng_data() is already an overiddable inline, so what
would the difference be?

Sorry if I'm a bit slow here to grok what's up.

Jason