Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

From: Nathan Chancellor <nathan@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	"Jason A. Donenfeld" <Jason@zx2c4.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, loongarch@lists.linux.dev,
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	linux-s390@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
Date: Fri, 25 Sep 2026 23:19:21 +0100	[thread overview]
Message-ID: <20260925221921.GA2296912@ax162> (raw)
In-Reply-To: <CAKwvOdmNZL3eFvV3v77GXDzL+XLgz+BT9TA=my0yTnbsHqVeSg@mail.gmail.com>

On Fri, Sep 25, 2026 at 03:00:46PM -0700, Nick Desaulniers wrote:
> > > > +static inline void *memset_inline(void *dst, int value, size_t size)
> > > > +{
> > > > +       char *d = dst;
> > > > +
> > > > +       while (size--)
> > > > +               *d++ = value;
> > > > +
> > > > +       return d;
> > > > +}
> > >
> > > Does this work?
> > > https://godbolt.org/z/Tx6EaGMfb
> >
> > Hmmm...possibly.
> > https://godbolt.org/z/ec9rzd9Yf
> > I don't get it...
> 
> Just keep it under 2 pages, it will be fine:
> https://godbolt.org/z/qqoMqfxev

Yeah, it might be worth a comment that this is only safe for small
values of size so it should not be blindly copied elsewhere (although I
guess if it is, the outlined version exists in the majority of other
places).

-- 
Cheers,
Nathan

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