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

From: David Laight <david.laight.linux@gmail.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
	Andreas Schwab <schwab@linux-m68k.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Nick Desaulniers <ndesaulniers@google.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,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	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: Sun, 27 Sep 2026 08:02:25 +0100	[thread overview]
Message-ID: <20260927080225.37534e7c@pumpkin> (raw)
In-Reply-To: <are7cfMIHZZn6jLz@zx2c4.com>

On Sat, 26 Sep 2026 14:32:49 +0200
"Jason A. Donenfeld" <Jason@zx2c4.com> wrote:

> On Sat, Sep 26, 2026 at 01:19:27PM +0100, Nathan Chancellor wrote:
> > On Sat, Sep 26, 2026 at 11:39:43AM +0200, Andreas Schwab wrote:  
> > > On Sep 25 2026, Nathan Chancellor wrote:
> > >   
> > > > +#if __has_builtin(__builtin_memset_inline)
> > > > +#define memset_inline(dst, value, size) __builtin_memset_inline(dst, value, size)
> > > > +#elif IS_ENABLED(CONFIG_CC_HAS_OPT_INLINE_MEMSET)
> > > > +#define memset_inline(dst, value, size) __builtin_memset(dst, value, size)
> > > > +#else
> > > > +static inline void *memset_inline(void *dst, int value, size_t size)
> > > > +{
> > > > +	char *d = dst;
> > > > +
> > > > +	while (size--)
> > > > +		*d++ = value;
> > > > +
> > > > +	return d;
> > > > +}
> > > > +#endif
> > > > +  
> > > 
> > > memset actually returns the original pointer, not the incremented one.
> > > Nothing uses the return value of memset_inline here, but it is
> > > confusing.  
> > 
> > Whoops, good catch, I guess my fingers gave up on those last two
> > characters :) I will fix this for v3 once there has been some time for
> > further review.  
> 
> Do we even need to return a value at all? Might as well just make the
> function two lines:
> 
> +       for (char *d = dst; size--;)
> +               *d++ = value;

Wouldn't it be better to add a barrier() or similar in there to
stop the compiler playing unwanted games>

David

> 
> (Technically that could even be one, but maybe that sort of golf loses
> clarity. Your choice.)
> 


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