linux-kernel - Re: [PATCH v2 05/17] vdso: Avoid call to memset() by getrandom

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Message-ID: <20240828123313.GD29862@gate.crashing.org>
Date: Wed, 28 Aug 2024 07:33:13 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Eric Biggers <ebiggers@...nel.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>, Naveen N Rao <naveen@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, "Theodore Ts'o" <tytso@....edu>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Shuah Khan <shuah@...nel.org>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-trace-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 05/17] vdso: Avoid call to memset() by getrandom

On Wed, Aug 28, 2024 at 01:18:34PM +0200, Jason A. Donenfeld wrote:
> On Tue, Aug 27, 2024 at 05:53:30PM -0500, Segher Boessenkool wrote:
> > On Tue, Aug 27, 2024 at 11:08:19AM -0700, Eric Biggers wrote:
> > > > +		for (i = 0; i < ARRAY_SIZE(params->reserved); i++)
> > > > +			params->reserved[i] = 0;

This is a loop.  With -ftree-loop-distribute-patterns, the default at
-O2, this is optimised to

    memset(params->reserved, 0, ...);

(which is perfectly fine, since memset is required to be there even
for freestanding environments, this is documented!)

> > -fno-tree-loop-distribute-patterns .  But, as always, read up on it, see
> > what it actually does (and how it avoids your problem, and mostly: learn
> > what the actual problem *was*!)
> 
> This might help with various loops, but it doesn't help with the matter
> that this patch fixes, which is struct initialization. I just tried it
> with the arm64 patch to no avail.

It very much *does* help.  Try harder?  Maybe you typoed?


Segher