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

From: Segher Boessenkool <[email-masked]>
Date: Wed, 28 Aug 2024 07:33:13 -0500

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