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

From: Segher Boessenkool <[email-masked]>
Date: Tue, 27 Aug 2024 17:53:30 -0500

On Tue, Aug 27, 2024 at 11:08:19AM -0700, Eric Biggers wrote:
> On Thu, Aug 22, 2024 at 09:13:13AM +0200, Christophe Leroy wrote:
> > With the current implementation, __cvdso_getrandom_data() calls
> > memset(), which is unexpected in the VDSO.
> > 
> > Rewrite opaque data initialisation to avoid memset().
> > 
> > Signed-off-by: Christophe Leroy <[email-masked]>
> > ---
> >  lib/vdso/getrandom.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> > 
> > diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
> > index cab153c5f9be..4a56f45141b4 100644
> > --- a/lib/vdso/getrandom.c
> > +++ b/lib/vdso/getrandom.c
> > @@ -4,6 +4,7 @@
> >   */
> >  
> >  #include <linux/minmax.h>
> > +#include <linux/array_size.h>
> >  #include <vdso/datapage.h>
> >  #include <vdso/getrandom.h>
> >  #include <vdso/unaligned.h>
> > @@ -74,11 +75,15 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_
> >  	u32 counter[2] = { 0 };
> >  
> >  	if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) {
> > -		*(struct vgetrandom_opaque_params *)opaque_state = (struct vgetrandom_opaque_params) {
> > -			.size_of_opaque_state = sizeof(*state),
> > -			.mmap_prot = PROT_READ | PROT_WRITE,
> > -			.mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS
> > -		};
> > +		struct vgetrandom_opaque_params *params = opaque_state;
> > +		int i;
> > +
> > +		params->size_of_opaque_state = sizeof(*state);
> > +		params->mmap_prot = PROT_READ | PROT_WRITE;
> > +		params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS;
> > +		for (i = 0; i < ARRAY_SIZE(params->reserved); i++)
> > +			params->reserved[i] = 0;
> > +
> >  		return 0;
> >  	}
> 
> Is there a compiler flag that could be used to disable the generation of calls
> to memset?

-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*!)

Have fun,

Segher