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

Segher Boessenkool
2024-08-27T23:00:49Z

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 <christophe.leroy@csgroup.eu>
> > ---
> > 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