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

Eric Biggers
2024-08-27T18:08:22Z

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?

- Eric