Re: [PATCH mm 1/4] kasan: clean up kasan_cache_create

[Marco Elver <elver@google.com>]

On Thu, 21 Dec 2023 at 19:35, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> Reorganize the code to avoid nested if/else checks to improve the
> readability.
>
> Also drop the confusing comments about KMALLOC_MAX_SIZE checks: they
> are relevant for both SLUB and SLAB (originally, the comments likely
> confused KMALLOC_MAX_SIZE with KMALLOC_MAX_CACHE_SIZE).
>
> Fixes: a5989d4ed40c ("kasan: improve free meta storage in Generic KASAN")
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  mm/kasan/generic.c | 67 +++++++++++++++++++++++++++-------------------
>  1 file changed, 39 insertions(+), 28 deletions(-)
>
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index 54e20b2bc3e1..769e43e05d0b 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -381,16 +381,11 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>
>         ok_size = *size;
>
> -       /* Add alloc meta into redzone. */
> +       /* Add alloc meta into the redzone. */
>         cache->kasan_info.alloc_meta_offset = *size;
>         *size += sizeof(struct kasan_alloc_meta);
>
> -       /*
> -        * If alloc meta doesn't fit, don't add it.
> -        * This can only happen with SLAB, as it has KMALLOC_MAX_SIZE equal
> -        * to KMALLOC_MAX_CACHE_SIZE and doesn't fall back to page_alloc for
> -        * larger sizes.
> -        */
> +       /* If alloc meta doesn't fit, don't add it. */
>         if (*size > KMALLOC_MAX_SIZE) {
>                 cache->kasan_info.alloc_meta_offset = 0;
>                 *size = ok_size;
> @@ -401,36 +396,52 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>         orig_alloc_meta_offset = cache->kasan_info.alloc_meta_offset;
>
>         /*
> -        * Add free meta into redzone when it's not possible to store
> +        * Store free meta in the redzone when it's not possible to store
>          * it in the object. This is the case when:
>          * 1. Object is SLAB_TYPESAFE_BY_RCU, which means that it can
>          *    be touched after it was freed, or
>          * 2. Object has a constructor, which means it's expected to
> -        *    retain its content until the next allocation, or
> -        * 3. Object is too small and SLUB DEBUG is enabled. Avoid
> -        *    free meta that exceeds the object size corrupts the
> -        *    SLUB DEBUG metadata.
> -        * Otherwise cache->kasan_info.free_meta_offset = 0 is implied.
> -        * If the object is smaller than the free meta and SLUB DEBUG
> -        * is not enabled, it is still possible to store part of the
> -        * free meta in the object.
> +        *    retain its content until the next allocation.
>          */
>         if ((cache->flags & SLAB_TYPESAFE_BY_RCU) || cache->ctor) {
>                 cache->kasan_info.free_meta_offset = *size;
>                 *size += sizeof(struct kasan_free_meta);
> -       } else if (cache->object_size < sizeof(struct kasan_free_meta)) {
> -               if (__slub_debug_enabled()) {
> -                       cache->kasan_info.free_meta_offset = *size;
> -                       *size += sizeof(struct kasan_free_meta);
> -               } else {
> -                       rem_free_meta_size = sizeof(struct kasan_free_meta) -
> -                                                                       cache->object_size;
> -                       *size += rem_free_meta_size;
> -                       if (cache->kasan_info.alloc_meta_offset != 0)
> -                               cache->kasan_info.alloc_meta_offset += rem_free_meta_size;
> -               }
> +               goto free_meta_added;
> +       }
> +
> +       /*
> +        * Otherwise, if the object is large enough to contain free meta,
> +        * store it within the object.
> +        */
> +       if (sizeof(struct kasan_free_meta) <= cache->object_size) {
> +               /* cache->kasan_info.free_meta_offset = 0 is implied. */
> +               goto free_meta_added;
>         }
>
> +       /*
> +        * For smaller objects, store the beginning of free meta within the
> +        * object and the end in the redzone. And thus shift the location of
> +        * alloc meta to free up space for free meta.
> +        * This is only possible when slub_debug is disabled, as otherwise
> +        * the end of free meta will overlap with slub_debug metadata.
> +        */
> +       if (!__slub_debug_enabled()) {
> +               rem_free_meta_size = sizeof(struct kasan_free_meta) -
> +                                                       cache->object_size;
> +               *size += rem_free_meta_size;
> +               if (cache->kasan_info.alloc_meta_offset != 0)
> +                       cache->kasan_info.alloc_meta_offset += rem_free_meta_size;
> +               goto free_meta_added;
> +       }
> +
> +       /*
> +        * If the object is small and slub_debug is enabled, store free meta
> +        * in the redzone after alloc meta.
> +        */
> +       cache->kasan_info.free_meta_offset = *size;
> +       *size += sizeof(struct kasan_free_meta);
> +
> +free_meta_added:
>         /* If free meta doesn't fit, don't add it. */
>         if (*size > KMALLOC_MAX_SIZE) {
>                 cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
> @@ -440,7 +451,7 @@ void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>
>         /* Calculate size with optimal redzone. */
>         optimal_size = cache->object_size + optimal_redzone(cache->object_size);
> -       /* Limit it with KMALLOC_MAX_SIZE (relevant for SLAB only). */
> +       /* Limit it with KMALLOC_MAX_SIZE. */
>         if (optimal_size > KMALLOC_MAX_SIZE)
>                 optimal_size = KMALLOC_MAX_SIZE;
>         /* Use optimal size if the size with added metas is not large enough. */
> --
> 2.25.1
>