Re: [PATCH v3 3/5] kfence: move saving stack trace of allocations into __kfence_alloc()

[Alexander Potapenko]

On Thu, Sep 23, 2021 at 12:48 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> Move the saving of the stack trace of allocations into __kfence_alloc(),
> so that the stack entries array can be used outside of
> kfence_guarded_alloc() and we avoid potentially unwinding the stack
> multiple times.
>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>

> ---
> v2:
> * New patch.
> ---
>  mm/kfence/core.c | 35 ++++++++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 11 deletions(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 249d75b7e5ee..db01814f8ff0 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -187,19 +187,26 @@ static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *m
>   * Update the object's metadata state, including updating the alloc/free stacks
>   * depending on the state transition.
>   */
> -static noinline void metadata_update_state(struct kfence_metadata *meta,
> -                                          enum kfence_object_state next)
> +static noinline void
> +metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state next,
> +                     unsigned long *stack_entries, size_t num_stack_entries)
>  {
>         struct kfence_track *track =
>                 next == KFENCE_OBJECT_FREED ? &meta->free_track : &meta->alloc_track;
>
>         lockdep_assert_held(&meta->lock);
>
> -       /*
> -        * Skip over 1 (this) functions; noinline ensures we do not accidentally
> -        * skip over the caller by never inlining.
> -        */
> -       track->num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
> +       if (stack_entries) {
> +               memcpy(track->stack_entries, stack_entries,
> +                      num_stack_entries * sizeof(stack_entries[0]));
> +       } else {
> +               /*
> +                * Skip over 1 (this) functions; noinline ensures we do not
> +                * accidentally skip over the caller by never inlining.
> +                */
> +               num_stack_entries = stack_trace_save(track->stack_entries, KFENCE_STACK_DEPTH, 1);
> +       }
> +       track->num_stack_entries = num_stack_entries;
>         track->pid = task_pid_nr(current);
>         track->cpu = raw_smp_processor_id();
>         track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
> @@ -261,7 +268,8 @@ static __always_inline void for_each_canary(const struct kfence_metadata *meta,
>         }
>  }
>
> -static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
> +static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp,
> +                                 unsigned long *stack_entries, size_t num_stack_entries)
>  {
>         struct kfence_metadata *meta = NULL;
>         unsigned long flags;
> @@ -320,7 +328,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
>         addr = (void *)meta->addr;
>
>         /* Update remaining metadata. */
> -       metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
> +       metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED, stack_entries, num_stack_entries);
>         /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
>         WRITE_ONCE(meta->cache, cache);
>         meta->size = size;
> @@ -400,7 +408,7 @@ static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool z
>                 memzero_explicit(addr, meta->size);
>
>         /* Mark the object as freed. */
> -       metadata_update_state(meta, KFENCE_OBJECT_FREED);
> +       metadata_update_state(meta, KFENCE_OBJECT_FREED, NULL, 0);
>
>         raw_spin_unlock_irqrestore(&meta->lock, flags);
>
> @@ -742,6 +750,9 @@ void kfence_shutdown_cache(struct kmem_cache *s)
>
>  void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>  {
> +       unsigned long stack_entries[KFENCE_STACK_DEPTH];
> +       size_t num_stack_entries;
> +
>         /*
>          * Perform size check before switching kfence_allocation_gate, so that
>          * we don't disable KFENCE without making an allocation.
> @@ -786,7 +797,9 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
>         if (!READ_ONCE(kfence_enabled))
>                 return NULL;
>
> -       return kfence_guarded_alloc(s, size, flags);
> +       num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 0);
> +
> +       return kfence_guarded_alloc(s, size, flags, stack_entries, num_stack_entries);
>  }
>
>  size_t kfence_ksize(const void *addr)
> --
> 2.33.0.464.g1972c5931b-goog
>


-- 
Alexander Potapenko
Software Engineer

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