Re: [PATCH 06/10] audit: Factor out chunk replacement code

[Paul Moore <paul@paul-moore.com>]

On Tue, Jul 10, 2018 at 6:02 AM Jan Kara <jack@suse.cz> wrote:
> Chunk replacement code is very similar for the cases where we grow or
> shrink chunk. Factor the code out into a common helper function.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  kernel/audit_tree.c | 86 +++++++++++++++++++++++++----------------------------
>  1 file changed, 40 insertions(+), 46 deletions(-)

Ooof.  That code sucks.  Not your code Jan, rather the code you are
replacing.  Thanks for doing this.

I believe this looks right, although see my previous comment/question
about RCU/barriers/etc. that affects some of this code (inherited from
a previous patch in the patchset).

> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index 6a01738d1ac2..f419fdfc25b4 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -235,6 +235,38 @@ static struct audit_chunk *find_chunk(struct node *p)
>         return container_of(p, struct audit_chunk, owners[0]);
>  }
>
> +static void replace_chunk(struct audit_chunk *new, struct audit_chunk *old,
> +                         struct node *skip)
> +{
> +       struct audit_tree *owner;
> +       int i, j;
> +
> +       new->key = old->key;
> +       list_replace_init(&old->trees, &new->trees);
> +       list_for_each_entry(owner, &new->trees, same_root)
> +               owner->root = new;
> +       for (i = j = 0; j < old->count; i++, j++) {
> +               if (&old->owners[j] == skip) {
> +                       i--;
> +                       continue;
> +               }
> +               owner = old->owners[j].owner;
> +               new->owners[i].owner = owner;
> +               new->owners[i].index = old->owners[j].index - j + i;
> +               if (!owner) /* result of earlier fallback */
> +                       continue;
> +               get_tree(owner);
> +               list_replace_init(&old->owners[j].list, &new->owners[i].list);
> +       }
> +       /*
> +        * Make sure chunk is fully initialized before making it visible in the
> +        * hash. Pairs with a data dependency barrier in READ_ONCE() in
> +        * audit_tree_lookup().
> +        */
> +       smp_wmb();
> +       list_replace_rcu(&old->hash, &new->hash);
> +}
> +
>  static void untag_chunk(struct node *p)
>  {
>         struct audit_chunk *chunk = find_chunk(p);
> @@ -242,7 +274,6 @@ static void untag_chunk(struct node *p)
>         struct audit_chunk *new = NULL;
>         struct audit_tree *owner;
>         int size = chunk->count - 1;
> -       int i, j;
>
>         fsnotify_get_mark(entry);
>
> @@ -291,38 +322,16 @@ static void untag_chunk(struct node *p)
>
>         chunk->dead = 1;
>         spin_lock(&hash_lock);
> -       new->key = chunk->key;
> -       list_replace_init(&chunk->trees, &new->trees);
>         if (owner->root == chunk) {
>                 list_del_init(&owner->same_root);
>                 owner->root = NULL;
>         }
> -
> -       for (i = j = 0; j <= size; i++, j++) {
> -               struct audit_tree *s;
> -               if (&chunk->owners[j] == p) {
> -                       list_del_init(&p->list);
> -                       i--;
> -                       continue;
> -               }
> -               s = chunk->owners[j].owner;
> -               new->owners[i].owner = s;
> -               new->owners[i].index = chunk->owners[j].index - j + i;
> -               if (!s) /* result of earlier fallback */
> -                       continue;
> -               get_tree(s);
> -               list_replace_init(&chunk->owners[j].list, &new->owners[i].list);
> -       }
> -
> -       list_for_each_entry(owner, &new->trees, same_root)
> -               owner->root = new;
> +       list_del_init(&p->list);
>         /*
> -        * Make sure chunk is fully initialized before making it visible in the
> -        * hash. Pairs with a data dependency barrier in READ_ONCE() in
> -        * audit_tree_lookup().
> +        * This has to go last when updating chunk as once replace_chunk() is
> +        * called, new RCU readers can see the new chunk.
>          */
> -       smp_wmb();
> -       list_replace_rcu(&chunk->hash, &new->hash);
> +       replace_chunk(new, chunk, p);
>         spin_unlock(&hash_lock);
>         fsnotify_detach_mark(entry);
>         mutex_unlock(&entry->group->mark_mutex);
> @@ -399,7 +408,6 @@ static int create_chunk(struct inode *inode, struct audit_tree *tree)
>  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
>  {
>         struct fsnotify_mark *old_entry, *chunk_entry;
> -       struct audit_tree *owner;
>         struct audit_chunk *chunk, *old;
>         struct node *p;
>         int n;
> @@ -464,35 +472,21 @@ static int tag_chunk(struct inode *inode, struct audit_tree *tree)
>                 fsnotify_put_mark(old_entry);
>                 return 0;
>         }
> -       chunk->key = old->key;
> -       list_replace_init(&old->trees, &chunk->trees);
> -       for (n = 0, p = chunk->owners; n < old->count; n++, p++) {
> -               struct audit_tree *s = old->owners[n].owner;
> -               p->owner = s;
> -               p->index = old->owners[n].index;
> -               if (!s) /* result of fallback in untag */
> -                       continue;
> -               get_tree(s);
> -               list_replace_init(&old->owners[n].list, &p->list);
> -       }
> +       p = &chunk->owners[chunk->count - 1];
>         p->index = (chunk->count - 1) | (1U<<31);
>         p->owner = tree;
>         get_tree(tree);
>         list_add(&p->list, &tree->chunks);
> -       list_for_each_entry(owner, &chunk->trees, same_root)
> -               owner->root = chunk;
>         old->dead = 1;
>         if (!tree->root) {
>                 tree->root = chunk;
>                 list_add(&tree->same_root, &chunk->trees);
>         }
>         /*
> -        * Make sure chunk is fully initialized before making it visible in the
> -        * hash. Pairs with a data dependency barrier in READ_ONCE() in
> -        * audit_tree_lookup().
> +        * This has to go last when updating chunk as once replace_chunk() is
> +        * called, new RCU readers can see the new chunk.
>          */
> -       smp_wmb();
> -       list_replace_rcu(&old->hash, &chunk->hash);
> +       replace_chunk(chunk, old, NULL);
>         spin_unlock(&hash_lock);
>         fsnotify_detach_mark(old_entry);
>         mutex_unlock(&audit_tree_group->mark_mutex);
> --
> 2.16.4

--
paul moore
www.paul-moore.com