audit: use kmem_cache to manage the audit_buffer cache

The audit subsystem implemented its own buffer cache mechanism which
is a bit silly these days when we could use the kmem_cache construct.

Some credit is due to Florian Westphal for originally proposing that
we remove the audit cache implementation in favor of simple
kmalloc()/kfree() calls, but I would rather have a dedicated slab
cache to ease debugging and future stats/performance work.

Cc: Florian Westphal <fw@strlen.de>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/audit.c

@@ -59,6 +59,7 @@
 #include <linux/mutex.h>
 #include <linux/gfp.h>
 #include <linux/pid.h>
+#include <linux/slab.h>
 
 #include <linux/audit.h>
 
@@ -152,12 +153,7 @@ static atomic_t	audit_lost = ATOMIC_INIT(0);
 /* Hash for inode-based rules */
 struct list_head audit_inode_hash[AUDIT_INODE_BUCKETS];
 
-/* The audit_freelist is a list of pre-allocated audit buffers (if more
- * than AUDIT_MAXFREE are in use, the audit buffer is freed instead of
- * being placed on the freelist). */
-static DEFINE_SPINLOCK(audit_freelist_lock);
-static int	   audit_freelist_count;
-static LIST_HEAD(audit_freelist);
+static struct kmem_cache *audit_buffer_cache;
 
 /* queue msgs to send via kauditd_task */
 static struct sk_buff_head audit_queue;
@@ -192,17 +188,12 @@ DEFINE_MUTEX(audit_cmd_mutex);
  * should be at least that large. */
 #define AUDIT_BUFSIZ 1024
 
-/* AUDIT_MAXFREE is the number of empty audit_buffers we keep on the
- * audit_freelist.  Doing so eliminates many kmalloc/kfree calls. */
-#define AUDIT_MAXFREE  (2*NR_CPUS)
-
 /* The audit_buffer is used when formatting an audit record.  The caller
  * locks briefly to get the record off the freelist or to allocate the
  * buffer, and locks briefly to send the buffer to the netlink layer or
  * to place it on a transmit queue.  Multiple audit_buffers can be in
  * use simultaneously. */
 struct audit_buffer {
-	struct list_head     list;
 	struct sk_buff       *skb;	/* formatted skb ready to send */
 	struct audit_context *ctx;	/* NULL or associated context */
 	gfp_t		     gfp_mask;
@@ -1486,6 +1477,10 @@ static int __init audit_init(void)
 	if (audit_initialized == AUDIT_DISABLED)
 		return 0;
 
+	audit_buffer_cache = kmem_cache_create("audit_buffer",
+					       sizeof(struct audit_buffer),
+					       0, SLAB_PANIC, NULL);
+
 	memset(&auditd_conn, 0, sizeof(auditd_conn));
 	spin_lock_init(&auditd_conn.lock);
 
@@ -1554,60 +1549,33 @@ __setup("audit_backlog_limit=", audit_backlog_limit_set);
 
 static void audit_buffer_free(struct audit_buffer *ab)
 {
-	unsigned long flags;
-
 	if (!ab)
 		return;
 
 	kfree_skb(ab->skb);
-	spin_lock_irqsave(&audit_freelist_lock, flags);
-	if (audit_freelist_count > AUDIT_MAXFREE)
-		kfree(ab);
-	else {
-		audit_freelist_count++;
-		list_add(&ab->list, &audit_freelist);
-	}
-	spin_unlock_irqrestore(&audit_freelist_lock, flags);
+	kmem_cache_free(audit_buffer_cache, ab);
 }
 
-static struct audit_buffer * audit_buffer_alloc(struct audit_context *ctx,
-						gfp_t gfp_mask, int type)
+static struct audit_buffer *audit_buffer_alloc(struct audit_context *ctx,
+					       gfp_t gfp_mask, int type)
 {
-	unsigned long flags;
-	struct audit_buffer *ab = NULL;
-	struct nlmsghdr *nlh;
-
-	spin_lock_irqsave(&audit_freelist_lock, flags);
-	if (!list_empty(&audit_freelist)) {
-		ab = list_entry(audit_freelist.next,
-				struct audit_buffer, list);
-		list_del(&ab->list);
-		--audit_freelist_count;
-	}
-	spin_unlock_irqrestore(&audit_freelist_lock, flags);
-
-	if (!ab) {
-		ab = kmalloc(sizeof(*ab), gfp_mask);
-		if (!ab)
-			goto err;
-	}
+	struct audit_buffer *ab;
 
-	ab->ctx = ctx;
-	ab->gfp_mask = gfp_mask;
+	ab = kmem_cache_alloc(audit_buffer_cache, gfp_mask);
+	if (!ab)
+		return NULL;
 
 	ab->skb = nlmsg_new(AUDIT_BUFSIZ, gfp_mask);
 	if (!ab->skb)
 		goto err;
+	if (!nlmsg_put(ab->skb, 0, 0, type, 0, 0))
+		goto err;
 
-	nlh = nlmsg_put(ab->skb, 0, 0, type, 0, 0);
-	if (!nlh)
-		goto out_kfree_skb;
+	ab->ctx = ctx;
+	ab->gfp_mask = gfp_mask;
 
 	return ab;
 
-out_kfree_skb:
-	kfree_skb(ab->skb);
-	ab->skb = NULL;
 err:
 	audit_buffer_free(ab);
 	return NULL;