apparmor: speed up transactional queries

The simple_transaction interface is slow. It requires 4 syscalls
(open, write, read, close) per query and shares a single lock for each
queries.

So replace its use with a compatible in multi_transaction interface.
It allows for a faster 2 syscall pattern per query. After an initial
open, an arbitrary number of writes and reads can be issued. Each
write will reset the query with new data that can be read. Reads do
not clear the data, and can be issued multiple times, and used with
seek, until a new write is performed which will reset the data
available and the seek position.

Note: this keeps the single lock design, if needed moving to a per
file lock will have to come later.
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/apparmorfs.c

@@ -673,6 +673,106 @@ static ssize_t query_data(char *buf, size_t buf_len,
 	return out - buf;
 }
 
+/*
+ * Transaction based IO.
+ * The file expects a write which triggers the transaction, and then
+ * possibly a read(s) which collects the result - which is stored in a
+ * file-local buffer. Once a new write is performed, a new set of results
+ * are stored in the file-local buffer.
+ */
+struct multi_transaction {
+	struct kref count;
+	ssize_t size;
+	char data[0];
+};
+
+#define MULTI_TRANSACTION_LIMIT (PAGE_SIZE - sizeof(struct multi_transaction))
+/* TODO: replace with per file lock */
+static DEFINE_SPINLOCK(multi_transaction_lock);
+
+static void multi_transaction_kref(struct kref *kref)
+{
+	struct multi_transaction *t;
+
+	t = container_of(kref, struct multi_transaction, count);
+	free_page((unsigned long) t);
+}
+
+static struct multi_transaction *
+get_multi_transaction(struct multi_transaction *t)
+{
+	if  (t)
+		kref_get(&(t->count));
+
+	return t;
+}
+
+static void put_multi_transaction(struct multi_transaction *t)
+{
+	if (t)
+		kref_put(&(t->count), multi_transaction_kref);
+}
+
+/* does not increment @new's count */
+static void multi_transaction_set(struct file *file,
+				  struct multi_transaction *new, size_t n)
+{
+	struct multi_transaction *old;
+
+	AA_BUG(n > MULTI_TRANSACTION_LIMIT);
+
+	new->size = n;
+	spin_lock(&multi_transaction_lock);
+	old = (struct multi_transaction *) file->private_data;
+	file->private_data = new;
+	spin_unlock(&multi_transaction_lock);
+	put_multi_transaction(old);
+}
+
+static struct multi_transaction *multi_transaction_new(struct file *file,
+						       const char __user *buf,
+						       size_t size)
+{
+	struct multi_transaction *t;
+
+	if (size > MULTI_TRANSACTION_LIMIT - 1)
+		return ERR_PTR(-EFBIG);
+
+	t = (struct multi_transaction *)get_zeroed_page(GFP_KERNEL);
+	if (!t)
+		return ERR_PTR(-ENOMEM);
+	kref_init(&t->count);
+	if (copy_from_user(t->data, buf, size))
+		return ERR_PTR(-EFAULT);
+
+	return t;
+}
+
+static ssize_t multi_transaction_read(struct file *file, char __user *buf,
+				       size_t size, loff_t *pos)
+{
+	struct multi_transaction *t;
+	ssize_t ret;
+
+	spin_lock(&multi_transaction_lock);
+	t = get_multi_transaction(file->private_data);
+	spin_unlock(&multi_transaction_lock);
+	if (!t)
+		return 0;
+
+	ret = simple_read_from_buffer(buf, size, pos, t->data, t->size);
+	put_multi_transaction(t);
+
+	return ret;
+}
+
+static int multi_transaction_release(struct inode *inode, struct file *file)
+{
+	put_multi_transaction(file->private_data);
+
+	return 0;
+}
+
 #define QUERY_CMD_DATA		"data\0"
 #define QUERY_CMD_DATA_LEN	5
 
@@ -700,36 +800,38 @@ static ssize_t query_data(char *buf, size_t buf_len,
 static ssize_t aa_write_access(struct file *file, const char __user *ubuf,
 			       size_t count, loff_t *ppos)
 {
-	char *buf;
+	struct multi_transaction *t;
 	ssize_t len;
 
 	if (*ppos)
 		return -ESPIPE;
 
-	buf = simple_transaction_get(file, ubuf, count);
-	if (IS_ERR(buf))
-		return PTR_ERR(buf);
+	t = multi_transaction_new(file, ubuf, count);
+	if (IS_ERR(t))
+		return PTR_ERR(t);
 
 	if (count > QUERY_CMD_DATA_LEN &&
-		   !memcmp(buf, QUERY_CMD_DATA, QUERY_CMD_DATA_LEN)) {
-		len = query_data(buf, SIMPLE_TRANSACTION_LIMIT,
-				 buf + QUERY_CMD_DATA_LEN,
+		   !memcmp(t->data, QUERY_CMD_DATA, QUERY_CMD_DATA_LEN)) {
+		len = query_data(t->data, MULTI_TRANSACTION_LIMIT,
+				 t->data + QUERY_CMD_DATA_LEN,
 				 count - QUERY_CMD_DATA_LEN);
 	} else
 		len = -EINVAL;
 
-	if (len < 0)
+	if (len < 0) {
+		put_multi_transaction(t);
 		return len;
+	}
 
-	simple_transaction_set(file, len);
+	multi_transaction_set(file, t, len);
 
 	return count;
 }
 
 static const struct file_operations aa_sfs_access = {
 	.write		= aa_write_access,
-	.read		= simple_transaction_read,
-	.release	= simple_transaction_release,
+	.read		= multi_transaction_read,
+	.release	= multi_transaction_release,
 	.llseek		= generic_file_llseek,
 };
 
@@ -1851,6 +1953,7 @@ static struct aa_sfs_entry aa_sfs_entry_policy[] = {
 
 static struct aa_sfs_entry aa_sfs_entry_query_label[] = {
 	AA_SFS_FILE_BOOLEAN("data",		1),
+	AA_SFS_FILE_BOOLEAN("multi_transaction",	1),
 	{ }
 };