fs: provide simple rcu-walk generic_check_acl implementation

This simple implementation just checks for no ACLs on the inode, and
if so, then the rcu-walk may proceed, otherwise fail it.

This could easily be extended to put acls under RCU and check them
under seqlock, if need be. But this implementation is enough to show
the rcu-walk aware permissions code for path lookups is working, and
will handle cases where there are no ACLs or ACLs in just the final
element.

This patch implicity converts tmpfs to rcu-aware permission check.
Subsequent patches onvert ext*, xfs, and, btrfs. Each of these uses
acl/permission code in a different way, so convert them all to provide
templates and proof of concept.
Signed-off-by: Nick Piggin <npiggin@kernel.dk>

fs/generic_acl.c

@@ -192,16 +192,18 @@ generic_acl_chmod(struct inode *inode)
 int
 generic_check_acl(struct inode *inode, int mask, unsigned int flags)
 {
-	struct posix_acl *acl;
-
-	if (flags & IPERM_FLAG_RCU)
-		return -ECHILD;
-
-	acl = get_cached_acl(inode, ACL_TYPE_ACCESS);
-	if (acl) {
-		int error = posix_acl_permission(inode, acl, mask);
-		posix_acl_release(acl);
-		return error;
+	if (flags & IPERM_FLAG_RCU) {
+		if (!negative_cached_acl(inode, ACL_TYPE_ACCESS))
+			return -ECHILD;
+	} else {
+		struct posix_acl *acl;
+
+		acl = get_cached_acl(inode, ACL_TYPE_ACCESS);
+		if (acl) {
+			int error = posix_acl_permission(inode, acl, mask);
+			posix_acl_release(acl);
+			return error;
+		}
 	}
 	return -EAGAIN;
 }

include/linux/posix_acl.h

@@ -108,6 +108,25 @@ static inline struct posix_acl *get_cached_acl(struct inode *inode, int type)
 	return acl;
 }
 
+static inline int negative_cached_acl(struct inode *inode, int type)
+{
+	struct posix_acl **p, *acl;
+	switch (type) {
+	case ACL_TYPE_ACCESS:
+		p = &inode->i_acl;
+		break;
+	case ACL_TYPE_DEFAULT:
+		p = &inode->i_default_acl;
+		break;
+	default:
+		BUG();
+	}
+	acl = ACCESS_ONCE(*p);
+	if (acl)
+		return 0;
+	return 1;
+}
+
 static inline void set_cached_acl(struct inode *inode,
 				  int type,
 				  struct posix_acl *acl)