fs: allow AT_EMPTY_PATH in linkat(), limit that to CAP_DAC_READ_SEARCH

We don't want to allow creation of private hardlinks by different application
using the fd passed to them via SCM_RIGHTS. So limit the null relative name
usage in linkat syscall to CAP_DAC_READ_SEARCH
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

fs/namei.c

@@ -2945,15 +2945,27 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
 	struct dentry *new_dentry;
 	struct nameidata nd;
 	struct path old_path;
+	int how = 0;
 	int error;
 	char *to;
 
-	if ((flags & ~AT_SYMLINK_FOLLOW) != 0)
+	if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
 		return -EINVAL;
+	/*
+	 * To use null names we require CAP_DAC_READ_SEARCH
+	 * This ensures that not everyone will be able to create
+	 * handlink using the passed filedescriptor.
+	 */
+	if (flags & AT_EMPTY_PATH) {
+		if (!capable(CAP_DAC_READ_SEARCH))
+			return -ENOENT;
+		how = LOOKUP_EMPTY;
+	}
+
+	if (flags & AT_SYMLINK_FOLLOW)
+		how |= LOOKUP_FOLLOW;
 
-	error = user_path_at(olddfd, oldname,
-			     flags & AT_SYMLINK_FOLLOW ? LOOKUP_FOLLOW : 0,
-			     &old_path);
+	error = user_path_at(olddfd, oldname, how, &old_path);
 	if (error)
 		return error;