ceph: canonicalize server path in place

syzbot reported that 4fbc0c71 ("ceph: remove the extra slashes in
the server path") had caused a regression where an allocation could be
done under a spinlock -- compare_mount_options() is called by sget_fc()
with sb_lock held.

We don't really need the supplied server path, so canonicalize it
in place and compare it directly.  To make this work, the leading
slash is kept around and the logic in ceph_real_mount() to skip it
is restored.  CEPH_MSG_CLIENT_SESSION now reports the same (i.e.
canonicalized) path, with the leading slash of course.

Fixes: 4fbc0c71 ("ceph: remove the extra slashes in the server path")
Reported-by: syzbot+98704a51af8e3d9425a9@syzkaller.appspotmail.com
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/ceph/super.c

@@ -202,6 +202,26 @@ struct ceph_parse_opts_ctx {
 	struct ceph_mount_options	*opts;
 };
 
+/*
+ * Remove adjacent slashes and then the trailing slash, unless it is
+ * the only remaining character.
+ *
+ * E.g. "//dir1////dir2///" --> "/dir1/dir2", "///" --> "/".
+ */
+static void canonicalize_path(char *path)
+{
+	int i, j = 0;
+
+	for (i = 0; path[i] != '\0'; i++) {
+		if (path[i] != '/' || j < 1 || path[j - 1] != '/')
+			path[j++] = path[i];
+	}
+
+	if (j > 1 && path[j - 1] == '/')
+		j--;
+	path[j] = '\0';
+}
+
 /*
  * Parse the source parameter.  Distinguish the server list from the path.
  *
@@ -224,15 +244,16 @@ static int ceph_parse_source(struct fs_parameter *param, struct fs_context *fc)
 
 	dev_name_end = strchr(dev_name, '/');
 	if (dev_name_end) {
-		kfree(fsopt->server_path);
-
 		/*
 		 * The server_path will include the whole chars from userland
 		 * including the leading '/'.
 		 */
+		kfree(fsopt->server_path);
 		fsopt->server_path = kstrdup(dev_name_end, GFP_KERNEL);
 		if (!fsopt->server_path)
 			return -ENOMEM;
+
+		canonicalize_path(fsopt->server_path);
 	} else {
 		dev_name_end = dev_name + strlen(dev_name);
 	}
@@ -456,73 +477,6 @@ static int strcmp_null(const char *s1, const char *s2)
 	return strcmp(s1, s2);
 }
 
-/**
- * path_remove_extra_slash - Remove the extra slashes in the server path
- * @server_path: the server path and could be NULL
- *
- * Return NULL if the path is NULL or only consists of "/", or a string
- * without any extra slashes including the leading slash(es) and the
- * slash(es) at the end of the server path, such as:
- * "//dir1////dir2///" --> "dir1/dir2"
- */
-static char *path_remove_extra_slash(const char *server_path)
-{
-	const char *path = server_path;
-	const char *cur, *end;
-	char *buf, *p;
-	int len;
-
-	/* if the server path is omitted */
-	if (!path)
-		return NULL;
-
-	/* remove all the leading slashes */
-	while (*path == '/')
-		path++;
-
-	/* if the server path only consists of slashes */
-	if (*path == '\0')
-		return NULL;
-
-	len = strlen(path);
-
-	buf = kmalloc(len + 1, GFP_KERNEL);
-	if (!buf)
-		return ERR_PTR(-ENOMEM);
-
-	end = path + len;
-	p = buf;
-	do {
-		cur = strchr(path, '/');
-		if (!cur)
-			cur = end;
-
-		len = cur - path;
-
-		/* including one '/' */
-		if (cur != end)
-			len += 1;
-
-		memcpy(p, path, len);
-		p += len;
-
-		while (cur <= end && *cur == '/')
-			cur++;
-		path = cur;
-	} while (path < end);
-
-	*p = '\0';
-
-	/*
-	 * remove the last slash if there has and just to make sure that
-	 * we will get something like "dir1/dir2"
-	 */
-	if (*(--p) == '/')
-		*p = '\0';
-
-	return buf;
-}
-
 static int compare_mount_options(struct ceph_mount_options *new_fsopt,
 				 struct ceph_options *new_opt,
 				 struct ceph_fs_client *fsc)
@@ -530,7 +484,6 @@ static int compare_mount_options(struct ceph_mount_options *new_fsopt,
 	struct ceph_mount_options *fsopt1 = new_fsopt;
 	struct ceph_mount_options *fsopt2 = fsc->mount_options;
 	int ofs = offsetof(struct ceph_mount_options, snapdir_name);
-	char *p1, *p2;
 	int ret;
 
 	ret = memcmp(fsopt1, fsopt2, ofs);
@@ -540,21 +493,12 @@ static int compare_mount_options(struct ceph_mount_options *new_fsopt,
 	ret = strcmp_null(fsopt1->snapdir_name, fsopt2->snapdir_name);
 	if (ret)
 		return ret;
+
 	ret = strcmp_null(fsopt1->mds_namespace, fsopt2->mds_namespace);
 	if (ret)
 		return ret;
 
-	p1 = path_remove_extra_slash(fsopt1->server_path);
-	if (IS_ERR(p1))
-		return PTR_ERR(p1);
-	p2 = path_remove_extra_slash(fsopt2->server_path);
-	if (IS_ERR(p2)) {
-		kfree(p1);
-		return PTR_ERR(p2);
-	}
-	ret = strcmp_null(p1, p2);
-	kfree(p1);
-	kfree(p2);
+	ret = strcmp_null(fsopt1->server_path, fsopt2->server_path);
 	if (ret)
 		return ret;
 
@@ -957,7 +901,9 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
 	mutex_lock(&fsc->client->mount_mutex);
 
 	if (!fsc->sb->s_root) {
-		const char *path, *p;
+		const char *path = fsc->mount_options->server_path ?
+				     fsc->mount_options->server_path + 1 : "";
+
 		err = __ceph_open_session(fsc->client, started);
 		if (err < 0)
 			goto out;
@@ -969,22 +915,11 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc,
 				goto out;
 		}
 
-		p = path_remove_extra_slash(fsc->mount_options->server_path);
-		if (IS_ERR(p)) {
-			err = PTR_ERR(p);
-			goto out;
-		}
-		/* if the server path is omitted or just consists of '/' */
-		if (!p)
-			path = "";
-		else
-			path = p;
 		dout("mount opening path '%s'\n", path);
 
 		ceph_fs_debugfs_init(fsc);
 
 		root = open_root_dentry(fsc, path, started);
-		kfree(p);
 		if (IS_ERR(root)) {
 			err = PTR_ERR(root);
 			goto out;

fs/ceph/super.h

@@ -91,7 +91,7 @@ struct ceph_mount_options {
 
 	char *snapdir_name;   /* default ".snap" */
 	char *mds_namespace;  /* default NULL */
-	char *server_path;    /* default  "/" */
+	char *server_path;    /* default NULL (means "/") */
 	char *fscache_uniq;   /* default NULL */
 };