sysfs: fix namespace refcnt leak

As mount() and kill_sb() is not a one-to-one match, we shoudn't get
ns refcnt unconditionally in sysfs_mount(), and instead we should
get the refcnt only when kernfs_mount() allocated a new superblock.

v2:
- Changed the name of the new argument, suggested by Tejun.
- Made the argument optional, suggested by Tejun.

v3:
- Make the new argument as second-to-last arg, suggested by Tejun.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
 ---
 fs/kernfs/mount.c      | 8 +++++++-
 fs/sysfs/mount.c       | 5 +++--
 include/linux/kernfs.h | 9 +++++----
 3 files changed, 15 insertions(+), 7 deletions(-)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 10 insertions, 3 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 0d6ce895a9ee..0f4152defe7b 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -94,6 +94,7 @@ const void *kernfs_super_ns(struct super_block *sb)
  * @fs_type: file_system_type of the fs being mounted
  * @flags: mount flags specified for the mount
  * @root: kernfs_root of the hierarchy being mounted
+ * @new_sb_created: tell the caller if we allocated a new superblock
  * @ns: optional namespace tag of the mount
  *
  * This is to be called from each kernfs user's file_system_type->mount()
@@ -104,7 +105,8 @@ const void *kernfs_super_ns(struct super_block *sb)
  * The return value can be passed to the vfs layer verbatim.
  */
 struct dentry *kernfs_mount_ns(struct file_system_type *fs_type, int flags,
-                               struct kernfs_root *root, const void *ns)
+                               struct kernfs_root *root, bool *new_sb_created,
+                               const void *ns)
 {
         struct super_block *sb;
         struct kernfs_super_info *info;
@@ -122,6 +124,10 @@ struct dentry *kernfs_mount_ns(struct file_system_type *fs_type, int flags,
                 kfree(info);
         if (IS_ERR(sb))
                 return ERR_CAST(sb);
+
+        if (new_sb_created)
+                *new_sb_created = !sb->s_root;
+
         if (!sb->s_root) {
                 error = kernfs_fill_super(sb);
                 if (error) {
diff --git a/fs/sysfs/mount.c b/fs/sysfs/mount.c
index 6211230814fd..3eaf5c6622eb 100644
--- a/fs/sysfs/mount.c
+++ b/fs/sysfs/mount.c
@@ -27,6 +27,7 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
 {
         struct dentry *root;
         void *ns;
+        bool new_sb;
 
         if (!(flags & MS_KERNMOUNT)) {
                 if (!capable(CAP_SYS_ADMIN) && !fs_fully_visible(fs_type))
@@ -37,8 +38,8 @@ static struct dentry *sysfs_mount(struct file_system_type *fs_type,
         }
 
         ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
-        root = kernfs_mount_ns(fs_type, flags, sysfs_root, ns);
-        if (IS_ERR(root))
+        root = kernfs_mount_ns(fs_type, flags, sysfs_root, &new_sb, ns);
+        if (IS_ERR(root) || !new_sb)
                 kobj_ns_drop(KOBJ_NS_TYPE_NET, ns);
         return root;
 }