[PATCH 4/4] fhandle: Improve error responses in name_to_handle_at()

[NeilBrown]

1/ Always return the mnt_id, even if some other error occurs.
   It can be useful without the file handle.
   An application can initialise the memory to, e.g. -1
   and if there is some other value after name_to_handle_at()
   returns, then it is a valid mnt_id.
   If the value is unchanged, then the kernel does not
   have this patch.

2/ Don't return -EINVAL if the requested handle_bytes is
   larger than MAX_HANDLE_SZ.  There is no need for an
   error and it causes unnecessary behavior change
   in the kernel ever needs to increase MAX_HANDLE_SZ.
   Simple limit handle_bytes to MAX_HANDLE_SZ silently.

Signed-off-by: NeilBrown <ne...@suse.com>
---
 fs/fhandle.c |   16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 0ace128f5d23..04afffaeb742 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -23,9 +23,16 @@ static long do_sys_name_to_handle(struct path *path,
        int handle_dwords, handle_bytes;
        struct file_handle *handle = NULL;
 
+       /*
+        * Always return the mnt_id, it might be useful even
+        * without the file handle
+        */
+       if (copy_to_user(mnt_id, &real_mount(path->mnt)->mnt_id,
+                        sizeof(*mnt_id)))
+               return -EFAULT;
        /*
         * We need to make sure whether the file system
-        * support decoding of the file handle
+        * supports decoding of the file handle.
         */
        if (!path->dentry->d_sb->s_export_op ||
            !path->dentry->d_sb->s_export_op->fh_to_dentry)
@@ -35,7 +42,7 @@ static long do_sys_name_to_handle(struct path *path,
                return -EFAULT;
 
        if (f_handle.handle_bytes > MAX_HANDLE_SZ)
-               return -EINVAL;
+               f_handle.handle_bytes = MAX_HANDLE_SZ;
 
        handle = kmalloc(sizeof(struct file_handle) + f_handle.handle_bytes,
                         GFP_KERNEL);
@@ -68,10 +75,7 @@ static long do_sys_name_to_handle(struct path *path,
                retval = -EOVERFLOW;
        } else
                retval = 0;
-       /* copy the mount id */
-       if (copy_to_user(mnt_id, &real_mount(path->mnt)->mnt_id,
-                        sizeof(*mnt_id)) ||
-           copy_to_user(ufh, handle,
+       if (copy_to_user(ufh, handle,
                         sizeof(struct file_handle) + handle_bytes))
                retval = -EFAULT;
        kfree(handle);