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

From: J. Bruce Fields
Date: Mon Dec 11 2017 - 11:08:34 EST


On Mon, Dec 11, 2017 at 05:04:05PM +1100, NeilBrown wrote:
> 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.

That's interesting--not the interface we might have chosen if we were
starting from scratch, but I can't see why it wouldn't work. OK!
ACK.--b.

>
> 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 <neilb@xxxxxxxx>
> ---
> 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);
>
>
> --
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