Re: [PATCH 3/3] btrfs: sysfs: protect reading label by lock

[Filipe Manana <fdmanana@gmail.com>]

On Tue, Apr 26, 2016 at 3:32 PM, David Sterba <dsterba@suse.com> wrote:
> If the label setting ioctl races with sysfs label handler, we could get
> mixed result in the output, part old part new. We should either get the
> old or new label. The chances to hit this race are low.
>
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/sysfs.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index 3d14618ce54b..7b0da1dcb6df 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -367,7 +367,12 @@ static ssize_t btrfs_label_show(struct kobject *kobj,
>  {
>         struct btrfs_fs_info *fs_info = to_fs_info(kobj);
>         char *label = fs_info->super_copy->label;
> -       return snprintf(buf, PAGE_SIZE, label[0] ? "%s\n" : "%s", label);
> +
> +       spin_lock(&fs_info->super_lock);
> +       snprintf(buf, PAGE_SIZE, label[0] ? "%s\n" : "%s", label);
> +       spin_unlock(&fs_info->super_lock);
> +
> +       return buf;

We should return a ssize_t value, not a char *. I.e. return what
snprintf returns. This should make gcc emit a warning.

>  }
>
>  static ssize_t btrfs_label_store(struct kobject *kobj,
> --
> 2.7.1
>
> --
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-- 
Filipe David Manana,

"Reasonable men adapt themselves to the world.
 Unreasonable men adapt the world to themselves.
 That's why all progress depends on unreasonable men."