Re: [PATCH] Btrfs: do not overwrite error return value in the device replace ioctl

[Anand Jain <anand.jain@oracle.com>]

On 12/15/2018 03:45 AM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> If the call to btrfs_dev_replace_by_ioctl() failed we would overwrite the
> error returned to user space with -EFAULT if the call to copy_to_user()
> failed as well. Fix that by calling copy_to_user() only if no error
> happened before.
> 
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   fs/btrfs/ioctl.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 0b101df822e9..abe45fd97ab5 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4401,7 +4401,7 @@ static long btrfs_ioctl_dev_replace(struct btrfs_fs_info *fs_info,
>   		break;
>   	}
>   
> -	if (copy_to_user(arg, p, sizeof(*p)))
> +	if (ret == 0 && copy_to_user(arg, p, sizeof(*p)))

  Its the same thing here too.. we copy ret to args->result even
  if it fails. Now with this patch, if ret is non-zero, its not
  in args->result at the userland anymore.  If there
  is any tool which just checks args->result will be at problem,
  which I think is rare, should be ok. It would have been nice
  to have this design as in this patch, in the original code.

------
  530         ret = btrfs_dev_replace_start(fs_info, 
args->start.tgtdev_name,
  531                                         args->start.srcdevid,
  532                                         args->start.srcdev_name,
  533 
args->start.cont_reading_from_srcdev_mode);
  534         args->result = ret;
--------



>   		ret = -EFAULT;
>   out:
>   	kfree(p);
>