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

[Filipe Manana <fdmanana@kernel.org>]

On Mon, Dec 17, 2018 at 8:25 AM Anand Jain <anand.jain@oracle.com> wrote:
>
>
>
> On 12/15/2018 03:45 AM, fdmanana@kernel.org wrote:
> > From: Filipe Manana <fdmanana@suse.com>
> >
> > If the call to btrfs_balance() 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 btrfs_balance()
> > returned success.
> >
> > 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 abe45fd97ab5..4ad7288f77d0 100644
> > --- a/fs/btrfs/ioctl.c
> > +++ b/fs/btrfs/ioctl.c
> > @@ -4707,7 +4707,7 @@ static long btrfs_ioctl_balance(struct file *file, void __user *arg)
> >       ret = btrfs_balance(fs_info, bctl, bargs);
> >       bctl = NULL;
> >
> > -     if (arg) {
> > +     if (ret == 0 && arg) {
> >               if (copy_to_user(arg, bargs, sizeof(*bargs)))
> >                       ret = -EFAULT;
> >       }
> >
>
> This changes the BTRFS_IOC_BALANCE_V2 semantics.
>
> Originally, if BTRFS_IOC_BALANCE_V2 is failed/canceled we do update
> bargs [1] and copy to user land, but after this patch we copy only
> if the ioctl is successful. I doubt if David is OK with that.

I thought about that. But that logic, if it's really in use by someone
(something) out there then they must be treating -EFAULT differently
from any other error.
Because if copy_to_user() fails it means it didn't copy all bytes it
was so supposed to (its return value is number of bytes not copied),
so the arguments structure ends up in an inconsistent state.


>
> [1]
> ----------
> 3717 int btrfs_balance(struct btrfs_fs_info *fs_info,
> 3718                   struct btrfs_balance_control *bctl,
> 3719                   struct btrfs_ioctl_balance_args *bargs)
> 3720 {
> ::
>
> 3855         ret = __btrfs_balance(fs_info);
> 3856
> 3857         mutex_lock(&fs_info->balance_mutex);
> 3858         clear_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags);
> 3859
> 3860         if (bargs) {
> 3861                 memset(bargs, 0, sizeof(*bargs));
> 3862                 btrfs_update_ioctl_balance_args(fs_info, bargs);
> 3863         }
>