Re: [patch 5/9] unprivileged mounts: allow unprivileged bind mounts

[Dave Hansen <haveblue@us.ibm.com>]

On Tue, 2008-01-08 at 12:35 +0100, Miklos Szeredi wrote:
> @@ -510,10 +533,16 @@ static struct vfsmount *clone_mnt(struct
>                                         int flag)
>  {
>         struct super_block *sb = old->mnt_sb;
> -       struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
> +       struct vfsmount *mnt;
> 
> +       if (flag & CL_SETUSER) {
> +               int err = reserve_user_mount();
> +               if (err)
> +                       return ERR_PTR(err);
> +       }
> +       mnt = alloc_vfsmnt(old->mnt_devname);
>         if (!mnt)
> -               return ERR_PTR(-ENOMEM);
> +               goto alloc_failed;
> 
>         mnt->mnt_flags = old->mnt_flags;
>         atomic_inc(&sb->s_active); 

I think there's a little race here.  We could have several users racing
to get to this point when nr_user_mounts==max_user_mounts-1.  One user
wins the race and gets their mount reserved.  The others get the error
out of reserve_user_mount(), and return.

But, the winner goes on to error out on some condition further down in
clone_mnt() and never actually instantiates the mount.

Do you think this is a problem?

I think just about the one solution is to block new mounters on a
sleepable lock until the race winner actually finishes their mount
operation.  

-- Dave