Re: [PATCH 2/6] nfsd: swap fs root in NFSd kthreads

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Dec 06, 2012 at 06:34:47PM +0300, Stanislav Kinsbursky wrote:
> NFSd does lookup. Lookup is done starting from current->fs->root.
> NFSd is a kthread, cloned by kthreadd, and thus have global (but luckely
> unshared) root.
> So we have to swap root to those, which process, started NFSd, has. Because
> that process can be in a container with it's own root.

This doesn't sound right to me.

Which lookups exactly do you see being done relative to
current->fs->root ?

--b.

> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
> ---
>  fs/nfsd/netns.h  |    1 +
>  fs/nfsd/nfssvc.c |   33 ++++++++++++++++++++++++++++++++-
>  2 files changed, 33 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index abfc97c..5c423c6 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -101,6 +101,7 @@ struct nfsd_net {
>  	struct timeval nfssvc_boot;
>  
>  	struct svc_serv *nfsd_serv;
> +	struct path	root;
>  };
>  
>  extern int nfsd_net_id;
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index cee62ab..177bb60 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -392,6 +392,7 @@ int nfsd_create_serv(struct net *net)
>  
>  	set_max_drc();
>  	do_gettimeofday(&nn->nfssvc_boot);		/* record boot time */
> +	get_fs_root(current->fs, &nn->root);
>  	return 0;
>  }
>  
> @@ -426,8 +427,10 @@ void nfsd_destroy(struct net *net)
>  	if (destroy)
>  		svc_shutdown_net(nn->nfsd_serv, net);
>  	svc_destroy(nn->nfsd_serv);
> -	if (destroy)
> +	if (destroy) {
> +		path_put(&nn->root);
>  		nn->nfsd_serv = NULL;
> +	}
>  }
>  
>  int nfsd_set_nrthreads(int n, int *nthreads, struct net *net)
> @@ -533,6 +536,25 @@ out:
>  	return error;
>  }
>  
> +/*
> + * This function is actually slightly modified set_fs_root()<F9>
> + */
> +static void nfsd_swap_root(struct net *net)
> +{
> +	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> +	struct fs_struct *fs = current->fs;
> +	struct path old_root;
> +
> +	path_get(&nn->root);
> +	spin_lock(&fs->lock);
> +	write_seqcount_begin(&fs->seq);
> +	old_root = fs->root;
> +	fs->root = nn->root;
> +	write_seqcount_end(&fs->seq);
> +	spin_unlock(&fs->lock);
> +	if (old_root.dentry)
> +		path_put(&old_root);
> +}
>  
>  /*
>   * This is the NFS server kernel thread
> @@ -559,6 +581,15 @@ nfsd(void *vrqstp)
>  	current->fs->umask = 0;
>  
>  	/*
> +	 * We have to swap NFSd kthread's fs->root.
> +	 * Why so? Because NFSd can be started in container, which has it's own
> +	 * root.
> +	 * And so what? NFSd lookup files, and lookup start from
> +	 * current->fs->root.
> +	 */
> +	nfsd_swap_root(net);
> +
> +	/*
>  	 * thread is spawned with all signals set to SIG_IGN, re-enable
>  	 * the ones that will bring down the thread
>  	 */
>