[Devel] Re: [PATCH v3] SUNRPC: set desired file system root before connecting local transports

[J. Bruce Fields]

Cc'ing Eric since I seem to recall he suggested doing it this way?

Seems OK to me, but maybe that swap_root should be in common code?  (Or
maybe we could use set_fs_root()?)

I'm assuming it's up to Trond to take this.--b.

On Mon, Oct 08, 2012 at 02:56:32PM +0400, Stanislav Kinsbursky wrote:
> Today, there is a problem in connecting of local SUNRPC thansports. These
> transports uses UNIX sockets and connection itself is done by rpciod
> workqueue.
> But all local transports are connecting in rpciod context. I.e. UNIX sockets
> lookup is done in context of process file system root.
> This works nice until we will try to mount NFS from process with other root -
> for example in container. This container can have it's own (nested) root and
> rcpbind process, listening on it's own unix sockets. But NFS mount attempt in
> this container will register new service (Lockd for example) in global rpcbind
> - not containers's one.
> This patch solves the problem by switching rpciod kernel thread's file system
> root to the right one (stored on transport) while connecting of local
> transports.
> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky at parallels.com>
> ---
>  net/sunrpc/xprtsock.c |   52 +++++++++++++++++++++++++++++++++++++++++++++++--
>  1 files changed, 50 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> index aaaadfb..ecbced1 100644
> --- a/net/sunrpc/xprtsock.c
> +++ b/net/sunrpc/xprtsock.c
> @@ -37,6 +37,7 @@
>  #include <linux/sunrpc/svcsock.h>
>  #include <linux/sunrpc/xprtsock.h>
>  #include <linux/file.h>
> +#include <linux/fs_struct.h>
>  #ifdef CONFIG_SUNRPC_BACKCHANNEL
>  #include <linux/sunrpc/bc_xprt.h>
>  #endif
> @@ -255,6 +256,11 @@ struct sock_xprt {
>  	void			(*old_state_change)(struct sock *);
>  	void			(*old_write_space)(struct sock *);
>  	void			(*old_error_report)(struct sock *);
> +
> +	/*
> +	 * Saved transport creator root. Required for local transports only.
> +	 */
> +	struct path		root;
>  };
>  
>  /*
> @@ -1876,6 +1882,30 @@ static int xs_local_finish_connecting(struct rpc_xprt *xprt,
>  	return kernel_connect(sock, xs_addr(xprt), xprt->addrlen, 0);
>  }
>  
> +/*
> + * __xs_local_swap_root - swap current root to tranport's root helper.
> + * @new_root - path to set to current fs->root.
> + * @old_root - optinal paoinet to save current root to
> + *
> + * This routine is requieed to connecting unix sockets to absolute path in
> + * proper root environment.
> + * Note: no path_get() will be called. I.e. caller have to hold reference to
> + * new_root.
> + */
> +static void __xs_local_swap_root(struct path *new_root, struct path *old_root)
> +{
> +	struct fs_struct *fs = current->fs;
> +
> +	spin_lock(&fs->lock);
> +	write_seqcount_begin(&fs->seq);
> +	if (old_root)
> +		*old_root = fs->root;
> +	fs->root = *new_root;
> +	write_seqcount_end(&fs->seq);
> +	spin_unlock(&fs->lock);
> +}
> +
> +
>  /**
>   * xs_local_setup_socket - create AF_LOCAL socket, connect to a local endpoint
>   * @xprt: RPC transport to connect
> @@ -1891,6 +1921,7 @@ static void xs_local_setup_socket(struct work_struct *work)
>  	struct rpc_xprt *xprt = &transport->xprt;
>  	struct socket *sock;
>  	int status = -EIO;
> +	struct path root;
>  
>  	current->flags |= PF_FSTRANS;
>  
> @@ -1907,7 +1938,10 @@ static void xs_local_setup_socket(struct work_struct *work)
>  	dprintk("RPC:       worker connecting xprt %p via AF_LOCAL to %s\n",
>  			xprt, xprt->address_strings[RPC_DISPLAY_ADDR]);
>  
> +	__xs_local_swap_root(&transport->root, &root);
>  	status = xs_local_finish_connecting(xprt, sock);
> +	__xs_local_swap_root(&root, NULL);
> +
>  	switch (status) {
>  	case 0:
>  		dprintk("RPC:       xprt %p connected to %s\n",
> @@ -2256,6 +2290,18 @@ static void xs_connect(struct rpc_task *task)
>  	}
>  }
>  
> +static void xs_local_destroy(struct rpc_xprt *xprt)
> +{
> +	struct sock_xprt *transport = container_of(xprt, struct sock_xprt, xprt);
> +	struct path root = transport->root;
> +
> +	dprintk("RPC:       xs_local_destroy xprt %p\n", xprt);
> +
> +	xs_destroy(xprt);
> +
> +	path_put(&root);
> +}
> +
>  /**
>   * xs_local_print_stats - display AF_LOCAL socket-specifc stats
>   * @xprt: rpc_xprt struct containing statistics
> @@ -2475,7 +2521,7 @@ static struct rpc_xprt_ops xs_local_ops = {
>  	.send_request		= xs_local_send_request,
>  	.set_retrans_timeout	= xprt_set_retrans_timeout_def,
>  	.close			= xs_close,
> -	.destroy		= xs_destroy,
> +	.destroy		= xs_local_destroy,
>  	.print_stats		= xs_local_print_stats,
>  };
>  
> @@ -2654,8 +2700,10 @@ static struct rpc_xprt *xs_setup_local(struct xprt_create *args)
>  	dprintk("RPC:       set up xprt to %s via AF_LOCAL\n",
>  			xprt->address_strings[RPC_DISPLAY_ADDR]);
>  
> -	if (try_module_get(THIS_MODULE))
> +	if (try_module_get(THIS_MODULE)) {
> +		get_fs_root(current->fs, &transport->root);
>  		return xprt;
> +	}
>  	ret = ERR_PTR(-EINVAL);
>  out_err:
>  	xprt_free(xprt);
>