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

[Stanislav Kinsbursky]

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);