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

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

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.

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
 	 */