nfs_localio: simplify interface to nfsd for getting nfsd_file

The nfsd_localio_operations structure contains nfsd_file_get() to get a
reference to an nfsd_file.  This is only used in one place, where
nfsd_open_local_fh() is also used.

This patch combines the two, calling nfsd_open_local_fh() passing a
pointer to where the nfsd_file pointer might be stored.  If there is a
pointer there an nfsd_file_get() can get a reference, that reference is
returned.  If not a new nfsd_file is acquired, stored at the pointer,
and returned.  When we store a reference we also increase the refcount
on the net, as that refcount is decrements when we clear the stored
pointer.

We now get an extra reference *before* storing the new nfsd_file at the
given location.  This avoids possible races with the nfsd_file being
freed before the final reference can be taken.

This patch moves the rcu_dereference() needed after fetching from
ro_file or rw_file into the nfsd code where the 'struct nfs_file' is
fully defined.  This avoids an error reported by older versions of gcc
such as gcc-8 which complain about rcu_dereference() use in contexts
where the structure (which will supposedly be accessed) is not fully
defined.

Reported-by: Pali Rohár <pali@kernel.org>
Reported-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Fixes: 86e00412254a ("nfs: cache all open LOCALIO nfsd_file(s) in client")
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/nfs_common/nfslocalio.c b/fs/nfs_common/nfslocalio.c
index f6821b2c87a2..503f85f64b76 100644
--- a/fs/nfs_common/nfslocalio.c
+++ b/fs/nfs_common/nfslocalio.c
@@ -237,6 +237,7 @@ static void nfs_uuid_add_file(nfs_uuid_t *nfs_uuid, struct nfs_file_localio *nfl
 struct nfsd_file *nfs_open_local_fh(nfs_uuid_t *uuid,
 		   struct rpc_clnt *rpc_clnt, const struct cred *cred,
 		   const struct nfs_fh *nfs_fh, struct nfs_file_localio *nfl,
+		   struct nfsd_file __rcu **pnf,
 		   const fmode_t fmode)
 {
 	struct net *net;
@@ -261,7 +262,7 @@ struct nfsd_file *nfs_open_local_fh(nfs_uuid_t *uuid,
 	rcu_read_unlock();
 	/* We have an implied reference to net thanks to nfsd_net_try_get */
 	localio = nfs_to->nfsd_open_local_fh(net, uuid->dom, rpc_clnt,
-					     cred, nfs_fh, fmode);
+					     cred, nfs_fh, pnf, fmode);
 	nfs_to_nfsd_net_put(net);
 	if (!IS_ERR(localio))
 		nfs_uuid_add_file(uuid, nfl);