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From: "NeilBrown" <neilb@suse.de>
To: Trond Myklebust <trond.myklebust@hammerspace.com>,
 Anna Schumaker <anna@kernel.org>
Cc: linux-nfs@vger.kernel.org
Subject: [PATCH] NFS: add atomic_open for NFSv3 to handle O_TRUNC correctly.
Date: Mon, 25 Mar 2024 16:36:05 +1100
Message-id: <171134496555.13576.1334297096866165638@noble.neil.brown.name>


With two clients, each with NFSv3 mounts of the same directory, the sequence:

   client1            client2
  ls -l afile
                      echo hello there > afile
  echo HELLO > afile
  cat afile

will show
   HELLO
   there

because the O_TRUNC requested in the final 'echo' doesn't take effect.
This is because the "Negative dentry, just create a file" section in
lookup_open() assumes that the file *does* get created since the dentry
was negative, so it sets FMODE_CREATED, and this causes do_open() to
clear O_TRUNC and so the file doesn't get truncated.

Even mounting with -o lookupcache=3Dnone does not help as
nfs_neg_need_reval() always returns false if LOOKUP_CREATE is set.

This patch fixes the problem by providing an atomic_open inode operation
for NFSv3 (and v2).  The code is largely the code from the branch in
lookup_open() when atomic_open is not provided.  The significant change
is that the O_TRUNC flag is passed a new nfs_do_create() which add
'trunc' handling to nfs_create().

With this change we also optimise away an unnecessary LOOKUP before the
file is created.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/nfs/dir.c           | 54 +++++++++++++++++++++++++++++++++++++++---
 fs/nfs/nfs3proc.c      |  1 +
 fs/nfs/proc.c          |  1 +
 include/linux/nfs_fs.h |  3 +++
 4 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index ac505671efbd..342930996226 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -56,6 +56,8 @@ static int nfs_readdir(struct file *, struct dir_context *);
 static int nfs_fsync_dir(struct file *, loff_t, loff_t, int);
 static loff_t nfs_llseek_dir(struct file *, loff_t, int);
 static void nfs_readdir_clear_array(struct folio *);
+static int nfs_do_create(struct inode *dir, struct dentry *dentry,
+			 umode_t mode, int open_flags);
=20
 const struct file_operations nfs_dir_operations =3D {
 	.llseek		=3D nfs_llseek_dir,
@@ -2243,6 +2245,41 @@ static int nfs4_lookup_revalidate(struct dentry *dentr=
y, unsigned int flags)
=20
 #endif /* CONFIG_NFSV4 */
=20
+int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
+			struct file *file, unsigned int open_flags,
+			umode_t mode)
+{
+
+	/* Same as look+open from lookup_open(), but with different O_TRUNC
+	 * handling.
+	 */
+	int error =3D 0;
+
+	if (open_flags & O_CREAT) {
+		file->f_mode |=3D FMODE_CREATED;
+		error =3D nfs_do_create(dir, dentry, mode, open_flags);
+		if (error)
+			return error;
+		return finish_open(file, dentry, NULL);
+	} else if (d_in_lookup(dentry)) {
+		/* The only flags nfs_lookup considers are
+		 * LOOKUP_EXCL and LOOKUP_RENAME_TARGET, and
+		 * we want those to be zero so the lookup isn't skipped.
+		 */
+		struct dentry *res =3D nfs_lookup(dir, dentry, 0);
+
+		d_lookup_done(dentry);
+		if (unlikely(res)) {
+			if (IS_ERR(res))
+				return PTR_ERR(res);
+			return finish_no_open(file, res);
+		}
+	}
+	return finish_no_open(file, NULL);
+
+}
+EXPORT_SYMBOL_GPL(nfs_atomic_open_v23);
+
 struct dentry *
 nfs_add_or_obtain(struct dentry *dentry, struct nfs_fh *fhandle,
 				struct nfs_fattr *fattr)
@@ -2303,18 +2340,23 @@ EXPORT_SYMBOL_GPL(nfs_instantiate);
  * that the operation succeeded on the server, but an error in the
  * reply path made it appear to have failed.
  */
-int nfs_create(struct mnt_idmap *idmap, struct inode *dir,
-	       struct dentry *dentry, umode_t mode, bool excl)
+static int nfs_do_create(struct inode *dir, struct dentry *dentry,
+			 umode_t mode, int open_flags)
 {
 	struct iattr attr;
-	int open_flags =3D excl ? O_CREAT | O_EXCL : O_CREAT;
 	int error;
=20
+	open_flags |=3D O_CREAT;
+
 	dfprintk(VFS, "NFS: create(%s/%lu), %pd\n",
 			dir->i_sb->s_id, dir->i_ino, dentry);
=20
 	attr.ia_mode =3D mode;
 	attr.ia_valid =3D ATTR_MODE;
+	if (open_flags & O_TRUNC) {
+		attr.ia_size =3D 0;
+		attr.ia_valid |=3D ATTR_SIZE;
+	}
=20
 	trace_nfs_create_enter(dir, dentry, open_flags);
 	error =3D NFS_PROTO(dir)->create(dir, dentry, &attr, open_flags);
@@ -2326,6 +2368,12 @@ int nfs_create(struct mnt_idmap *idmap, struct inode *=
dir,
 	d_drop(dentry);
 	return error;
 }
+
+int nfs_create(struct mnt_idmap *idmap, struct inode *dir,
+	       struct dentry *dentry, umode_t mode, bool excl)
+{
+	return nfs_do_create(dir, dentry, mode, excl ? O_EXCL : 0);
+}
 EXPORT_SYMBOL_GPL(nfs_create);
=20
 /*
diff --git a/fs/nfs/nfs3proc.c b/fs/nfs/nfs3proc.c
index cbbe3f0193b8..74bda639a7cf 100644
--- a/fs/nfs/nfs3proc.c
+++ b/fs/nfs/nfs3proc.c
@@ -986,6 +986,7 @@ static int nfs3_have_delegation(struct inode *inode, fmod=
e_t flags)
=20
 static const struct inode_operations nfs3_dir_inode_operations =3D {
 	.create		=3D nfs_create,
+	.atomic_open	=3D nfs_atomic_open_v23,
 	.lookup		=3D nfs_lookup,
 	.link		=3D nfs_link,
 	.unlink		=3D nfs_unlink,
diff --git a/fs/nfs/proc.c b/fs/nfs/proc.c
index ad3a321ae997..d105e5b2659d 100644
--- a/fs/nfs/proc.c
+++ b/fs/nfs/proc.c
@@ -695,6 +695,7 @@ static int nfs_have_delegation(struct inode *inode, fmode=
_t flags)
 static const struct inode_operations nfs_dir_inode_operations =3D {
 	.create		=3D nfs_create,
 	.lookup		=3D nfs_lookup,
+	.atomic_open	=3D nfs_atomic_open_v23,
 	.link		=3D nfs_link,
 	.unlink		=3D nfs_unlink,
 	.symlink	=3D nfs_symlink,
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index d59116ac8209..039898d70954 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -561,6 +561,9 @@ extern int nfs_may_open(struct inode *inode, const struct=
 cred *cred, int openfl
 extern void nfs_access_zap_cache(struct inode *inode);
 extern int nfs_access_get_cached(struct inode *inode, const struct cred *cre=
d,
 				 u32 *mask, bool may_block);
+extern int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
+			       struct file *file, unsigned int open_flags,
+			       umode_t mode);
=20
 /*
  * linux/fs/nfs/symlink.c
--=20
2.44.0