From: NeilBrown <neilb@suse.de>
Date: Mon, 4 Mar 2024 10:54:34 +1100
Subject: [PATCH] NFS: add atomic_open for NFSv3 to handle O_TRUNC correctly.
Patch-mainline: Submitted -  05mar2024 linux-nfs
References: bsc#1219847 bsc#1221862

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=none 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>
Acked-by: NeilBrown <neilb@suse.com>

---
 fs/nfs/dir.c           |   56 ++++++++++++++++++++++++++++++++++++++++++++++---
 fs/nfs/nfs3proc.c      |    1 
 fs/nfs/proc.c          |    1 
 include/linux/nfs_fs.h |    3 ++
 4 files changed, 58 insertions(+), 3 deletions(-)

--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -53,6 +53,8 @@ static int nfs_readdir(struct file *, st
 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 page*);
+static int nfs_do_create(struct inode *dir, struct dentry *dentry,
+			 umode_t mode, int open_flags);
 
 const struct file_operations nfs_dir_operations = {
 	.llseek		= nfs_llseek_dir,
@@ -1721,6 +1723,43 @@ static int nfs4_lookup_revalidate(struct
 }
 
 #endif /* CONFIG_NFSV4 */
+int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
+			struct file *file, unsigned int open_flags,
+			umode_t mode, int *opened)
+{
+
+	/* Same as look+open from lookup_open(), but with different O_TRUNC
+	 * handling.
+	 */
+	int error = 0;
+
+	if (dentry->d_name.len > NFS_SERVER(dir)->namelen)
+		return -ENAMETOOLONG;
+
+	if (open_flags & O_CREAT) {
+		*opened |= FILE_CREATED;
+		error = nfs_do_create(dir, dentry, mode, open_flags);
+		if (error)
+			return error;
+		return finish_open(file, dentry, NULL, opened);
+	}
+
+	if (d_in_lookup(dentry)) {
+		/* The only flag that nfs_lookup uses is LOOKUP_EXCL,
+		 * and we don't want that set as it bypasses the lookup.
+		 */
+		struct dentry *res = 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);
 
 /*
  * Code common to create, mkdir, and mknod.
@@ -1771,18 +1810,23 @@ 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 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 = excl ? O_CREAT | O_EXCL : O_CREAT;
 	int error;
 
+	open_flags |= O_CREAT;
+
 	dfprintk(VFS, "NFS: create(%s/%lu), %pd\n",
 			dir->i_sb->s_id, dir->i_ino, dentry);
 
 	attr.ia_mode = mode;
 	attr.ia_valid = ATTR_MODE;
+	if (open_flags & O_TRUNC) {
+		attr.ia_size = 0;
+		attr.ia_valid |= ATTR_SIZE;
+	}
 
 	trace_nfs_create_enter(dir, dentry, open_flags);
 	error = NFS_PROTO(dir)->create(dir, dentry, &attr, open_flags);
@@ -1794,6 +1838,12 @@ out_err:
 	d_drop(dentry);
 	return error;
 }
+
+int nfs_create(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);
 
 /*
--- a/fs/nfs/nfs3proc.c
+++ b/fs/nfs/nfs3proc.c
@@ -937,6 +937,7 @@ static int nfs3_return_delegation(struct
 
 static const struct inode_operations nfs3_dir_inode_operations = {
 	.create		= nfs_create,
+	.atomic_open	= nfs_atomic_open_v23,
 	.lookup		= nfs_lookup,
 	.link		= nfs_link,
 	.unlink		= nfs_unlink,
--- a/fs/nfs/proc.c
+++ b/fs/nfs/proc.c
@@ -682,6 +682,7 @@ static int nfs_return_delegation(struct
 static const struct inode_operations nfs_dir_inode_operations = {
 	.create		= nfs_create,
 	.lookup		= nfs_lookup,
+	.atomic_open	= nfs_atomic_open_v23,
 	.link		= nfs_link,
 	.unlink		= nfs_unlink,
 	.symlink	= nfs_symlink,
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -457,6 +457,9 @@ extern int nfs_instantiate(struct dentry
 			struct nfs_fattr *fattr, struct nfs4_label *label);
 extern int nfs_may_open(struct inode *inode, struct rpc_cred *cred, int openflags);
 extern void nfs_access_zap_cache(struct inode *inode);
+extern int nfs_atomic_open_v23(struct inode *dir, struct dentry *dentry,
+			       struct file *file, unsigned int open_flags,
+			       umode_t mode, int *opened);
 
 /*
  * linux/fs/nfs/symlink.c