NFS: add atomic_open for NFSv3 to handle O_TRUNC correctly.

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 <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>

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);
 
 const struct file_operations nfs_dir_operations = {
  .llseek = nfs_llseek_dir,
@@ -2243,6 +2245,41 @@ static int nfs4_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 
 #endif /* CONFIG_NFSV4 */
 
+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 = 0;
+
+ if (open_flags & O_CREAT) {
+ file->f_mode |= FMODE_CREATED;
+ error = 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 = 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 = 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);
@@ -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);
 
 /*

fs/nfs/nfs3proc.c

@@ -986,6 +986,7 @@ static int nfs3_have_delegation(struct inode *inode, fmode_t flags)
 
 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,

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 = {
  .create = nfs_create,
  .lookup = nfs_lookup,
+ .atomic_open = nfs_atomic_open_v23,
  .link = nfs_link,
  .unlink = nfs_unlink,
  .symlink = nfs_symlink,

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 *cred,
  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);
 
 /*
  * linux/fs/nfs/symlink.c