NFSD: Instantiate a struct file when creating a regular NFSv4 file

There have been reports of races that cause NFSv4 OPEN(CREATE) to
return an error even though the requested file was created. NFSv4
does not provide a status code for this case.

To mitigate some of these problems, reorganize the NFSv4
OPEN(CREATE) logic to allocate resources before the file is actually
created, and open the new file while the parent directory is still
locked.

Two new APIs are added:

+ Add an API that works like nfsd_file_acquire() but does not open
the underlying file. The OPEN(CREATE) path can use this API when it
already has an open file.

+ Add an API that is kin to dentry_open(). NFSD needs to create a
file and grab an open "struct file *" atomically. The
alloc_empty_file() has to be done before the inode create. If it
fails (for example, because the NFS server has exceeded its
max_files limit), we avoid creating the file and can still return
an error to the NFS client.

BugLink: https://bugzilla.linux-nfs.org/show_bug.cgi?id=382
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: JianHong Yin <jiyin@redhat.com>

fs/nfsd/filecache.c

@@ -897,9 +897,9 @@ nfsd_file_is_cached(struct inode *inode)
 	return ret;
 }
 
-__be32
-nfsd_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
-		  unsigned int may_flags, struct nfsd_file **pnf)
+static __be32
+nfsd_do_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
+		     unsigned int may_flags, struct nfsd_file **pnf, bool open)
 {
 	__be32	status;
 	struct net *net = SVC_NET(rqstp);
@@ -994,10 +994,13 @@ open_file:
 		nfsd_file_gc();
 
 	nf->nf_mark = nfsd_file_mark_find_or_create(nf);
-	if (nf->nf_mark)
-		status = nfsd_open_verified(rqstp, fhp, may_flags,
-					    &nf->nf_file);
-	else
+	if (nf->nf_mark) {
+		if (open)
+			status = nfsd_open_verified(rqstp, fhp, may_flags,
+						    &nf->nf_file);
+		else
+			status = nfs_ok;
+	} else
 		status = nfserr_jukebox;
 	/*
 	 * If construction failed, or we raced with a call to unlink()
@@ -1017,6 +1020,40 @@ open_file:
 	goto out;
 }
 
+/**
+ * nfsd_file_acquire - Get a struct nfsd_file with an open file
+ * @rqstp: the RPC transaction being executed
+ * @fhp: the NFS filehandle of the file to be opened
+ * @may_flags: NFSD_MAY_ settings for the file
+ * @pnf: OUT: new or found "struct nfsd_file" object
+ *
+ * Returns nfs_ok and sets @pnf on success; otherwise an nfsstat in
+ * network byte order is returned.
+ */
+__be32
+nfsd_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
+		  unsigned int may_flags, struct nfsd_file **pnf)
+{
+	return nfsd_do_file_acquire(rqstp, fhp, may_flags, pnf, true);
+}
+
+/**
+ * nfsd_file_create - Get a struct nfsd_file, do not open
+ * @rqstp: the RPC transaction being executed
+ * @fhp: the NFS filehandle of the file just created
+ * @may_flags: NFSD_MAY_ settings for the file
+ * @pnf: OUT: new or found "struct nfsd_file" object
+ *
+ * Returns nfs_ok and sets @pnf on success; otherwise an nfsstat in
+ * network byte order is returned.
+ */
+__be32
+nfsd_file_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
+		 unsigned int may_flags, struct nfsd_file **pnf)
+{
+	return nfsd_do_file_acquire(rqstp, fhp, may_flags, pnf, false);
+}
+
 /*
  * Note that fields may be added, removed or reordered in the future. Programs
  * scraping this file for info should test the labels to ensure they're

fs/nfsd/filecache.h

@@ -59,5 +59,7 @@ void nfsd_file_close_inode_sync(struct inode *inode);
 bool nfsd_file_is_cached(struct inode *inode);
 __be32 nfsd_file_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
 		  unsigned int may_flags, struct nfsd_file **nfp);
+__be32 nfsd_file_create(struct svc_rqst *rqstp, struct svc_fh *fhp,
+		  unsigned int may_flags, struct nfsd_file **nfp);
 int	nfsd_file_cache_stats_open(struct inode *, struct file *);
 #endif /* _FS_NFSD_FILECACHE_H */

fs/nfsd/nfs4proc.c

@@ -243,6 +243,37 @@ static inline bool nfsd4_create_is_exclusive(int createmode)
 		createmode == NFS4_CREATE_EXCLUSIVE4_1;
 }
 
+static __be32
+nfsd4_vfs_create(struct svc_fh *fhp, struct dentry *child,
+		 struct nfsd4_open *open)
+{
+	struct file *filp;
+	struct path path;
+	int oflags;
+
+	oflags = O_CREAT | O_LARGEFILE;
+	switch (open->op_share_access & NFS4_SHARE_ACCESS_BOTH) {
+	case NFS4_SHARE_ACCESS_WRITE:
+		oflags |= O_WRONLY;
+		break;
+	case NFS4_SHARE_ACCESS_BOTH:
+		oflags |= O_RDWR;
+		break;
+	default:
+		oflags |= O_RDONLY;
+	}
+
+	path.mnt = fhp->fh_export->ex_path.mnt;
+	path.dentry = child;
+	filp = dentry_create(&path, oflags, open->op_iattr.ia_mode,
+			     current_cred());
+	if (IS_ERR(filp))
+		return nfserrno(PTR_ERR(filp));
+
+	open->op_filp = filp;
+	return nfs_ok;
+}
+
 /*
  * Implement NFSv4's unchecked, guarded, and exclusive create
  * semantics for regular files. Open state for this new file is
@@ -355,11 +386,9 @@ nfsd4_create_file(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	if (!IS_POSIXACL(inode))
 		iap->ia_mode &= ~current_umask();
 
-	host_err = vfs_create(&init_user_ns, inode, child, iap->ia_mode, true);
-	if (host_err < 0) {
-		status = nfserrno(host_err);
+	status = nfsd4_vfs_create(fhp, child, open);
+	if (status != nfs_ok)
 		goto out;
-	}
 	open->op_created = true;
 
 	/* A newly created file already has a file size of zero. */
@@ -517,6 +546,8 @@ nfsd4_open(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 		(int)open->op_fnamelen, open->op_fname,
 		open->op_openowner);
 
+	open->op_filp = NULL;
+
 	/* This check required by spec. */
 	if (open->op_create && open->op_claim_type != NFS4_OPEN_CLAIM_NULL)
 		return nfserr_inval;
@@ -613,6 +644,10 @@ nfsd4_open(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	if (reclaim && !status)
 		nn->somebody_reclaimed = true;
 out:
+	if (open->op_filp) {
+		fput(open->op_filp);
+		open->op_filp = NULL;
+	}
 	if (resfh && resfh != &cstate->current_fh) {
 		fh_dup2(&cstate->current_fh, resfh);
 		fh_put(resfh);

fs/nfsd/nfs4state.c

@@ -5093,9 +5093,19 @@ static __be32 nfs4_get_vfs_file(struct svc_rqst *rqstp, struct nfs4_file *fp,
 
 	if (!fp->fi_fds[oflag]) {
 		spin_unlock(&fp->fi_lock);
-		status = nfsd_file_acquire(rqstp, cur_fh, access, &nf);
-		if (status)
-			goto out_put_access;
+
+		if (!open->op_filp) {
+			status = nfsd_file_acquire(rqstp, cur_fh, access, &nf);
+			if (status != nfs_ok)
+				goto out_put_access;
+		} else {
+			status = nfsd_file_create(rqstp, cur_fh, access, &nf);
+			if (status != nfs_ok)
+				goto out_put_access;
+			nf->nf_file = open->op_filp;
+			open->op_filp = NULL;
+		}
+
 		spin_lock(&fp->fi_lock);
 		if (!fp->fi_fds[oflag]) {
 			fp->fi_fds[oflag] = nf;

fs/nfsd/xdr4.h

@@ -273,6 +273,7 @@ struct nfsd4_open {
 	bool		op_truncate;        /* used during processing */
 	bool		op_created;         /* used during processing */
 	struct nfs4_openowner *op_openowner; /* used during processing */
+	struct file	*op_filp;           /* used during processing */
 	struct nfs4_file *op_file;          /* used during processing */
 	struct nfs4_ol_stateid *op_stp;	    /* used during processing */
 	struct nfs4_clnt_odstate *op_odstate; /* used during processing */

fs/open.c

@@ -981,6 +981,48 @@ struct file *dentry_open(const struct path *path, int flags,
 }
 EXPORT_SYMBOL(dentry_open);
 
+/**
+ * dentry_create - Create and open a file
+ * @path: path to create
+ * @flags: O_ flags
+ * @mode: mode bits for new file
+ * @cred: credentials to use
+ *
+ * Caller must hold the parent directory's lock, and have prepared
+ * a negative dentry, placed in @path->dentry, for the new file.
+ *
+ * Caller sets @path->mnt to the vfsmount of the filesystem where
+ * the new file is to be created. The parent directory and the
+ * negative dentry must reside on the same filesystem instance.
+ *
+ * On success, returns a "struct file *". Otherwise a ERR_PTR
+ * is returned.
+ */
+struct file *dentry_create(const struct path *path, int flags, umode_t mode,
+			   const struct cred *cred)
+{
+	struct file *f;
+	int error;
+
+	validate_creds(cred);
+	f = alloc_empty_file(flags, cred);
+	if (IS_ERR(f))
+		return f;
+
+	error = vfs_create(mnt_user_ns(path->mnt),
+			   d_inode(path->dentry->d_parent),
+			   path->dentry, mode, true);
+	if (!error)
+		error = vfs_open(path, f);
+
+	if (unlikely(error)) {
+		fput(f);
+		return ERR_PTR(error);
+	}
+	return f;
+}
+EXPORT_SYMBOL(dentry_create);
+
 struct file *open_with_fake_path(const struct path *path, int flags,
 				struct inode *inode, const struct cred *cred)
 {

include/linux/fs.h

@@ -2640,6 +2640,8 @@ static inline struct file *file_open_root_mnt(struct vfsmount *mnt,
 			      name, flags, mode);
 }
 extern struct file * dentry_open(const struct path *, int, const struct cred *);
+extern struct file *dentry_create(const struct path *path, int flags,
+				  umode_t mode, const struct cred *cred);
 extern struct file * open_with_fake_path(const struct path *, int,
 					 struct inode*, const struct cred *);
 static inline struct file *file_clone_open(struct file *file)