cifs: Use file_dentry()

CIFS may be used as lower layer of overlayfs and accessing f_path.dentry can
lead to a crash.

Fix by replacing direct access of file->f_path.dentry with the
file_dentry() accessor, which will always return a native object.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Acked-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/file.c

@@ -271,7 +271,7 @@ struct cifsFileInfo *
 cifs_new_fileinfo(struct cifs_fid *fid, struct file *file,
 		  struct tcon_link *tlink, __u32 oplock)
 {
-	struct dentry *dentry = file->f_path.dentry;
+	struct dentry *dentry = file_dentry(file);
 	struct inode *inode = d_inode(dentry);
 	struct cifsInodeInfo *cinode = CIFS_I(inode);
 	struct cifsFileInfo *cfile;
@@ -461,7 +461,7 @@ int cifs_open(struct inode *inode, struct file *file)
 	tcon = tlink_tcon(tlink);
 	server = tcon->ses->server;
 
-	full_path = build_path_from_dentry(file->f_path.dentry);
+	full_path = build_path_from_dentry(file_dentry(file));
 	if (full_path == NULL) {
 		rc = -ENOMEM;
 		goto out;

fs/cifs/readdir.c

@@ -303,7 +303,7 @@ initiate_cifs_search(const unsigned int xid, struct file *file)
 	cifsFile->invalidHandle = true;
 	cifsFile->srch_inf.endOfSearch = false;
 
-	full_path = build_path_from_dentry(file->f_path.dentry);
+	full_path = build_path_from_dentry(file_dentry(file));
 	if (full_path == NULL) {
 		rc = -ENOMEM;
 		goto error_exit;
@@ -762,7 +762,7 @@ static int cifs_filldir(char *find_entry, struct file *file,
 		 */
 		fattr.cf_flags |= CIFS_FATTR_NEED_REVAL;
 
-	cifs_prime_dcache(file->f_path.dentry, &name, &fattr);
+	cifs_prime_dcache(file_dentry(file), &name, &fattr);
 
 	ino = cifs_uniqueid_to_ino_t(fattr.cf_uniqueid);
 	return !dir_emit(ctx, name.name, name.len, ino, fattr.cf_dtype);