[patch 2/2] cifs: dereferencing first then checking

[Dan Carpenter]

Smatch complained about a couple checking for NULL after dereferencing
bugs.  I'm not super familiar with the code so I did the conservative
thing and move the dereferences after the checks.

The dereferences in cifs_lock() and cifs_fsync() were added in
ba00ba64cf0 "cifs: make various routines use the cifsFileInfo->tcon
pointer".  The dereference in find_writable_file() was added in 
6508d904e6f "cifs: have find_readable/writable_file filter by fsuid".
The comments there say it's possible to trigger the NULL dereference
under stress.

Signed-off-by: Dan Carpenter <error27 at gmail.com>

diff --git a/fs/cifs/file.c b/fs/cifs/file.c
index 06a006b..33a19b4 100644
--- a/fs/cifs/file.c
+++ b/fs/cifs/file.c
@@ -775,13 +775,13 @@ int cifs_lock(struct file *file, int cmd, struct file_lock *pfLock)
 		cFYI(1, "Unknown type of lock");
 
 	cifs_sb = CIFS_SB(file->f_path.dentry->d_sb);
-	tcon = tlink_tcon(((struct cifsFileInfo *)file->private_data)->tlink);
 
 	if (file->private_data == NULL) {
 		rc = -EBADF;
 		FreeXid(xid);
 		return rc;
 	}
+	tcon = tlink_tcon(((struct cifsFileInfo *)file->private_data)->tlink);
 	netfid = ((struct cifsFileInfo *)file->private_data)->netfid;
 
 	if ((tcon->ses->capabilities & CAP_UNIX) &&
@@ -1179,7 +1179,7 @@ struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *cifs_inode,
 					bool fsuid_only)
 {
 	struct cifsFileInfo *open_file;
-	struct cifs_sb_info *cifs_sb = CIFS_SB(cifs_inode->vfs_inode.i_sb);
+	struct cifs_sb_info *cifs_sb;
 	bool any_available = false;
 	int rc;
 
@@ -1192,6 +1192,7 @@ struct cifsFileInfo *find_writable_file(struct cifsInodeInfo *cifs_inode,
 		dump_stack();
 		return NULL;
 	}
+	cifs_sb = CIFS_SB(cifs_inode->vfs_inode.i_sb);
 
 	/* only filter by fsuid on multiuser mounts */
 	if (!(cifs_sb->mnt_cifs_flags & CIFS_MOUNT_MULTIUSER))
@@ -1628,15 +1629,26 @@ int cifs_fsync(struct file *file, int datasync)
 		file->f_path.dentry->d_name.name, datasync);
 
 	rc = filemap_write_and_wait(inode->i_mapping);
-	if (rc == 0) {
-		rc = CIFS_I(inode)->write_behind_rc;
-		CIFS_I(inode)->write_behind_rc = 0;
-		tcon = tlink_tcon(smbfile->tlink);
-		if (!rc && tcon && smbfile &&
-		   !(CIFS_SB(inode->i_sb)->mnt_cifs_flags & CIFS_MOUNT_NOSSYNC))
-			rc = CIFSSMBFlush(xid, tcon, smbfile->netfid);
-	}
+	if (rc)
+		goto err_out;
+
+	rc = CIFS_I(inode)->write_behind_rc;
+	CIFS_I(inode)->write_behind_rc = 0;
+	if (rc)
+		goto err_out;
+
+	if (CIFS_SB(inode->i_sb)->mnt_cifs_flags & CIFS_MOUNT_NOSSYNC)
+		goto done;
+	if (!smbfile)
+		goto done;
+	tcon = tlink_tcon(smbfile->tlink);
+	if (!tcon)
+		goto done;
+
+	rc = CIFSSMBFlush(xid, tcon, smbfile->netfid);
 
+err_out:
+done:
 	FreeXid(xid);
 	return rc;
 }