make build_open_flags() treat O_CREAT | O_EXCL as implying O_NOFOLLOW

O_CREAT | O_EXCL means "-EEXIST if we run into a trailing symlink".
As it is, we might or might not have LOOKUP_FOLLOW in op->intent
in that case - that depends upon having O_NOFOLLOW in open flags.
It doesn't matter, since we won't be checking it in that case -
do_last() bails out earlier.

However, making sure it's not set (i.e. acting as if we had an explicit
O_NOFOLLOW) makes the behaviour more explicit and allows to reorder the
check for O_CREAT | O_EXCL in do_last() with the call of step_into()
immediately following it.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -3396,22 +3396,17 @@ static int do_last(struct nameidata *nd,
 	if (unlikely(error < 0))
 		return error;
 
-	/*
-	 * create/update audit record if it already exists.
-	 */
-	audit_inode(nd->name, path.dentry, 0);
-
-	if (unlikely((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))) {
-		path_to_nameidata(&path, nd);
-		return -EEXIST;
-	}
-
 	seq = 0;	/* out of RCU mode, so the value doesn't matter */
 	inode = d_backing_inode(path.dentry);
 finish_lookup:
 	error = step_into(nd, &path, 0, inode, seq);
 	if (unlikely(error))
 		return error;
+
+	if (unlikely((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))) {
+		audit_inode(nd->name, nd->path.dentry, 0);
+		return -EEXIST;
+	}
 finish_open:
 	/* Why this, you ask?  _Now_ we might have grown LOOKUP_JUMPED... */
 	error = complete_walk(nd);

fs/open.c

@@ -1049,8 +1049,10 @@ inline int build_open_flags(const struct open_how *how, struct open_flags *op)
 
 	if (flags & O_CREAT) {
 		op->intent |= LOOKUP_CREATE;
-		if (flags & O_EXCL)
+		if (flags & O_EXCL) {
 			op->intent |= LOOKUP_EXCL;
+			flags |= O_NOFOLLOW;
+		}
 	}
 
 	if (flags & O_DIRECTORY)