[PATCH 2/2] fs/kernfs/dir: obey S_ISGID

[Max Kellermann <max.kellermann@ionos.com>]

Handling of S_ISGID is usually done by inode_init_owner() in all other
filesystems, but kernfs doesn't use that function.  In kernfs, struct
kernfs_node is the primary data structure, and struct inode is only
created from it on demand.  Therefore, inode_init_owner() can't be
used and we need to imitate its behavior.

S_ISGID support is useful for the cgroup filesystem; it allows
subtrees managed by an unprivileged process to retain a certain owner
gid, which then enables sharing access to the subtree with another
unprivileged process.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/kernfs/dir.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8b2bd65d70e7..7580cc340d28 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -676,6 +676,17 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
 {
 	struct kernfs_node *kn;
 
+	if (parent->mode & S_ISGID) {
+		/* this code block imitates inode_init_owner() for
+		 * kernfs */
+
+		if (parent->iattr)
+			gid = parent->iattr->ia_gid;
+
+		if (flags & KERNFS_DIR)
+			mode |= S_ISGID;
+	}
+
 	kn = __kernfs_new_node(kernfs_root(parent), parent,
 			       name, mode, uid, gid, flags);
 	if (kn) {
-- 
2.39.2