[PATCH AUTOSEL 5.4 04/11] fs/kernfs/dir: obey S_ISGID

Sasha Levin
2024-01-28T16:16:45Z

From: Max Kellermann <max.kellermann@ionos.com>

[ Upstream commit 5133bee62f0ea5d4c316d503cc0040cac5637601 ]

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.

--
v1 -> v2: minor coding style fix (comment)

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20231208093310.297233-2-max.kellermann@ionos.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/kernfs/dir.c | 12 ++++++++++++
1 file changed, 12 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index d2068566c0b8..d3a602ea795b 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -702,6 +702,18 @@ 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) {
--
2.43.0