fs/kernfs/dir: obey S_ISGID

[ 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>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8b2bd65..62d39ec 100644 (file)
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -676,6 +676,18 @@ 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) {