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    Subject[PATCH AUTOSEL 6.1 09/27] fs/kernfs/dir: obey S_ISGID
    Date
    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 44842e6cf0a9..a00e11ebfa77 100644
    --- a/fs/kernfs/dir.c
    +++ b/fs/kernfs/dir.c
    @@ -669,6 +669,18 @@ 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

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