fs: add umh argument to struct kernel_clone_args

Add a umh field to struct kernel_clone_args. When set, copy_fs() copies
from pid 1's fs_struct instead of the kthread's fs_struct. This ensures
usermodehelper threads always get init's filesystem state regardless of
their parent's (kthreadd's) fs.

Usermodehelper threads are not allowed to create mount namespaces
(CLONE_NEWNS), share filesystem state (CLONE_FS), or be started from
a non-initial mount namespace. No usermodehelper currently does this so
we don't need to worry about this restriction.

Set .umh = 1 in user_mode_thread(). At this stage pid 1's fs points to
rootfs which is the same as kthreadd's fs, so this is functionally
equivalent.

Link: https://patch.msgid.link/20260601-work-kthread-nullfs-v4-20-77ee053060e0@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 41ed884cffc9..e0c1ca8c6a18 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -31,6 +31,7 @@ struct kernel_clone_args {
 	u32 io_thread:1;
 	u32 user_worker:1;
 	u32 no_files:1;
+	u32 umh:1;
 	unsigned long stack;
 	unsigned long stack_size;
 	unsigned long tls;