kmod: don't run async usermode helper as a child of kworker thread

call_usermodehelper_exec_sync() does fork() + wait() with "unignored"
SIGCHLD.  What we have missed is that this worker thread can have other
children previously forked by call_usermodehelper_exec_work() without
UMH_WAIT_PROC.  If such a child exits in between it becomes a zombie
because auto-reaping only works if SIGCHLD is ignored, and nobody can
reap it (unless/until this worker thread exits too).

Change the !UMH_WAIT_PROC case to use CLONE_PARENT.

Note: this is only first step.  All PF_KTHREAD tasks, even created by
kernel_thread() should have ->parent == kthreadd by default.

Fixes: bb304a5c6fc63d8506c ("kmod: handle UMH_WAIT_PROC from system unbound workqueue")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/kmod.c b/kernel/kmod.c
index da98d0593de24206d68222d787d059a5a2b025a1..0277d1216f80ae1adeed84a686ed34c9b2931fc2 100644 (file)
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -327,9 +327,13 @@ static void call_usermodehelper_exec_work(struct work_struct *work)
                call_usermodehelper_exec_sync(sub_info);
        } else {
                pid_t pid;
-
+               /*
+                * Use CLONE_PARENT to reparent it to kthreadd; we do not
+                * want to pollute current->children, and we need a parent
+                * that always ignores SIGCHLD to ensure auto-reaping.
+                */
                pid = kernel_thread(call_usermodehelper_exec_async, sub_info,
-                                   SIGCHLD);
+                                   CLONE_PARENT | SIGCHLD);
                if (pid < 0) {
                        sub_info->retval = pid;
                        umh_complete(sub_info);