linux-kernel - [PATCH] kthread: Make it clear that create_kthread() might be terminated by any fatal signal

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Message-Id: <20220310093455.15176-1-pmladek@suse.com>
Date:   Thu, 10 Mar 2022 10:34:55 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>
Subject: [PATCH] kthread: Make it clear that create_kthread() might be terminated by any fatal signal

The comments in kernel/kthread.c create a feeling that only SIGKILL
is able to break create_kthread().

In reality, wait_for_completion_killable() might be killed by any
fatal signal that does not have a custom handler:

	(!siginmask(signr, SIG_KERNEL_IGNORE_MASK|SIG_KERNEL_STOP_MASK) && \
	 (t)->sighand->action[(signr)-1].sa.sa_handler == SIG_DFL)

static inline void signal_wake_up(struct task_struct *t, bool resume)
{
	signal_wake_up_state(t, resume ? TASK_WAKEKILL : 0);
}

static void complete_signal(int sig, struct task_struct *p, enum pid_type type)
{
[...]
	/*
	 * Found a killable thread.  If the signal will be fatal,
	 * then start taking the whole group down immediately.
	 */
	if (sig_fatal(p, sig) ...) {
		if (!sig_kernel_coredump(sig)) {
		[...]
			do {
				task_clear_jobctl_pending(t, JOBCTL_PENDING_MASK);
				sigaddset(&t->pending.signal, SIGKILL);
				signal_wake_up(t, 1);
			} while_each_thread(p, t);
			return;
		}
	}
}

Update the comments in kernel/kthread.c to make this more obvious.

The motivation for this change was debugging why a module initialization
failed. The module was being loaded from initrd. It "magically" failed
when systemd was switching to the real root. The clean up operations
sent SIGTERM to various pending processed that were started from initrd.

Signed-off-by: Petr Mladek <pmladek@...e.com>
---
 kernel/kthread.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 38c6dd822da8..6f994c354adb 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -341,7 +341,7 @@ static int kthread(void *_create)
 
 	self = to_kthread(current);
 
-	/* If user was SIGKILLed, I release the structure. */
+	/* Release the structure when caller killed by a fatal signal. */
 	done = xchg(&create->done, NULL);
 	if (!done) {
 		kfree(create);
@@ -399,7 +399,7 @@ static void create_kthread(struct kthread_create_info *create)
 	/* We want our own signal handler (we take no signals by default). */
 	pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD);
 	if (pid < 0) {
-		/* If user was SIGKILLed, I release the structure. */
+		/* Release the structure when caller killed by a fatal signal. */
 		struct completion *done = xchg(&create->done, NULL);
 
 		if (!done) {
@@ -441,9 +441,9 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
 	 */
 	if (unlikely(wait_for_completion_killable(&done))) {
 		/*
-		 * If I was SIGKILLed before kthreadd (or new kernel thread)
-		 * calls complete(), leave the cleanup of this structure to
-		 * that thread.
+		 * If I was killed by a fatal signal before kthreadd (or new
+		 * kernel thread) calls complete(), leave the cleanup of this
+		 * structure to that thread.
 		 */
 		if (xchg(&create->done, NULL))
 			return ERR_PTR(-EINTR);
@@ -877,7 +877,7 @@ __kthread_create_worker(int cpu, unsigned int flags,
  *
  * Returns a pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
  * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
- * when the worker was SIGKILLed.
+ * when the caller was killed by a fatal signal.
  */
 struct kthread_worker *
 kthread_create_worker(unsigned int flags, const char namefmt[], ...)
@@ -926,7 +926,7 @@ EXPORT_SYMBOL(kthread_create_worker);
  * Return:
  * The pointer to the allocated worker on success, ERR_PTR(-ENOMEM)
  * when the needed structures could not get allocated, and ERR_PTR(-EINTR)
- * when the worker was SIGKILLed.
+ * when the caller was killed by a fatal signal.
  */
 struct kthread_worker *
 kthread_create_worker_on_cpu(int cpu, unsigned int flags,
-- 
2.26.2