Show a warning if task comm is truncated. Below is the result
of my test case:

truncated kthread comm:I-am-a-kthread-with-a-l (pid:178) by 8 characters

As we have extended task comm to 24, all the existing in-tree
kthreads/workqueues are not truncated anymore. So this warning will only
be printed for the newly introduced one if it has a long name.

Suggested-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Yafang Shao <redacted>
Reviewed-by: Kees Cook <redacted>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Arnaldo Carvalho de Melo <redacted>
Cc: Alexei Starovoitov <redacted>
Cc: Andrii Nakryiko <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Kees Cook <redacted>
Cc: Petr Mladek <pmladek@suse.com>
---
 kernel/kthread.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 5b37a8567168..63f38d3a4f62 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c

@@ -399,12 +399,17 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
 	if (!IS_ERR(task)) {
 		static const struct sched_param param = { .sched_priority = 0 };
 		char name[TASK_COMM_LEN];
+		int len;
 
 		/*
 		 * task is already visible to other tasks, so updating
 		 * COMM must be protected.
 		 */
-		vsnprintf(name, sizeof(name), namefmt, args);
+		len = vsnprintf(name, sizeof(name), namefmt, args);
+		if (len >= TASK_COMM_LEN) {
+			pr_warn("truncated kthread comm:%s (pid:%d) by %d characters\n",
+				name, task->pid, len - TASK_COMM_LEN + 1);
+		}
 		set_task_comm(task, name);
 		/*
 		 * root may have changed our (kthreadd's) priority or CPU mask.

-- 
2.17.1