On Tue, Oct 26, 2021 at 5:35 AM Kees Cook [off-list ref] wrote:

On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:

quoted

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

truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters

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: 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..46b924c92078 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) {

And since this failure case is slow-path, we could improve the warning
as other had kind of suggested earlier with something like this instead:

It Makes sense to me.  I will do it as you suggested.

                        char *full_comm;

                        full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
                        pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
                                full_comm, name);

                        kfree(full_comm);
                }

quoted

              set_task_comm(task, name);
              /*
               * root may have changed our (kthreadd's) priority or CPU mask.
--
2.17.1

--
Kees Cook

-- 
Thanks
Yafang