On Thu, Oct 28, 2021 at 4:10 AM Petr Mladek [off-list ref] wrote:

On Mon 2021-10-25 14:35:42, Kees Cook wrote:

quoted

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:

                      char *full_comm;

                      full_comm = kvasprintf(GFP_KERNEL, namefmt, args);

You need to use va_copy()/va_end() if you want to use the same va_args
twice.

Now I understand it.
So the patch will be:

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 5b37a8567168..c1ff67283725 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c

@@ -399,12 +399,29 @@ 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];
+               char *full_comm;
+               va_list aq;
+               int len;

                /*
                 * task is already visible to other tasks, so updating
                 * COMM must be protected.
                 */
-               vsnprintf(name, sizeof(name), namefmt, args);
+               va_copy(aq, args);
+               len = vsnprintf(name, sizeof(name), namefmt, aq);
+               va_end(aq);
+               if (len >= TASK_COMM_LEN) {
+                       full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
+                       if (full_comm) {
+                               pr_warn("truncated kthread comm '%s'
to '%s' (pid:%d)\n",
+                                       full_comm, name, task->pid);
+                               kfree(full_comm);
+                       } else {
+                               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.

That seems a little overkill to me.
I prefer to keep the v6 as-is.

For example, see how kvasprintf() is implemented. It calls
vsnprintf() twice and it uses va_copy()/va_end() around the the first call.

kvasprintf() could also return NULL if there is not enough memory.

quoted

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

BTW: Is this message printed during normal boot? I did not tried the
patchset myself.

We should add this warning only if there is a good solution how to
avoid the truncated names. And we should me sure that the most common
kthreads/workqueues do not trigger it. It would be ugly to print many
warnings during boot if people could not get rid of them easily.

quoted

                      kfree(full_comm);
              }

quoted

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

Best Regards,
Petr

-- 
Thanks
Yafang