On Mon 2021-11-08 08:41:42, Yafang Shao wrote:
> When I was implementing a new per-cpu kthread cfs_migration, I found the
> comm of it "cfs_migration/%u" is truncated due to the limitation of
> TASK_COMM_LEN. For example, the comm of the percpu thread on CPU10~19 are
> all with the same name "cfs_migration/1", which will confuse the user. This
> issue is not critical, because we can get the corresponding CPU from the
> task's Cpus_allowed. But for kthreads correspoinding to other hardware
> devices, it is not easy to get the detailed device info from task comm,
> for example,
>
> After this change, the full name of these truncated kthreads will be
> displayed via /proc/[pid]/comm:
>
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -102,6 +103,8 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
>
> if (p->flags & PF_WQ_WORKER)
> wq_worker_comm(tcomm, sizeof(tcomm), p);
Just for record. I though that this patch obsoleted wq_worker_comm()
but it did not. wq_worker_comm() returns different values
depending on the last proceed work item and has to stay.
> + else if (p->flags & PF_KTHREAD)
> + get_kthread_comm(tcomm, sizeof(tcomm), p);
> else
> __get_task_comm(tcomm, sizeof(tcomm), p);
>
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -121,6 +135,7 @@ void free_kthread_struct(struct task_struct *k)
Hmm, there is the following comment:
/*
* Can be NULL if this kthread was created by kernel_thread()
* or if kmalloc() in kthread() failed.
*/
kthread = to_kthread(k);
And indeed, set_kthread_struct() is called only by kthread()
and init_idle().
For example, call_usermodehelper_exec_sync() calls kernel_thread()
but given @fn does not call set_kthread_struct(). Also init_idle()
continues even when the allocation failed.
> #ifdef CONFIG_BLK_CGROUP
> WARN_ON_ONCE(kthread && kthread->blkcg_css);
> #endif
> + kfree(kthread->full_name);
Hence, we have to make sure that it is not NULL here. I suggest
something like:
void free_kthread_struct(struct task_struct *k)
{
struct kthread *kthread;
/*
* Can be NULL if this kthread was created by kernel_thread()
* or if kmalloc() in kthread() failed.
*/
kthread = to_kthread(k);
if (!kthread)
return;
#ifdef CONFIG_BLK_CGROUP
WARN_ON_ONCE(kthread->blkcg_css);
#endif
kfree(kthread->full_name);
kfree(kthread);
}
Side note: The possible NULL pointer looks dangerous to
me. to_kthread() is dereferenced without any check on
several locations.
For example, kthread_create_on_cpu() looks safe. It is a kthread
crated by kthread(). It will exists only when the allocation
succeeded.
kthread_stop() is probably safe only because it used only for
the classic kthreads created by kthread(). But the API
is not safe.
kthread_use_mm() is probably used only by classic kthreads as
well. But it is less clear to me.
All this unsafe APIs looks like a ticking bomb to me. But
it is beyond this patchset.
> kfree(kthread);
> }
>
Best Regards,
Petr
