Re: [PATCH] kthread: dynamically allocate memory to store kthread's full name

[Petr Mladek]

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