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

[Yafang Shao]

On Fri, Nov 12, 2021 at 11:34 PM Petr Mladek <pmladek@xxxxxxxx> wrote:
>
> 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.
>

Right. worker comm is changed dynamically, which is combined by
(task_comm+worker_desc) or (task_comm-worker_desc).
I planned to remove the whole worker->desc and set it dynamically to
the new kthread full_name but I found it may not be a good idea.


> > +     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.
>

Yes, it really can be NULL.

>
> >  #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:
>

Agreed.  I will do it.

> 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.
>

I will analyze it in depth and try to dismantle this ticking bomb.


-- 
Thanks
Yafang