Re: [PATCH v6 mm-new 03/10] mm: thp: add a new kfunc bpf_mm_get_task()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Thu, Aug 28, 2025 at 02:47:34PM +0800, Yafang Shao wrote:
> On Wed, Aug 27, 2025 at 11:42 PM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > On Tue, Aug 26, 2025 at 03:19:41PM +0800, Yafang Shao wrote:
> > > We will utilize this new kfunc bpf_mm_get_task() to retrieve the
> > > associated task_struct from the given @mm. The obtained task_struct must
> > > be released by calling bpf_task_release() as a paired operation.
> >
> > You're basically describing the patch you're not saying why - yeah you're
> > getting a task struct from an mm (only if CONFIG_MEMCG which you don't
> > mention here), but not for what purpose you intend to use this?
>
> For example, we could retrieve task->comm or other attributes and make
> decisions based on that information. I’ll provide a clearer
> description in the next revision.

Thanks!

>
> >
> > >
> > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > ---
> > >  mm/bpf_thp.c | 34 ++++++++++++++++++++++++++++++++++
> > >  1 file changed, 34 insertions(+)
> > >
> > > diff --git a/mm/bpf_thp.c b/mm/bpf_thp.c
> > > index b757e8f425fd..46b3bc96359e 100644
> > > --- a/mm/bpf_thp.c
> > > +++ b/mm/bpf_thp.c
> > > @@ -205,11 +205,45 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
> > >  #endif
> > >  }
> > >
> > > +/**
> > > + * bpf_mm_get_task - Get the task struct associated with a mm_struct.
> > > + * @mm: The mm_struct to query
> > > + *
> > > + * The obtained task_struct must be released by calling bpf_task_release().
> >
> > Hmmm so now bpf programs can cause kernel bugs by keeping a reference around?
> >
> > This feels extremely dodgy, I don't like this at all.
> >
> > I thought the whole point of BPF was that this kind of thing couldn't possibly
> > happen?
> >
> > Or would this be a kernel bug?
> >
> > If a bpf program can lead to a refcount not being put, this is not
> > upstreamable surely?
>
> As explained by Andrii, the BPF verifier can protect it.

Yeah that's nice!

>
> >
> > > + *
> > > + * Return: The associated task_struct on success, or NULL on failure. Note that
> > > + * this function depends on CONFIG_MEMCG being enabled - it will always return
> > > + * NULL if CONFIG_MEMCG is not configured.
> > > + */
> > > +__bpf_kfunc struct task_struct *bpf_mm_get_task(struct mm_struct *mm)
> > > +{
> > > +#ifdef CONFIG_MEMCG
> > > +     struct task_struct *task;
> > > +
> > > +     if (!mm)
> > > +             return NULL;
> > > +     rcu_read_lock();
> > > +     task = rcu_dereference(mm->owner);
> >
> > > +     if (!task)
> > > +             goto out;
> > > +     if (!refcount_inc_not_zero(&task->rcu_users))
> > > +             goto out;
> > > +
> > > +     rcu_read_unlock();
> > > +     return task;
> > > +
> > > +out:
> > > +     rcu_read_unlock();
> > > +#endif
> >
> > This #ifdeffery is horrid, can we please just have separate functions instead of
> > inside the one? Thanks.
> >
> > > +     return NULL;
> >
> > So we can't tell the difference between this failling due to CONFIG_MEMCG
> > not being set (in which case it will _always_ fail) or we couldn't get a
> > task or we couldn't get a refcount on the task.
> >
> > Maybe this doesn't matter since perhaps we are only using this if
> > CONFIG_MEMCG but in that case why even expose this if !CONFIG_MEMCG?
> >
>
> As suggested by Andrii, I will remove this kfunc and mark mm->owner as
> BTF_TYPE_SAFE_TRUSTED_OR_NULL.

OK thanks!

>
> Thanks for your comments.

You're welcome :)

>
> --
> Regards
> Yafang

Cheers, Lorenzo