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

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

On Wed, Aug 27, 2025 at 02:50:36PM -0700, Andrii Nakryiko wrote:
> On Wed, Aug 27, 2025 at 8:48 AM 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?
> >
> > >
> > > 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?
>
> BPF verifier will reject any program that cannot guarantee that
> bpf_task_release() will always be called. So there shouldn't be any
> problem here.

Ah that's nice!

What specifically here is enforcing that? Apologies again - BPF is new to me.