linux-kernel - Re: [PATCH v9 mm-new 07/11] bpf: mark vma->vm_mm as __safe_trusted_or

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Message-ID: <CALOAHbD5OM+_iUg3DXw+BqYU+PFae6DcQGnE1CrLoKmb9OV9Ug@mail.gmail.com>
Date: Tue, 7 Oct 2025 17:05:07 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com, 
	baolin.wang@...ux.alibaba.com, lorenzo.stoakes@...cle.com, 
	Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com, 
	dev.jain@....com, hannes@...xchg.org, usamaarif642@...il.com, 
	gutierrez.asier@...wei-partners.com, willy@...radead.org, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, ameryhung@...il.com, 
	rientjes@...gle.com, corbet@....net, 21cnbao@...il.com, 
	shakeel.butt@...ux.dev, tj@...nel.org, lance.yang@...ux.dev, 
	rdunlap@...radead.org, bpf@...r.kernel.org, linux-mm@...ck.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Mykyta Yatsenko <mykyta.yatsenko5@...il.com>
Subject: Re: [PATCH v9 mm-new 07/11] bpf: mark vma->vm_mm as __safe_trusted_or_null

On Tue, Oct 7, 2025 at 5:07 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Sep 29, 2025 at 11:00 PM Yafang Shao <laoar.shao@...il.com> wrote:
> >
> > The vma->vm_mm might be NULL and it can be accessed outside of RCU. Thus,
> > we can mark it as trusted_or_null. With this change, BPF helpers can safely
> > access vma->vm_mm to retrieve the associated mm_struct from the VMA.
> > Then we can make policy decision from the VMA.
> >
> > The "trusted" annotation enables direct access to vma->vm_mm within kfuncs
> > marked with KF_TRUSTED_ARGS or KF_RCU, such as bpf_task_get_cgroup1() and
> > bpf_task_under_cgroup(). Conversely, "null" enforcement requires all
> > callsites using vma->vm_mm to perform NULL checks.
> >
> > The lsm selftest must be modified because it directly accesses vma->vm_mm
> > without a NULL pointer check; otherwise it will break due to this
> > change.
> >
> > For the VMA based THP policy, the use case is as follows,
> >
> >   @mm = @vma->vm_mm; // vm_area_struct::vm_mm is trusted or null
> >   if (!@mm)
> >       return;
> >   bpf_rcu_read_lock(); // rcu lock must be held to dereference the owner
> >   @owner = @mm->owner; // mm_struct::owner is rcu trusted or null
> >   if (!@...er)
> >     goto out;
> >   @cgroup1 = bpf_task_get_cgroup1(@owner, MEMCG_HIERARCHY_ID);
> >
> >   /* make the decision based on the @cgroup1 attribute */
> >
> >   bpf_cgroup_release(@cgroup1); // release the associated cgroup
> > out:
> >   bpf_rcu_read_unlock();
> >
> > PSI memory information can be obtained from the associated cgroup to inform
> > policy decisions. Since upstream PSI support is currently limited to cgroup
> > v2, the following example demonstrates cgroup v2 implementation:
> >
> >   @owner = @mm->owner;
> >   if (@owner) {
> >       // @ancestor_cgid is user-configured
> >       @ancestor = bpf_cgroup_from_id(@ancestor_cgid);
> >       if (bpf_task_under_cgroup(@owner, @ancestor)) {
> >           @psi_group = @ancestor->psi;
> >
> >           /* Extract PSI metrics from @psi_group and
> >            * implement policy logic based on the values
> >            */
> >
> >       }
> >   }
> >
> > Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> > Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> > ---
> >  kernel/bpf/verifier.c                   | 5 +++++
> >  tools/testing/selftests/bpf/progs/lsm.c | 8 +++++---
> >  2 files changed, 10 insertions(+), 3 deletions(-)
> >
>
> Hey Yafang,
>
> This looks like a generally useful change, so I think it would be best
> if you can send it as a stand-alone patch to bpf-next to land it
> sooner.

Sure. I will do it.

>
> Also, am I imagining this, or did you have similar change for the
> vm_file field as well? Any reasons to not mark vm_file as trusted as
> well?

Marking vm_file as trusted will directly support our follow-up work on
file-backed THP policies, where we need to apply different policies to
different files in production. I will include this change in the same
stand-alone patch. Thanks for the suggestion.

-- 
Regards
Yafang