linux-kernel - Re: [RFC PATCH v10 mm-new 6/9] bpf: mark mm->owner as __safe_rcu_or

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Message-ID: <CAEf4BzZYk+LyR0WTQ+TinEqC0Av8MuO-tKxqhEFbOw=Gu+D_gQ@mail.gmail.com>
Date: Wed, 15 Oct 2025 09:35:52 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yafang Shao <laoar.shao@...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
Subject: Re: [RFC PATCH v10 mm-new 6/9] bpf: mark mm->owner as __safe_rcu_or_null

On Wed, Oct 15, 2025 at 7:18 AM Yafang Shao <laoar.shao@...il.com> wrote:
>
> When CONFIG_MEMCG is enabled, we can access mm->owner under RCU. The
> owner can be NULL. With this change, BPF helpers can safely access
> mm->owner to retrieve the associated task from the mm. We can then make
> policy decision based on the task attribute.
>
> The typical use case is as follows,
>
>   bpf_rcu_read_lock(); // rcu lock must be held for rcu trusted field
>   @owner = @mm->owner; // mm_struct::owner is rcu trusted or null
>   if (!@...er)
>       goto out;
>
>   /* Do something based on the task attribute */
>
> out:
>   bpf_rcu_read_unlock();
>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Acked-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>  kernel/bpf/verifier.c | 3 +++
>  1 file changed, 3 insertions(+)
>

I thought you were going to send this and next patches outside of your
thp patch set to land them sooner, as they don't have dependency on
the rest of the patches and are useful on their own?

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index c4f69a9e9af6..d400e18ee31e 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7123,6 +7123,9 @@ BTF_TYPE_SAFE_RCU(struct cgroup_subsys_state) {
>  /* RCU trusted: these fields are trusted in RCU CS and can be NULL */
>  BTF_TYPE_SAFE_RCU_OR_NULL(struct mm_struct) {
>         struct file __rcu *exe_file;
> +#ifdef CONFIG_MEMCG
> +       struct task_struct __rcu *owner;
> +#endif
>  };
>
>  /* skb->sk, req->sk are not RCU protected, but we mark them as such
> --
> 2.47.3
>