From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
To: Matt Bobrowski <mattbobrowski@google.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Tejun Heo <tj@kernel.org>,
Roman Gushchin <roman.gushchin@linux.dev>,
bpf <bpf@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
JP Kobryn <inwardvessel@gmail.com>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Shakeel Butt <shakeel.butt@linux.dev>,
Michal Hocko <mhocko@kernel.org>,
Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH bpf-next v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc
Date: Tue, 6 Jan 2026 16:13:24 +0100 [thread overview]
Message-ID: <CAP01T75ATFb_gjy5_fSwt6=QMxt7kGSS+12SJN9rz9SfJQ7Qyg@mail.gmail.com> (raw)
In-Reply-To: <aVwnUUXmgE1uOOj4@google.com>
On Mon, 5 Jan 2026 at 22:04, Matt Bobrowski <mattbobrowski@google.com> wrote:
>
> On Mon, Jan 05, 2026 at 08:05:54AM -0800, Alexei Starovoitov wrote:
> > On Sun, Jan 4, 2026 at 11:49 PM Matt Bobrowski <mattbobrowski@google.com> wrote:
> > >
> > > >
> > > > No need for a new KF flag. Any struct returned by kfunc should be
> > > > trusted or trusted_or_null if KF_RET_NULL was specified.
> > > > I don't remember off the top of my head, but this behavior
> > > > is already implemented or we discussed making it this way.
> > >
> > > Hm, I do not see any evidence of this kind of semantic currently
> > > implemented, so perhaps it was only discussed at some point. Would you
> > > like me to put forward a patch that introduces this kind of implicit
> > > trust semantic for BPF kfuncs returning pointer to struct types?
> >
> > Hmm. What about these:
> > BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
> > BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_RET_NULL)
> > BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_RET_NULL | KF_RCU_PROTECTED)
> >
> > I thought they're returning a trusted pointer without acquiring it.
> > iirc the last one returns trusted in RCU CS,
> > but the first two return just a legacy ptr_to_btf_id ?
> > This is something to fix asap then.
>
> No, AFAIU they do not. These simply return a regular pointer to BTF ID
> (PTR_TO_BTF_ID), rather than a formally "trusted" pointer (which would
> carry the PTR_TRUSTED flag or a ref_obj_id). scx_bpf_cpu_curr returns
> a MEM_RCU pointer (via KF_RCU_PROTECTED), which is somewhat considered
> to be trusted within a RCU read-side critical section *ONLY*.
>
> Kumar/Tejun,
Yeah, they don't return a trusted pointer. I think it would make sense
to change the behavior here by default.
A non-trusted pointer cannot be passed to kfuncs taking trusted
arguments, so hopefully it will only make things more permissive and
doesn't break anything.
>
> Please keep me honest here.
next prev parent reply other threads:[~2026-01-06 15:14 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-23 4:41 [PATCH bpf-next v4 0/6] mm: bpf kfuncs to access memcg data Roman Gushchin
2025-12-23 4:41 ` [PATCH bpf-next v4 1/6] mm: declare memcg_page_state_output() in memcontrol.h Roman Gushchin
2025-12-23 4:41 ` [PATCH bpf-next v4 2/6] mm: introduce BPF kfuncs to deal with memcg pointers Roman Gushchin
2025-12-23 4:41 ` [PATCH bpf-next v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc Roman Gushchin
2025-12-30 20:27 ` Matt Bobrowski
2025-12-30 21:00 ` Roman Gushchin
2025-12-31 7:41 ` Matt Bobrowski
2025-12-31 17:02 ` Roman Gushchin
2025-12-31 17:32 ` Alexei Starovoitov
2026-01-05 7:49 ` Matt Bobrowski
2026-01-05 16:05 ` Alexei Starovoitov
2026-01-05 21:04 ` Matt Bobrowski
2026-01-06 15:13 ` Kumar Kartikeya Dwivedi [this message]
2026-01-07 9:29 ` Matt Bobrowski
2025-12-23 4:41 ` [PATCH bpf-next v4 4/6] mm: introduce BPF kfuncs to access memcg statistics and events Roman Gushchin
2025-12-23 4:41 ` [PATCH bpf-next v4 5/6] bpf: selftests: selftests for memcg stat kfuncs Roman Gushchin
2025-12-23 4:41 ` [PATCH bpf-next v4 6/6] MAINTAINERS: add an entry for MM BPF extensions Roman Gushchin
2025-12-23 19:25 ` [PATCH bpf-next v4 0/6] mm: bpf kfuncs to access memcg data Alexei Starovoitov
2025-12-23 19:57 ` Roman Gushchin
2025-12-24 3:41 ` Konstantin Ryabitsev
2025-12-23 19:46 ` Shakeel Butt
2025-12-24 3:01 ` Yafang Shao
2025-12-25 1:16 ` Roman Gushchin
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