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Message-ID: <Zw1fN1WqjvoCeT_s@google.com>
Date: Mon, 14 Oct 2024 11:13:11 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
Yonghong Song <yonghong.song@...ux.dev>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Vlastimil Babka <vbabka@...e.cz>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm <linux-mm@...ck.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Kees Cook <kees@...nel.org>,
"Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v5 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc
Hi Alexei,
On Fri, Oct 11, 2024 at 12:14:14PM -0700, Namhyung Kim wrote:
> On Fri, Oct 11, 2024 at 11:35:27AM -0700, Alexei Starovoitov wrote:
> > On Thu, Oct 10, 2024 at 4:25 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > The bpf_get_kmem_cache() is to get a slab cache information from a
> > > virtual address like virt_to_cache(). If the address is a pointer
> > > to a slab object, it'd return a valid kmem_cache pointer, otherwise
> > > NULL is returned.
> > >
> > > It doesn't grab a reference count of the kmem_cache so the caller is
> > > responsible to manage the access. The returned point is marked as
> > > PTR_UNTRUSTED. And the kfunc has KF_RCU_PROTECTED as the slab object
> > > might be protected by RCU.
> >
> > ...
> > > +BTF_ID_FLAGS(func, bpf_get_kmem_cache, KF_RCU_PROTECTED)
> >
> > This flag is unnecessary. PTR_UNTRUSTED can point to absolutely any memory.
> > In this case it likely points to a valid kmem_cache, but
> > the verifier will guard all accesses with probe_read anyway.
> >
> > I can remove this flag while applying.
>
> Ok, I'd be happy if you would remove it.
You will need to update the bpf_rcu_read_lock/unlock() in the test code
(patch 3). I can send v6 with that and Vlastimil's Ack if you want.
Thanks,
Namhyung
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