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Date: Mon, 14 Oct 2024 11:13:11 -0700
From: Namhyung Kim <namhyung@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@fomichev.me>,
	Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm <linux-mm@kvack.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Kees Cook <kees@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH v5 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc
Message-ID: <Zw1fN1WqjvoCeT_s@google.com>
References: <20241010232505.1339892-1-namhyung@kernel.org>
 <20241010232505.1339892-3-namhyung@kernel.org>
 <CAADnVQLN1De95WqUu2ESAdX-wNvaGhSNeboar1k-O+z_d7-dNA@mail.gmail.com>
 <Zwl5BkB-SawgQ9KY@google.com>
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In-Reply-To: <Zwl5BkB-SawgQ9KY@google.com>

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@kernel.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