linux-kernel - Re: [PATCH v5 bpf-next 2/3] mm/bpf: Add bpf_get_kmem

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Message-ID: <CAADnVQLN1De95WqUu2ESAdX-wNvaGhSNeboar1k-O+z_d7-dNA@mail.gmail.com>
Date: Fri, 11 Oct 2024 11:35:27 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Namhyung Kim <namhyung@...nel.org>
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

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.