Re: [PATCH] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

From: Kees Cook <keescook@chromium.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Stanislav Fomichev <sdf@google.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>, Haowen Bai <baihaowen@meizu.com>,
	bpf <bpf@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array
Date: Thu, 9 Feb 2023 09:48:18 -0800	[thread overview]
Message-ID: <63e531e3.170a0220.3a46a.3262@mx.google.com> (raw)
In-Reply-To: <CAADnVQK-sW51SsC7FYDqLaO8c5xj=MAgcu_6p8L-JN9kAsrzeA@mail.gmail.com>

On Thu, Feb 09, 2023 at 08:55:23AM -0800, Alexei Starovoitov wrote:
> On Thu, Feb 9, 2023 at 8:36 AM Kees Cook <keescook@chromium.org> wrote:
> > This approach is, perhaps, the best way to go? Besides the selftest,
> > what things in userspace consumes struct bpf_lpm_trie_key?
> 
> Plenty of bpf progs use it:
> https://github.com/cilium/cilium/blob/master/bpf/lib/common.h#L352

Thanks for the pointer! Yeah, it seems the "data" member is not
directly used, but is expected to be there for static initializers and
offsetof() use. For example:

cilium:
        struct egress_gw_policy_key in_key = {
                .lpm_key = { 32 + 24, {} },
                .saddr   = CLIENT_IP,
                .daddr   = EXTERNAL_SVC_IP & 0Xffffff,
        };

systemd:
	ipv6_map_fd = bpf_map_new(
			BPF_MAP_TYPE_LPM_TRIE,
			offsetof(struct bpf_lpm_trie_key, data) + sizeof(uint32_t)*4,
			sizeof(uint64_t),
			...

All the others searches in Debian I could find were just copies of UAPI
headers:
https://codesearch.debian.net/search?q=struct+bpf_lpm_trie_key&literal=1&perpkg=1

-- 
Kees Cook