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

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Date:   Thu, 9 Feb 2023 09:48:18 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>,
        Shuah Khan <shuah@...nel.org>,
        Haowen Bai <baihaowen@...zu.com>, bpf <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Replace bpf_lpm_trie_key 0-length array with
 flexible array

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@...omium.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