linux-kernel - Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie

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Date:   Thu, 9 Feb 2023 12:05:45 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        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@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, llvm@...ts.linux.dev,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key

On Thu, Feb 09, 2023 at 11:52:10AM -0800, Andrii Nakryiko wrote:
> Do we need to add a new type to UAPI at all here? We can make this new
> struct internal to kernel code (e.g. struct bpf_lpm_trie_key_kern) and
> point out that it should match the layout of struct bpf_lpm_trie_key.
> User-space can decide whether to use bpf_lpm_trie_key as-is, or if
> just to ensure their custom struct has the same layout (I see some
> internal users at Meta do just this, just make sure that they have
> __u32 prefixlen as first member).

The uses outside the kernel seemed numerous enough to justify a new UAPI
struct (samples, selftests, etc). It also paves a single way forward
when the userspace projects start using modern compiler options (e.g.
systemd is usually pretty quick to adopt new features).

> This whole union work-around seems like just extra cruft that we don't
> really need in UAPI.

The union is really only there so that possible uses of container_of()
would be happy. But I did add a BUILD_BUG_ON() test for member offset
equality, so a hard cast would be safe too. I'm happy to drop it if
that's preferred?

-- 
Kees Cook