Message-ID: <63e5521a.170a0220.297d7.3a80@mx.google.com>
Date:   Thu, 9 Feb 2023 12:05:45 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        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@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, llvm@lists.linux.dev,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key
References: <20230209192337.never.690-kees@kernel.org>
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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