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

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Date:   Thu, 9 Feb 2023 12:50:28 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        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 <bpf@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <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>, LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key

On Thu, Feb 9, 2023 at 12:05 PM Kees Cook <keescook@...omium.org> wrote:
>
> 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).

I don't understand how the new uapi struct bpf_lpm_trie_key_u8 helps.
cilium progs and progs/map_ptr_kern.c
cannot do s/bpf_lpm_trie_key/bpf_lpm_trie_key_u8/.
They will fail to build, so they're stuck with bpf_lpm_trie_key.

Can we do just
struct bpf_lpm_trie_key_kern {
  __u32   prefixlen;
  __u8    data[];
};
and use it in the kernel?
What is the disadvantage?