Date:   Sat, 11 Feb 2023 09:55:09 -0800
From:   Kees Cook <kees@kernel.org>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Kees Cook <keescook@chromium.org>
CC:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        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 <bpf@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <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>, LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        clang-built-linux <llvm@lists.linux.dev>,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key
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On February 9, 2023 2:01:15 PM PST, Alexei Starovoitov <alexei=2Estarovoito=
v@gmail=2Ecom> wrote:
>On Thu, Feb 9, 2023 at 1:12 PM Kees Cook <keescook@chromium=2Eorg> wrote:
>>
>> On Thu, Feb 09, 2023 at 12:50:28PM -0800, Alexei Starovoitov wrote:
>> > On Thu, Feb 9, 2023 at 12:05 PM Kees Cook <keescook@chromium=2Eorg> w=
rote:
>> > >
>> > > 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 thi=
s new
>> > > > struct internal to kernel code (e=2Eg=2E struct bpf_lpm_trie_key_=
kern) and
>> > > > point out that it should match the layout of struct bpf_lpm_trie_=
key=2E
>> > > > User-space can decide whether to use bpf_lpm_trie_key as-is, or i=
f
>> > > > just to ensure their custom struct has the same layout (I see som=
e
>> > > > internal users at Meta do just this, just make sure that they hav=
e
>> > > > __u32 prefixlen as first member)=2E
>> > >
>> > > The uses outside the kernel seemed numerous enough to justify a new=
 UAPI
>> > > struct (samples, selftests, etc)=2E It also paves a single way forw=
ard
>> > > when the userspace projects start using modern compiler options (e=
=2Eg=2E
>> > > systemd is usually pretty quick to adopt new features)=2E
>> >
>> > I don't understand how the new uapi struct bpf_lpm_trie_key_u8 helps=
=2E
>> > cilium progs and progs/map_ptr_kern=2Ec
>> > cannot do s/bpf_lpm_trie_key/bpf_lpm_trie_key_u8/=2E
>> > They will fail to build, so they're stuck with bpf_lpm_trie_key=2E
>>
>> Right -- I'm proposing not changing bpf_lpm_trie_key=2E I'm proposing
>> _adding_ bpf_lpm_trie_key_u8 for new users who will be using modern
>> compiler options (i=2Ee=2E where "data[0]" is nonsense)=2E
>>
>> > Can we do just
>> > struct bpf_lpm_trie_key_kern {
>> >   __u32   prefixlen;
>> >   __u8    data[];
>> > };
>> > and use it in the kernel?
>>
>> Yeah, I can do that if that's preferred, but it leaves userspace hangin=
g
>> when they eventually trip over this in their code when they enable
>> -fstrict-flex-arrays=3D3 too=2E
>>
>> > What is the disadvantage?
>>
>> It seemed better to give a working example of how to migrate this code=
=2E
>
>I understand and agree with intent, but I'm still missing
>how you're going to achieve this migration=2E
>bpf_lpm_trie_key_u8 doesn't provide a migration path to cilium progs
>and pretty much all bpf progs that use LPM map=2E
>Sure, one can change the user space part, like you did in test_lpm_map=2E=
c,
>but it doesn't address the full scope=2E
>imo half way is worse than not doing it=2E

Maybe I'm missing something, but if a program isn't building with -fstrict=
-flex-arrays=3D3, it can keep on using struct bpf_lpm_trie_key as before=2E=
 If/when it starts using -fsfa, if can use struct bpf_lpm_trie_key in compo=
site structs as a header just like before, but if it has places using the "=
data" member as an array of u8, it can switch to something using struct bpf=
_lpm_trie_key_u8, either directly or as a union with whatever ever struct t=
hey have=2E (And this replacement is what I did for all the samples/selftes=
ts=2E)


--=20
Kees Cook