Re: [PATCH] bpf: Deprecate "data" member of bpf_lpm_trie_key

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Feb 9, 2023 at 1:12 PM Kees Cook <keescook@chromium.org> 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.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.
>
> Right -- I'm proposing not changing bpf_lpm_trie_key. I'm proposing
> _adding_ bpf_lpm_trie_key_u8 for new users who will be using modern
> compiler options (i.e. where "data[0]" is nonsense).
>
> > 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 hanging
> when they eventually trip over this in their code when they enable
> -fstrict-flex-arrays=3 too.
>
> > What is the disadvantage?
>
> It seemed better to give a working example of how to migrate this code.

I understand and agree with intent, but I'm still missing
how you're going to achieve this migration.
bpf_lpm_trie_key_u8 doesn't provide a migration path to cilium progs
and pretty much all bpf progs that use LPM map.
Sure, one can change the user space part, like you did in test_lpm_map.c,
but it doesn't address the full scope.
imo half way is worse than not doing it.