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

[Kees Cook]

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@xxxxxxxxxxxx> 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.

Regardless, I can just make this specific to the kernel code if that's
what's wanted.

-- 
Kees Cook