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

[Kees Cook <keescook@chromium.org>]

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