Re: [PATCH] bpf: Replace bpf_lpm_trie_key 0-length array with flexible array

[Alexei Starovoitov]

On Thu, Feb 9, 2023 at 8:36 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Mon, Feb 06, 2023 at 11:17:06AM -0800, Stanislav Fomichev wrote:
> > It's my understanding that it's the intended use-case. Users are
> > expected to use this struct as a header; at least we've been using it
> > that way :-)
> >
> > For me, both return the same:
> > sizeof(struct { __u32 prefix; __u8 data[0]; })
> > sizeof(struct { __u32 prefix; __u8 data[]; })
> >
> > So let's do s/data[0]/data[]/ in the UAPI only? What's wrong with
> > using this struct as a header?
>
> For the whole struct, yup, the above sizeof()s are correct. However:
>
> sizeof(foo->data) == 0             // when data[0]
> sizeof(foo->data) == compile error // when data[]
>
> The [0]-array GNU extension doesn't have consistent behavior, so it's
> being removed from the kernel in favor of the proper C99 [] flexible
> arrays, so we can enable -fstrict-flex-arrays=3 to remove all the
> ambiguities with array bounds:
> https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
> https://people.kernel.org/kees/bounded-flexible-arrays-in-c
>
> As a header, this kind of overlap isn't well supported. Clang already
> warns, and GCC is going to be removing support for overlapping composite
> structs with a flex array in the middle (and also warns under -pedantic):
> https://godbolt.org/z/vWzqs41h6
>
> I talk about dealing with these specific cases in my recent write-up
> on array bounds checking -- see "Overlapping composite structure members"
> in the people.kernel.org post above.
>
> > > Perhaps better might be:
> > >
> > > struct bpf_lpm_trie_key {
> > >     __u32   prefixlen;      /* up to 32 for AF_INET, 128 for AF_INET6 */
> > > };
> > >
> > > struct bpf_lpm_trie_key_raw {
> > >     struct bpf_lpm_trie_key_prefix prefix;
> > >     u8 data[];
> > > };
> > >
> > > struct my_key {
> > >     struct bpf_lpm_trie_key_prefix prefix;
> > >     int a, b, c;
> > > };
>
> This approach is, perhaps, the best way to go? Besides the selftest,
> what things in userspace consumes struct bpf_lpm_trie_key?

Plenty of bpf progs use it:
https://github.com/cilium/cilium/blob/master/bpf/lib/common.h#L352