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

[Kees Cook <keescook@chromium.org>]

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?

-- 
Kees Cook