Re: [PATCH bpf-next] bpf: Add document for 'dst_port' of 'struct bpf_sock'

[Martin KaFai Lau <kafai@fb.com>]

On Tue, Jan 25, 2022 at 03:02:37PM -0800, Alexei Starovoitov wrote:
> On Tue, Jan 25, 2022 at 2:45 PM Martin KaFai Lau <kafai@fb.com> wrote:
> >
> > On Tue, Jan 25, 2022 at 08:24:27PM +0100, Jakub Sitnicki wrote:
> > > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > > index b0383d371b9a..891a182a749a 100644
> > > > --- a/include/uapi/linux/bpf.h
> > > > +++ b/include/uapi/linux/bpf.h
> > > > @@ -5500,7 +5500,11 @@ struct bpf_sock {
> > > >     __u32 src_ip4;
> > > >     __u32 src_ip6[4];
> > > >     __u32 src_port;         /* host byte order */
> > > > -   __u32 dst_port;         /* network byte order */
> > > > +   __u32 dst_port;         /* low 16-bits are in network byte order,
> > > > +                            * and high 16-bits are filled by 0.
> > > > +                            * So the real port in host byte order is
> > > > +                            * bpf_ntohs((__u16)dst_port).
> > > > +                            */
> > > >     __u32 dst_ip4;
> > > >     __u32 dst_ip6[4];
> > > >     __u32 state;
> > >
> > > I'm probably missing something obvious, but is there anything stopping
> > > us from splitting the field, so that dst_ports is 16-bit wide?
> > >
> > > I gave a quick check to the change below and it seems to pass verifier
> > > checks and sock_field tests.
> > >
> > > IDK, just an idea. Didn't give it a deeper thought.
> > >
> > > --8<--
> > >
> > > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > > index 4a2f7041ebae..344d62ccafba 100644
> > > --- a/include/uapi/linux/bpf.h
> > > +++ b/include/uapi/linux/bpf.h
> > > @@ -5574,7 +5574,8 @@ struct bpf_sock {
> > >       __u32 src_ip4;
> > >       __u32 src_ip6[4];
> > >       __u32 src_port;         /* host byte order */
> > > -     __u32 dst_port;         /* network byte order */
> > > +     __u16 unused;
> > > +     __u16 dst_port;         /* network byte order */
> > This will break the existing bpf prog.
> 
> I think Jakub's idea is partially expressed:
> +       case offsetof(struct bpf_sock, dst_port):
> +               bpf_ctx_record_field_size(info, sizeof(__u16));
> +               return bpf_ctx_narrow_access_ok(off, size, sizeof(__u16));
> 
> Either 'unused' needs to be after dst_port or
> bpf_sock_is_valid_access() needs to allow offset at 'unused'
> and at 'dst_port'.
> And allow u32 access though the size is actually u16.
> Then the existing bpf progs (without recompiling) should work?
Yes, I think that should work with the existing bpf progs.
I suspect putting 'dst_port' first and then followed by 'unused'
may be easier.  That will also serve as a natural doc for the
current behavior (the value is in the lower 16 bits).

It can be extended to bpf_sk_lookup? bpf_sk_lookup can read at any
offset of these 4 bytes, so may need to read 0 during
convert_ctx_accesses?