Re: [PATCH bpf-next v2 3/4] bpf: Add support for writing to nf_conn:mark

[Martin KaFai Lau]

On Wed, Aug 17, 2022 at 02:30:01PM -0700, Alexei Starovoitov wrote:
> On Wed, Aug 17, 2022 at 11:43 AM Daniel Xu <dxu@xxxxxxxxx> wrote:
> >
> > +/* Check writes into `struct nf_conn` */
> > +int nf_conntrack_btf_struct_access(struct bpf_verifier_log *log,
> > +                                  const struct btf *btf,
> > +                                  const struct btf_type *t, int off,
> > +                                  int size, enum bpf_access_type atype,
> > +                                  u32 *next_btf_id,
> > +                                  enum bpf_type_flag *flag)
> > +{
> > +       const struct btf_type *nct = READ_ONCE(nf_conn_type);
> > +       s32 type_id;
> > +       size_t end;
> > +
> > +       if (!nct) {
> > +               type_id = btf_find_by_name_kind(btf, "nf_conn", BTF_KIND_STRUCT);
> > +               if (type_id < 0)
> > +                       return -EINVAL;
> > +
> > +               nct = btf_type_by_id(btf, type_id);
> > +               WRITE_ONCE(nf_conn_type, nct);
> > +       }
> > +
> > +       if (t != nct) {
> > +               bpf_log(log, "only read is supported\n");
> > +               return -EACCES;
> > +       }
> > +
> > +       switch (off) {
> > +#if defined(CONFIG_NF_CONNTRACK_MARK)
> > +       case offsetof(struct nf_conn, mark):
> > +               end = offsetofend(struct nf_conn, mark);
> > +               break;
> > +#endif
> > +       default:
> > +               bpf_log(log, "no write support to nf_conn at off %d\n", off);
> > +               return -EACCES;
> > +       }
> > +
> > +       if (off + size > end) {
> > +               bpf_log(log,
> > +                       "write access at off %d with size %d beyond the member of nf_conn ended at %zu\n",
> > +                       off, size, end);
> > +               return -EACCES;
> > +       }
> > +
> > +       return NOT_INIT;
> 
> Took me a long time to realize that this is a copy-paste
> from net/ipv4/bpf_tcp_ca.c.
> It's not wrong, but misleading.
> When atype == BPF_READ the return value from
> btf_struct_access should only be error<0, SCALAR_VALUE, PTR_TO_BTF_ID.
> For atype == BPF_WRITE we should probably standardize on
> error<0, or 0.
> 
> The NOT_INIT happens to be zero, but explicit 0
> is cleaner to avoid confusion that this is somehow enum bpf_reg_type.
> 
> Martin,
> since you've added this code in bpf_tcp_ca, wdyt?
Yep, sgtm.  This will be less confusing.