Subject: Re: [PATCH bpf-next RFC 2/3] bpf: devmap dynamic map-value storage area based on BTF

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 3 Jun 2020 11:11:58 +0200

On Tue, 2 Jun 2020 11:27:03 -0700
Alexei Starovoitov <[email-masked]> wrote:

> On Tue, Jun 2, 2020 at 12:00 AM Jesper Dangaard Brouer
> <[email-masked]> wrote:
> >
> > On Mon, 1 Jun 2020 14:30:12 -0700
> > Alexei Starovoitov <[email-masked]> wrote:
> >  
> > > On Fri, May 29, 2020 at 05:59:45PM +0200, Jesper Dangaard Brouer wrote:  
> > > > +
> > > > +/* Expected BTF layout that match struct bpf_devmap_val */
> > > > +static const struct expect layout[] = {
> > > > +   {BTF_KIND_INT,          true,    0,      4,     "ifindex"},
> > > > +   {BTF_KIND_UNION,        false,  32,      4,     "bpf_prog"},
> > > > +   {BTF_KIND_STRUCT,       false,  -1,     -1,     "storage"}
> > > > +};
> > > > +
> > > > +static int dev_map_check_btf(const struct bpf_map *map,
> > > > +                        const struct btf *btf,
> > > > +                        const struct btf_type *key_type,
> > > > +                        const struct btf_type *value_type)
> > > > +{
> > > > +   struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> > > > +   u32 found_members_cnt = 0;
> > > > +   u32 int_data;
> > > > +   int off;
> > > > +   u32 i;
> > > > +
> > > > +   /* Validate KEY type and size */
> > > > +   if (BTF_INFO_KIND(key_type->info) != BTF_KIND_INT)
> > > > +           return -EOPNOTSUPP;
> > > > +
> > > > +   int_data = *(u32 *)(key_type + 1);
> > > > +   if (BTF_INT_BITS(int_data) != 32 || BTF_INT_OFFSET(int_data) != 0)
> > > > +           return -EOPNOTSUPP;
> > > > +
> > > > +   /* Validate VALUE have layout that match/map-to struct bpf_devmap_val
> > > > +    * - With a flexible size of member 'storage'.
> > > > +    */
> > > > +
> > > > +   if (BTF_INFO_KIND(value_type->info) != BTF_KIND_STRUCT)
> > > > +           return -EOPNOTSUPP;
> > > > +
> > > > +   /* Struct/union members in BTF must not exceed (max) expected members */
> > > > +   if (btf_type_vlen(value_type) > ARRAY_SIZE(layout))
> > > > +                   return -E2BIG;
> > > > +
> > > > +   for (i = 0; i < ARRAY_SIZE(layout); i++) {
> > > > +           off = btf_find_expect_layout_offset(btf, value_type, &layout[i]);
> > > > +
> > > > +           if (off < 0 && layout[i].mandatory)
> > > > +                   return -EUCLEAN;
> > > > +
> > > > +           if (off >= 0)
> > > > +                   found_members_cnt++;
> > > > +
> > > > +           /* Transfer layout config to map */
> > > > +           switch (i) {
> > > > +           case 0:
> > > > +                   dtab->cfg.btf_offset.ifindex = off;
> > > > +                   break;
> > > > +           case 1:
> > > > +                   dtab->cfg.btf_offset.bpf_prog = off;
> > > > +                   break;
> > > > +           default:
> > > > +                   break;
> > > > +           }
> > > > +   }
> > > > +
> > > > +   /* Detect if BTF/vlen have members that were not found */
> > > > +   if (btf_type_vlen(value_type) > found_members_cnt)
> > > > +           return -E2BIG;
> > > > +
> > > > +   return 0;
> > > > +}  
> > >
> > > This layout validation looks really weird to me.
> > > That layout[] array sort of complements BTF to describe the data,
> > > but double describe of the layout feels like hack.  
> >
> > This is the kind of feedback I'm looking for.  I want to make the
> > map-value more dynamic.  It seems so old school to keep extending the
> > map-value with a size and fixed binary layout, when we have BTF
> > available.  I'm open to input on how to better verify/parse/desc the
> > expected BTF layout for kernel-code side.
> >
> > The patch demonstrates that this is possible, I'm open for changes.
> > E.g. devmap is now extended with a bpf_prog, but most end-users will
> > not be using this feature. Today they can use value_size=4 to avoid
> > using this field. When we extend map-value again, then end-users are
> > force into providing 'bpf_prog.fd' if they want to use the newer
> > options.  In this patch end-users don't need to provide 'bpf_prog' if
> > they don't use it. Via BTF we can see this struct member can be skipped.  
> 
> I think 'struct bpf_devmap_val' should be in uapi/bpf.h.

I disagree.

> That's what it is and it will be extended with new fields at the end
> just like all other structs in uapi/bpf.h

This only works when new fields added will be zero, meaning that
default value of zero means the feature is not used.  In this specific
case devmap adds a file-descriptor field, that have to be -1 for the
feature to be unused.

Thus, when programs gets compiled with this new UAPI header, they will
start to fail, because they try to map-insert file-descriptor zero.

> I don't think BTF can become a substitute for uapi
> where uapi struct has to have all fields defined and backwards supported
> by the kernel.
> BTF is for flexible structs where fields may disappear.

Then BTF is perfect for this, as e.g. I want to remove field/member
'ifindex' for the HASH-variant of devmap, and instead use the key as
the ifindex.

> BTF is there to define a meaning of a binary blob.
> 'struct bpf_devmap_val' is not such thing. It's very much known with
> fixed fields and fixed meaning.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer