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: Tue, 2 Jun 2020 09:00:05 +0200

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.

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