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Wed, 3 Jun 2020 09:12:05 +0000 (UTC) Received: from carbon (unknown [10.40.208.9]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0C2205D9CD; Wed, 3 Jun 2020 09:11:59 +0000 (UTC) Date: Wed, 3 Jun 2020 11:11:58 +0200 From: Jesper Dangaard Brouer To: Alexei Starovoitov Cc: David Ahern , bpf , Network Development , Daniel Borkmann , Andrii Nakryiko , Lorenzo Bianconi , brouer@redhat.com Subject: Re: [PATCH bpf-next RFC 2/3] bpf: devmap dynamic map-value storage area based on BTF Message-ID: <20200603111158.2cfd99e5@carbon> In-Reply-To: References: <159076794319.1387573.8722376887638960093.stgit@firesoul> <159076798566.1387573.8417040652693679408.stgit@firesoul> <20200601213012.vgt7oqplfbzeddzm@ast-mbp.dhcp.thefacebook.com> <20200602090005.5a6eb50c@carbon> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Tue, 2 Jun 2020 11:27:03 -0700 Alexei Starovoitov wrote: > On Tue, Jun 2, 2020 at 12:00 AM Jesper Dangaard Brouer > wrote: > > > > On Mon, 1 Jun 2020 14:30:12 -0700 > > Alexei Starovoitov 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