Re: [PATCH bpf-next 2/4] netfilter: add bpf_xdp_flow_offload_lookup kfunc

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, 15 May 2024 at 23:13, Lorenzo Bianconi <lorenzo@kernel.org> wrote:
>
> Introduce bpf_xdp_flow_offload_lookup kfunc in order to perform the
> lookup of a given flowtable entry based on a fib tuple of incoming
> traffic.
> bpf_xdp_flow_offload_lookup can be used as building block to offload
> in xdp the processing of sw flowtable when hw flowtable is not
> available.
>
> Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
> ---
>  include/net/netfilter/nf_flow_table.h | 10 +++
>  net/netfilter/Makefile                |  5 ++
>  net/netfilter/nf_flow_table_bpf.c     | 95 +++++++++++++++++++++++++++
>  net/netfilter/nf_flow_table_inet.c    |  2 +
>  4 files changed, 112 insertions(+)
>  create mode 100644 net/netfilter/nf_flow_table_bpf.c
>
> diff --git a/include/net/netfilter/nf_flow_table.h b/include/net/netfilter/nf_flow_table.h
> index 0bbe6ea8e0651..085660cbcd3f2 100644
> --- a/include/net/netfilter/nf_flow_table.h
> +++ b/include/net/netfilter/nf_flow_table.h
> @@ -312,6 +312,16 @@ unsigned int nf_flow_offload_ip_hook(void *priv, struct sk_buff *skb,
>  unsigned int nf_flow_offload_ipv6_hook(void *priv, struct sk_buff *skb,
>                                        const struct nf_hook_state *state);
>
> +#if (IS_BUILTIN(CONFIG_NF_FLOW_TABLE) && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) || \
> +    (IS_MODULE(CONFIG_NF_FLOW_TABLE) && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES))
> +extern int nf_flow_offload_register_bpf(void);
> +#else
> +static inline int nf_flow_offload_register_bpf(void)
> +{
> +       return 0;
> +}
> +#endif
> +
>  #define MODULE_ALIAS_NF_FLOWTABLE(family)      \
>         MODULE_ALIAS("nf-flowtable-" __stringify(family))
>
> diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
> index 614815a3ed738..18b09cec92024 100644
> --- a/net/netfilter/Makefile
> +++ b/net/netfilter/Makefile
> @@ -144,6 +144,11 @@ obj-$(CONFIG_NF_FLOW_TABLE)        += nf_flow_table.o
>  nf_flow_table-objs             := nf_flow_table_core.o nf_flow_table_ip.o \
>                                    nf_flow_table_offload.o
>  nf_flow_table-$(CONFIG_NF_FLOW_TABLE_PROCFS) += nf_flow_table_procfs.o
> +ifeq ($(CONFIG_NF_FLOW_TABLE),m)
> +nf_flow_table-$(CONFIG_DEBUG_INFO_BTF_MODULES) += nf_flow_table_bpf.o
> +else ifeq ($(CONFIG_NF_FLOW_TABLE),y)
> +nf_flow_table-$(CONFIG_DEBUG_INFO_BTF) += nf_flow_table_bpf.o
> +endif
>
>  obj-$(CONFIG_NF_FLOW_TABLE_INET) += nf_flow_table_inet.o
>
> diff --git a/net/netfilter/nf_flow_table_bpf.c b/net/netfilter/nf_flow_table_bpf.c
> new file mode 100644
> index 0000000000000..836a1127e4052
> --- /dev/null
> +++ b/net/netfilter/nf_flow_table_bpf.c
> @@ -0,0 +1,95 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Unstable Flow Table Helpers for XDP hook
> + *
> + * These are called from the XDP programs.
> + * Note that it is allowed to break compatibility for these functions since
> + * the interface they are exposed through to BPF programs is explicitly
> + * unstable.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/init.h>
> +#include <linux/module.h>
> +#include <net/netfilter/nf_flow_table.h>
> +#include <linux/bpf.h>
> +#include <linux/btf.h>
> +#include <net/xdp.h>
> +
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +                 "Global functions as their definitions will be in nf_flow_table BTF");
> +
> +static struct flow_offload_tuple_rhash *
> +bpf_xdp_flow_offload_tuple_lookup(struct net_device *dev,
> +                                 struct flow_offload_tuple *tuple,
> +                                 __be16 proto)
> +{
> +       struct flow_offload_tuple_rhash *tuplehash;
> +       struct nf_flowtable *flow_table;
> +       struct flow_offload *flow;
> +
> +       flow_table = nf_flowtable_by_dev(dev);
> +       if (!flow_table)
> +               return ERR_PTR(-ENOENT);
> +
> +       tuplehash = flow_offload_lookup(flow_table, tuple);
> +       if (!tuplehash)
> +               return ERR_PTR(-ENOENT);

This is fine to do, but the caller should catch it using IS_ERR_PTR
and return NULL.
BPF side cannot distinguish ERR_PTR from normal pointer, so this will
cause a bad deref in the program.

> +
> +       flow = container_of(tuplehash, struct flow_offload,
> +                           tuplehash[tuplehash->tuple.dir]);
> +       flow_offload_refresh(flow_table, flow, false);
> +
> +       return tuplehash;
> +}
> +
> +__bpf_kfunc struct flow_offload_tuple_rhash *
> +bpf_xdp_flow_offload_lookup(struct xdp_md *ctx,
> +                           struct bpf_fib_lookup *fib_tuple,
> +                           u32 fib_tuple__sz)

Do you think the __sz has the intended effect? It only works when the
preceding parameter is a void *.
If you have a type like struct bpf_fib_lookup, I think it should work
fine without taking a size at all.

> +{
> +       struct xdp_buff *xdp = (struct xdp_buff *)ctx;
> +       struct flow_offload_tuple tuple = {
> +               .iifidx = fib_tuple->ifindex,
> +               .l3proto = fib_tuple->family,
> +               .l4proto = fib_tuple->l4_protocol,
> +               .src_port = fib_tuple->sport,
> +               .dst_port = fib_tuple->dport,
> +       };
> +       __be16 proto;
> +
> +       switch (fib_tuple->family) {
> +       case AF_INET:
> +               tuple.src_v4.s_addr = fib_tuple->ipv4_src;
> +               tuple.dst_v4.s_addr = fib_tuple->ipv4_dst;
> +               proto = htons(ETH_P_IP);
> +               break;
> +       case AF_INET6:
> +               tuple.src_v6 = *(struct in6_addr *)&fib_tuple->ipv6_src;
> +               tuple.dst_v6 = *(struct in6_addr *)&fib_tuple->ipv6_dst;
> +               proto = htons(ETH_P_IPV6);
> +               break;
> +       default:
> +               return ERR_PTR(-EINVAL);

Likewise. While you check IS_ERR_VALUE in selftest, direct dereference
will be allowed by verifier, which would crash the kernel.
It's better to do something like conntrack kfuncs, where they set
opts->error when returning NULL, allowing better debugging in case
lookup fails.

> +       }
> +
> +       return bpf_xdp_flow_offload_tuple_lookup(xdp->rxq->dev, &tuple, proto);
> +}
> +
> +__diag_pop()
> +
> +BTF_KFUNCS_START(nf_ft_kfunc_set)
> +BTF_ID_FLAGS(func, bpf_xdp_flow_offload_lookup)
> +BTF_KFUNCS_END(nf_ft_kfunc_set)
> +
> +static const struct btf_kfunc_id_set nf_flow_offload_kfunc_set = {
> +       .owner = THIS_MODULE,
> +       .set   = &nf_ft_kfunc_set,
> +};
> +
> +int nf_flow_offload_register_bpf(void)
> +{
> +       return register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP,
> +                                        &nf_flow_offload_kfunc_set);
> +}

We should probably also expose it to skb? We just need net_device, so
it can work with both XDP and TC progs.
That would be similar to how we expose conntrack kfuncs to both XDP
and TC progs.

> +EXPORT_SYMBOL_GPL(nf_flow_offload_register_bpf);
> diff --git a/net/netfilter/nf_flow_table_inet.c b/net/netfilter/nf_flow_table_inet.c
> index 6eef15648b7b0..b13587238eceb 100644
> --- a/net/netfilter/nf_flow_table_inet.c
> +++ b/net/netfilter/nf_flow_table_inet.c
> @@ -98,6 +98,8 @@ static int __init nf_flow_inet_module_init(void)
>         nft_register_flowtable_type(&flowtable_ipv6);
>         nft_register_flowtable_type(&flowtable_inet);
>
> +       nf_flow_offload_register_bpf();
> +

Error checking needed here? Kfunc registration can fail.

>         return 0;
>  }
>
> --
> 2.45.0
>
>