Re: [PATCH bpf-next V11 4/7] bpf: add BPF-helper for MTU checking

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Jan 12, 2021 at 9:49 AM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> This BPF-helper bpf_check_mtu() works for both XDP and TC-BPF programs.
>
> The SKB object is complex and the skb->len value (accessible from
> BPF-prog) also include the length of any extra GRO/GSO segments, but
> without taking into account that these GRO/GSO segments get added
> transport (L4) and network (L3) headers before being transmitted. Thus,
> this BPF-helper is created such that the BPF-programmer don't need to
> handle these details in the BPF-prog.
>
> The API is designed to help the BPF-programmer, that want to do packet
> context size changes, which involves other helpers. These other helpers
> usually does a delta size adjustment. This helper also support a delta
> size (len_diff), which allow BPF-programmer to reuse arguments needed by
> these other helpers, and perform the MTU check prior to doing any actual
> size adjustment of the packet context.
>
> It is on purpose, that we allow the len adjustment to become a negative
> result, that will pass the MTU check. This might seem weird, but it's not
> this helpers responsibility to "catch" wrong len_diff adjustments. Other
> helpers will take care of these checks, if BPF-programmer chooses to do
> actual size adjustment.
>
> V9:
> - Use dev->hard_header_len (instead of ETH_HLEN)
> - Annotate with unlikely req from Daniel
> - Fix logic error using skb_gso_validate_network_len from Daniel
>
> V6:
> - Took John's advice and dropped BPF_MTU_CHK_RELAX
> - Returned MTU is kept at L3-level (like fib_lookup)
>
> V4: Lot of changes
>  - ifindex 0 now use current netdev for MTU lookup
>  - rename helper from bpf_mtu_check to bpf_check_mtu
>  - fix bug for GSO pkt length (as skb->len is total len)
>  - remove __bpf_len_adj_positive, simply allow negative len adj
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  include/uapi/linux/bpf.h       |   67 ++++++++++++++++++++++
>  net/core/filter.c              |  122 ++++++++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |   67 ++++++++++++++++++++++
>  3 files changed, 256 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 649586d656b6..fa2e99351758 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3833,6 +3833,61 @@ union bpf_attr {
>   *     Return
>   *             A pointer to a struct socket on success or NULL if the file is
>   *             not a socket.
> + *
> + * int bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)

should return long, same as most other helpers

> + *     Description
> + *             Check ctx packet size against MTU of net device (based on
> + *             *ifindex*).  This helper will likely be used in combination with
> + *             helpers that adjust/change the packet size.  The argument
> + *             *len_diff* can be used for querying with a planned size
> + *             change. This allows to check MTU prior to changing packet ctx.
> + *

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