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

[Jesper Dangaard Brouer <brouer@redhat.com>]

On Tue, 12 Jan 2021 11:23:33 -0800
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> 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

Is it enough to change it here?
(as this will be used for generating the helpers header file,
via ./scripts/bpf_helpers_doc.py --header)

Or do I also need to change bpf_func_proto.ret_type ?

> > + *     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.
> > + *  
> 
> [...]
> 



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