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

[Yonghong Song <yhs@fb.com>]

On 1/14/21 6:52 AM, Jesper Dangaard Brouer wrote:
> 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)

Just change here is enough.

> 
> Or do I also need to change bpf_func_proto.ret_type ?

There is no need to change bpf_func_proto.ret_type.
RET_INTEGER already implies 64bit scalar.

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