Re: [bpf-next PATCH v3 07/18] bpf: sockmap, add msg_cork_bytes() helper

[John Fastabend <john.fastabend@gmail.com>]

On 03/19/2018 09:30 AM, Alexei Starovoitov wrote:
> On Sun, Mar 18, 2018 at 12:57:20PM -0700, John Fastabend wrote:
>> In the case where we need a specific number of bytes before a
>> verdict can be assigned, even if the data spans multiple sendmsg
>> or sendfile calls. The BPF program may use msg_cork_bytes().
>>
>> The extreme case is a user can call sendmsg repeatedly with
>> 1-byte msg segments. Obviously, this is bad for performance but
>> is still valid. If the BPF program needs N bytes to validate
>> a header it can use msg_cork_bytes to specify N bytes and the
>> BPF program will not be called again until N bytes have been
>> accumulated. The infrastructure will attempt to coalesce data
>> if possible so in many cases (most my use cases at least) the
>> data will be in a single scatterlist element with data pointers
>> pointing to start/end of the element. However, this is dependent
>> on available memory so is not guaranteed. So BPF programs must
>> validate data pointer ranges, but this is the case anyways to
>> convince the verifier the accesses are valid.
>>
>> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
>> ---
>>  include/uapi/linux/bpf.h |    3 ++-
>>  net/core/filter.c        |   16 ++++++++++++++++
>>  2 files changed, 18 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index a557a2a..1765cfb 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -792,7 +792,8 @@ struct bpf_stack_build_id {
>>  	FN(override_return),		\
>>  	FN(sock_ops_cb_flags_set),	\
>>  	FN(msg_redirect_map),		\
>> -	FN(msg_apply_bytes),
>> +	FN(msg_apply_bytes),		\
>> +	FN(msg_cork_bytes),
>>  
>>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>>   * function eBPF program intends to call
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 17d6775..0c9daf6 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -1942,6 +1942,20 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>>  	.arg2_type      = ARG_ANYTHING,
>>  };
>>  
>> +BPF_CALL_2(bpf_msg_cork_bytes, struct sk_msg_buff *, msg, u32, bytes)
>> +{
>> +	msg->cork_bytes = bytes;
>> +	return 0;
>> +}
> 
> my understanding that setting it here and in the other helper *_bytes to zero
> will be effectively a nop. Right?
> 

Correct, setting cork_bytes or apply_bytes to zero is just a nop.

> Acked-by: Alexei Starovoitov <ast@kernel.org>
>