Re: [dpdk-dev] [PATCH v3 1/4] ip_frag: ensure minimum v4 fragmentation length

[Aaron Conole <aconole@redhat.com>]

"Ananyev, Konstantin" <konstantin.ananyev@intel.com> writes:

>> 
>> "Ananyev, Konstantin" <konstantin.ananyev@intel.com> writes:
>> 
>> >>
>> >> The IPv4 specification says that each fragment must at least the size of
>> >> an IP header plus 8 octets.  When attempting to run ipfrag using a
>> >> smaller size, the fragment library will return successful completion,
>> >> even though it is a violation of RFC791 (and updates).
>> >>
>> >> Signed-off-by: Aaron Conole <aconole@redhat.com>
>> >> ---
>> >>  lib/librte_ip_frag/rte_ipv4_fragmentation.c | 6 ++++++
>> >>  1 file changed, 6 insertions(+)
>> >>
>> >> diff --git a/lib/librte_ip_frag/rte_ipv4_fragmentation.c b/lib/librte_ip_frag/rte_ipv4_fragmentation.c
>> >> index 9e9f986cc5..4baaf6355c 100644
>> >> --- a/lib/librte_ip_frag/rte_ipv4_fragmentation.c
>> >> +++ b/lib/librte_ip_frag/rte_ipv4_fragmentation.c
>> >> @@ -76,6 +76,12 @@ rte_ipv4_fragment_packet(struct rte_mbuf *pkt_in,
>> >>  	uint16_t fragment_offset, flag_offset, frag_size;
>> >>  	uint16_t frag_bytes_remaining;
>> >>
>> >> +	/*
>> >> +	 * Ensure the IP fragmentation size is at least iphdr length + 8 octets
>> >> +	 */
>> >> +	if (unlikely(mtu_size < (sizeof(struct rte_ipv4_hdr) + 8*sizeof(char))))
>> >> +		return -EINVAL;
>> >> +
>> >
>> > Same comment as for ipv6: ipv4 min MTU is 68B.
>> 
>> I can change it.  I suspected that if I went with 68 here and 1280 in
>> the v6 code, I would get pushback, but I should have just submitted it
>> that way to begin.
>> 
>> > Why do we need extra checking here?
>> 
>> These are error conditions to submit to fragmentation module.  Someone
>> needs to do the check - either it is done in the application or the
>> library.  If the library doesn't, and the application writer doesn't
>> know they must write these checks (it isn't documented anywhere), then
>> we get non compliant behavior.  By putting it in the library, we can
>> clearly signal the application writer such a case has occurred.
>> 
>> Should we not do error checking?
>
> It depends I think...
> In many data-path functions we skip parameter checking.
> These fragment() functions are data-path too.
> Agree, it is not stated clearly in these functions formal comments,
> as it should be.

I'll add documentation as another patch.

> After another thought - these functions are quite heavy-weighed anyway,
> so probably formal parameter checking, something like:
> if (pkt_in == NULL || pkts_out == NULL || pool_direct == NULL ||
> 		pool_indirect == NULL || mtu < MIN_MTU)
> 	return -EINVAL;
>
> wouldn't introduce any real slowdown.

Agreed.

> About more intense checking - like parsing all extension
> headers, etc. - I think it would be too much overhead.
> Again for that there is a special function that user can call directly:
> rte_ipv6_frag_get_ipv6_fragment_header
> (though its current implementation also checks only first extension header).
> So, I think we just need to document that it is a user responsibility to
> provide not fragmented yet packet, without any pre-fragment headers.

Makes sense.  Then again, the v6 frag code will need to preserve many of
the headers anyway, so since we have to read them, maybe it makes
sense to do the check here anyway.  WDYT?

>  Konstantin
>
>> 
>> >>  	/*
>> >>  	 * Ensure the IP payload length of all fragments is aligned to a
>> >>  	 * multiple of 8 bytes as per RFC791 section 2.3.
>> >> --
>> >> 2.25.1