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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> >> >> 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?

If we want to make this function fully compliant to what rfc8200 says,
then yes - extra changes is required in current implementation:
1. somehow obtain information about pre-fragment extensions length
2. use info from #1 to put fragment header at proper location.
And extra testing of course.
Probably safer and easier, for that patch just add formal parameter checking.
And if you feel like that - have the hard part as a separate patch.

> 
> >  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