[dpdk-dev] ipv4 fragmentation bug?

[Александр Киселев]

While playing with function rte_ipv4_fragment_packet I found that it
incorrectly fragments packets.
For example if the function takes 1200 bytes packet and mtu size 1000 it
will produces two fragments. And when those fragments are reassembled back
the resulting packet will be 4 bytes shorter than it should be.

I played with linux ping program and it reports that a reply is truncated.
    1204 bytes from 192.168.125.1: icmp_seq=1 ttl=64 (truncated)

Looking at the source of rte_ipv4_fragment_packet I discovered the cause of
the above behavior.

Function makes the following assumption and the whole calculations are
bases on that assumption.

/* Fragment size should be a multiply of 8. */
IP_FRAG_ASSERT((frag_size & IPV4_HDR_FO_MASK) == 0);

The problem is that this assert doesn’t make any sense. It's true that
fragment size should be a multiply of 8, but what this line real checks is
that
the size of mtu minus 20 bytes should be multiply of 8. In other words
it constrains the size of the mtu. So, if I take valid mtu value, say 1504,
it will produce incorrect fragments when asserts are off.

P.S.
I am using DPDK v 2.2.0

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Kiselev Alexander