Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx> writes: > On little-endian systems, doing subtraction after htons() > leads to interesting results: > > Given: > MAGIC_BYTES = 123 = 0x007B aka. in big endian: 0x7B00 = 31488 > sizeof(struct iphdr) = 20 > > Before this patch: > __bpf_constant_htons(MAGIC_BYTES) - sizeof(struct iphdr) = 0x7AEC > 0x7AEC = htons(0xEC7A) = htons(60538) > > So these were outer IP packets with a total length of 123 bytes, > containing an inner IP packet with a total length of 60538 bytes. It's just using bag of holding technology! > After this patch: > __bpf_constant_htons(MAGIC_BYTES - sizeof(struct iphdr)) = htons(103) > > Now these packets are outer IP packets with a total length of 123 bytes, > containing an inner IP packet with a total length of 103 bytes. > > Signed-off-by: Asbjørn Sloth Tønnesen <ast@xxxxxxxxxxx> Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
