Asbjørn Sloth Tønnesen ast@fiberby.net 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@fiberby.net
Reviewed-by: Toke Høiland-Jørgensen toke@kernel.org