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92ec8b9367
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. 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> Link: https://lore.kernel.org/r/20240808075906.1849564-1-ast@fiberby.net Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>