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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.