Date: Mon, 6 Feb 2012 12:12:34 -0800 From: Adrian Chadd <adrian@freebsd.org> To: Bernhard Schmidt <bschmidt@freebsd.org> Cc: freebsd-wireless@freebsd.org Subject: Re: [net80211] support vendor bitmap entries; teach if_ath to export PHY error code in error frames Message-ID: <CAJ-Vmo=6xF8pWceBQVb4x3PYbLoGiiiAh9u2ZA8A=bAQdrOFdg@mail.gmail.com> In-Reply-To: <201202062059.16816.bschmidt@freebsd.org> References: <CAJ-VmonSrcSQmZeENgH1fagnn6DOMhX6rC-n9=dOtypTabcSLg@mail.gmail.com> <CAAgh0_aAijDoguQCJO8jR=cZ5LrZGLfxpGruZWGsjGf987-zOw@mail.gmail.com> <CAJ-Vmon%2BpQ9U7-YEQ1pYVbHM=5D=cp-z6eoPGJhV_GU3r7hy8A@mail.gmail.com> <201202062059.16816.bschmidt@freebsd.org>
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On 6 February 2012 11:59, Bernhard Schmidt <bschmidt@freebsd.org> wrote: > Given that the calculation of the offsets is totally unrelated to the length of the actually vendor specific data, couldn't you just add the length of the additional ext field + padding based on the presents of VENDOREXT? Well, the problem is the lack of documented alignment requirements for a variable-length vendor data chunk. I'm aligning the vendor header @ a 4 byte boundary (which wireshark seems to accept) and johannes' reference radiotap parser comes with vendor chunks which are also 4 byte aligned. But: * the vendor header is 6 bytes, so * there's two bytes between it and the next 4 / 8 byte boundary; * there's no requirement for the vendor header to be 8 byte aligned, so you can't store uint64_t's inside the vendor data blob and assume they'll be "naturally aligned." For now I've just added a 2 byte pad (1 byte version, one byte rx chainmask) to pad it out to a 4 byte array, then all my sub-fields assume 4 byte alignment. Along with plenty of comments about alignment assumptions. I haven't yet tested my patch out on MIPS to ensure that I haven't busted alignment constraints. I'll do that before I finally commit it. Adrian
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