Date: Fri, 6 Jun 2008 17:52:10 +1000 From: Peter Jeremy <peterjeremy@optushome.com.au> To: Marc =?iso-8859-1?Q?L=F6rner?= <marc.loerner@hob.de> Cc: freebsd-net@freebsd.org Subject: Re: Probable Bug in tcp.h Message-ID: <20080606075210.GD67629@server.vk2pj.dyndns.org> In-Reply-To: <200806060930.28527.marc.loerner@hob.de> References: <200806051712.47048.marc.loerner@hob.de> <20080605155646.GC6864@epsilon.local> <200806060930.28527.marc.loerner@hob.de>
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--RASg3xLB4tUQ4RcS Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2008-Jun-06 09:30:28 +0200, Marc L=F6rner <marc.loerner@hob.de> wrote: >th_x2 and th_off are created as a bitfield. But C-Standard says that >bitfields are accessed as integers =3D> 4-bytes > >On itanium integers are read with ld4-command but the address of >th_x2/th_off may not be aligned to 4-bytes =3D> we get an unaligned >reference fault. If the C compiler chooses to implement bitfields as a subset of a 32-bit integers, it is up to it to load an aligned 32-bit integer and shift/mask the result appropriately to extract the fields. In this particular case, th_x2/th_off are immediately preceeded by a tcp_seq (u_int32_t) field and so will have 32-bit alignment. Note that the presence of 32-bit fields in the definition for struct tcphdr means that the struct must be aligned to at least 32 bits. >If we'd change to 1 byte-accesses =3D> I won't get any misaligned faults= =20 >anymore. I gather from this comment that you have some code using struct tcphdr that is getting alignment errors. struct tcphdr is extensively used in the TCP stack within the kernel so it's likely that any layout or alignment problem with it would show up there. I suspect you are dereferencing a mis-aligned struct tcphdr. --=20 Peter Jeremy Please excuse any delays as the result of my ISP's inability to implement an MTA that is either RFC2821-compliant or matches their claimed behaviour. --RASg3xLB4tUQ4RcS Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (FreeBSD) iEYEARECAAYFAkhI7KoACgkQ/opHv/APuIccbwCgtGkXwOIifLt4K4DN0V0VQU6H k6IAn1S0C0soIZdtaesp62Y9g/xVOwN3 =5suD -----END PGP SIGNATURE----- --RASg3xLB4tUQ4RcS--
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