Date: Tue, 29 Sep 2009 14:12:56 -0600 (MDT) From: "M. Warner Losh" <imp@bsdimp.com> To: xcllnt@mac.com Cc: marcel@freebsd.org, jhb@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, rwatson@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r197608 - head/sys/geom/part Message-ID: <20090929.141256.-981704958.imp@bsdimp.com> In-Reply-To: <EF993D67-F575-45CC-98EC-A1163EF15B50@mac.com> References: <1F22EA6A-2793-45CE-9669-3FFD550CF4E2@mac.com> <alpine.BSF.2.00.0909291759180.94746@fledge.watson.org> <EF993D67-F575-45CC-98EC-A1163EF15B50@mac.com>
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In message: <EF993D67-F575-45CC-98EC-A1163EF15B50@mac.com> Marcel Moolenaar <xcllnt@mac.com> writes: : : On Sep 29, 2009, at 10:00 AM, Robert Watson wrote: : : > : > On Tue, 29 Sep 2009, Marcel Moolenaar wrote: : > : >>> Why do you check for zeros at all? AFAIK, the only real check is : >>> for AA55 at the end of the sector (and having an MBR or other : >>> Extended MBR partition entry point at the sector in question). : >>> The '96' thing seems rather arbitrary in the code, and I think the : >>> zero's check is overly restrictive. : >> : >> Only checking for a signature that 99% of the boot blocks have : >> isn't enough. The msdos file system has that signature and the : >> check for all-zeroes is to prevent false positives there. : > : > And, as I recall, the msdosfs check has been gradually getting : > weaker over time as the constraints it places on things like : > cylinder counts become obsolete. It's not quite that we'll mount : > msdosfs on any random pile of bytes, but it might be getting there... : : We really need to get to a point where we treat partition types : seriously and : use it to help avoid false positives. Reducing or eliminating false : positives : is critical if we ever want to go towards DWIM or auto-mounting. With : the : partition type taken into consideration, we may be able to eliminate : ad hoc : checks, like the zero check in g_part_ebr, without creating false : positives. Part of the problem too is that the msdosfs code checks too many things, making it hard to use on raw images :( Of course, I'm about 12 sigma from the mean on the images I use, but still... Warner
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