Re: [PATCH] Fix when installing on pationless but partionable medium

[Colin Watson]

On Fri, Jul 17, 2009 at 06:41:59PM +0200, Vladimir 'phcoder' Serbinenko wrote:
> Sometimes a media that can be partioned isn't really partioned. E.g.
> usb sticks. This is a patch to handle this situation. Unfortunately
> such medium is often formated with a flavour of FAT which shares its
> signature with MBR so it may be easily misidentified as
> pc_partition_table. Furthermore the same signature is shared with
> bootsectors including grub. One possibility is to try interpret disk
> as known filesystems and see if we succeed. But the problem is that
> the check for FAT are light and may result in false positives too. The
> only more or less advanced check there is a check for FATXX string.
> But I was about to propose to eliminate this check since I encountered
> a FAT filesystem without this string on friend's SD card formatted
> with symbian which he wanted to use as liveusb. Does anyone has a
> better idea?

When checking for an MBR filesystem label, parted checks whether each of
partitions 1-4 has a boot indicator that's either 0 or 0x80, since as
you point out checking for the FAT signature suffers false positives; I
believe this algorithm matches that in the Linux kernel. Look at
libparted/labels/dos.c:msdos_probe(), which is already FSF-copyrighted
and GPLv3+. GRUB should use the same algorithm, and then the worst case
is that things will fail consistently.

-- 
Colin Watson                                       address@hidden