linux-kernel - Re: [PATCH] fs: FAT: Add support for DOS 1.x formatted volumes

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Date:	Sun, 30 Mar 2014 02:56:46 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Conrad Meyer <cemeyer@...edu>
Cc:	linux-kernel@...r.kernel.org, Mark <markk@...ra.co.uk>,
	Conrad Meyer <cse.cem@...il.com>
Subject: Re: [PATCH] fs: FAT: Add support for DOS 1.x formatted volumes

Conrad Meyer <cemeyer@...edu> writes:

Hi,

> When possible, infer DOS 2.x BIOS Parameter Block from block device
> geometry (for floppies and floppy images). Update in-memory only. We
> only perform this update when the entire BPB region is zeroed, like
> produced by DOS 1.x-era FORMAT (and other OEM variations on DOS).
>
> Fixes kernel.org bug #42617.
>
> BPB default values are inferred from media size and a table.[0] Media
> size is assumed to be static for archaic FAT volumes. See also [1].
>
> [0]: https://en.wikipedia.org/wiki/File_Allocation_Table#Exceptions
> [1]: http://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html

[...]

> +static void fat_update_archaic_boot_sector(struct super_block *sb,
> +	struct fat_boot_sector *b)
> +{
> +	sector_t bd_sects;
> +
> +	if (get_unaligned_le16(&b->sector_size) != 0 || b->sec_per_clus != 0 ||
> +		b->reserved != 0 || b->fats != 0 ||
> +		get_unaligned_le16(&b->dir_entries) != 0 ||
> +		get_unaligned_le16(&b->sectors) != 0 || b->media != 0 ||
> +		b->fat_length != 0 || b->secs_track != 0 || b->heads != 0 ||
> +		b->secs_track != 0 || b->heads != 0)
> +		return;
> +
> +	bd_sects = part_nr_sects_read(sb->s_bdev->bd_part);
> +	switch (bd_sects) {
> +	case 160 * KB_IN_SECTORS:
> +		b->sec_per_clus = 1;
> +		put_unaligned_le16(64, &b->dir_entries);
> +		b->media = 0xFE;
> +		b->fat_length = cpu_to_le16(1);
> +		break;

[...]

Hm, this looks like check the volume size. But if there is newer fat
format on same volume size, how to detect it? Or, it is conflicting?

[BTW, we should avoid to mount if it doesn't seem fatfs, to prevent
mis-mount as fatfs (auto mount is depending on this detection).]

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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