linux-kernel - Re: [PATCH] fat: Relax checks for sector size and media type

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Date:   Mon, 3 Sep 2018 09:40:05 +0200
From:   Pali Rohár <pali.rohar@...il.com>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fat: Relax checks for sector size and media type

On Monday 03 September 2018 16:17:26 OGAWA Hirofumi wrote:
> Pali Rohár <pali.rohar@...il.com> writes:
> 
> > Windows fastfat.sys driver accepts also media types 0x00 and 0x01 and
> > sector sizes 128 and 256 bytes. Linux mkfs.fat can format disk also to
> > larger FAT sector sizes then 4096 bytes, therefore relax also upper limit
> > restriction.
> 
> > -	if (!is_power_of_2(bpb->fat_sector_size)
> > -	    || (bpb->fat_sector_size < 512)
> > -	    || (bpb->fat_sector_size > 4096)) {
> > +	if (!is_power_of_2(bpb->fat_sector_size)) {
> 
> Just relaxing validation doesn't work. The block layer doesn't support
> smaller than 512, and lager than PAGE_SIZE.  (And in specification, fat
> doesn't support lager than 4096.)

Hi! I just sent this patch for discussion, with links to (now open
source) Windows implementation. I guess that Windows driver
implementation is more "authoritative" then Microsoft's own
specification. It is known that Windows implementation does not match
Microsoft specification.

I know at least 3 FAT specifications (MS EFI FAT, MS/SD card FAT,
ECMA-107) and you are right that Microsoft's one does not allow sector
sizes larger then 4096.

If there is limitation by block layer, then:

1) Why we do not check for PAGE_SIZE?

2) Is check in fat driver really needed (if block layer checks it)?

> >  static inline int fat_valid_media(u8 media)
> >  {
> > -	return 0xf8 <= media || media == 0xf0;
> > +	return 0xf8 <= media || media == 0xf0 || media == 0x00 || media == 0x01;
> >  }
> >  #endif /* !_LINUX_MSDOS_FS_H */
> 
> This is ok though, this would be for ancient floppy media.

Ok.

-- 
Pali Rohár
pali.rohar@...il.com