Re: [RFC][FAT] Handle MBR on the first sector

[Sascha Hauer <s.hauer@pengutronix.de>]

Hi Franck,

On Thu, Sep 13, 2012 at 02:13:29PM +0200, Franck Jullien wrote:
> *** Did not send this email with git because it's no working where I am **
> 
> We may have some disk with MBR as a first sector. In this case, the
> current FAT check returns an error. However, the FAT sector exist and
> the MBR can tell us where it is.
> 
> This patch make the FAT fs try to find the FAT boot sector on the first
> sector of the first partition in case it is not on sector 0.

The patch looks good. Not applying it because it's RFC, but I would if
you give your ok.

Sascha

> 
> Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
> ---
>  fs/fat/ff.c |   24 ++++++++++++++++++++----
>  1 files changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/fat/ff.c b/fs/fat/ff.c
> index 66db1d6..6a4ae32 100644
> --- a/fs/fat/ff.c
> +++ b/fs/fat/ff.c
> @@ -191,6 +191,7 @@
>  #define	FSI_Nxt_Free		492	/* FSI: Last allocated cluster (4) */
>  #define MBR_Table		446	/* MBR: Partition table offset (2) */
>  #define	SZ_PTE			16	/* MBR: Size of a partition table entry */
> +#define MBR_StartSector		8	/* MBR: Offset of Starting Sector in
> Partition Table Entry */
>  #define BS_55AA			510	/* Boot sector signature (2) */
> 
>  #define	DIR_Name		0	/* Short file name (11) */
> @@ -1533,11 +1534,15 @@ int follow_path (	/* 0(0): successful, !=0:
> error code */
>   */
>  static int check_fs (	/* 0:The FAT BR, 1:Valid BR but not an FAT,
> 2:Not a BR, 3:Disk error */
>  	FATFS *fs,	/* File system object */
> -	DWORD sect	/* Sector# (lba) to check if it is an FAT boot record or not */
> +	DWORD sect,	/* Sector# (lba) to check if it is an FAT boot record or not */
> +	DWORD *bootsec
>  )
>  {
>  	int ret;
> 
> +	if(bootsec)
> +		*bootsec = 0;
> +
>  	/* Load boot record */
>  	ret = disk_read(fs, fs->win, sect, 1);
>  	if (ret)
> @@ -1553,6 +1558,12 @@ static int check_fs (	/* 0:The FAT BR, 1:Valid
> BR but not an FAT, 2:Not a BR, 3:
>  	if ((LD_DWORD(&fs->win[BS_FilSysType32]) & 0xFFFFFF) == 0x544146)
>  		return 0;
> 
> +	if(bootsec)
> +		/* This must be an MBR, so return the starting sector of the
> +		 * first partition so we could check if there is a FAT boot
> +		 * sector there */
> +		*bootsec = LD_WORD(&fs->win[MBR_Table + MBR_StartSector]);
> +
>  	return -ENODEV;
>  }
> 
> @@ -1565,6 +1576,7 @@ static int chk_mounted (	/* 0(0): successful,
> !=0: any error occurred */
>  )
>  {
>  	BYTE fmt, b;
> +	DWORD first_boot_sect;
>  	DWORD bsect, fasize, tsect, sysect, nclst, szbfat;
>  	WORD nrsv;
> 
> @@ -1579,9 +1591,13 @@ static int chk_mounted (	/* 0(0): successful,
> !=0: any error occurred */
>  		return -EIO;
>  #endif
>  	/* Search FAT partition on the drive. Supports only generic
> partitionings, FDISK and SFD. */
> -	fmt = check_fs(fs, bsect = 0);	/* Check sector 0 if it is a VBR */
> -	if (fmt)
> -		return fmt; /* No FAT volume is found */
> +	fmt = check_fs(fs, bsect = 0, &first_boot_sect);	/* Check sector 0
> if it is a VBR */
> +	if (fmt && first_boot_sect != 0) {
> +		/* Sector 0 is an MBR, now check for FAT in the first partition */
> +		fmt = check_fs(fs, bsect = first_boot_sect, NULL);
> +		if(fmt)
> +			return fmt; /* No FAT volume is found */
> +	}
> 
>  	/* Following code initializes the file system object */
> 
> -- 
> 1.7.1
> 
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