[PATCH 1/2] [v2] filetype: Improve FAT detection

[Sascha Hauer]

Hi Franck,

On Sun, Sep 16, 2012 at 10:33:32PM +0200, Franck Jullien wrote:
> 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 add to file_name_detect_type function the ability to find
> the FAT boot sector on the first sector of the first partition in case
> it is not on sector 0.
> 
> It also introduce is_fat_boot_sector to check if a buffer is a FAT boot
> sector.

I think it would be cleaner to introduce a filetype MBR as a first
step...

> @@ -110,6 +128,7 @@ enum filetype file_name_detect_type(const char *filename)
>  	int fd, ret;
>  	void *buf;
>  	enum filetype type = filetype_unknown;
> +	unsigned long bootsec;
>  
>  	fd = open(filename, O_RDONLY);
>  	if (fd < 0)
> @@ -123,6 +142,25 @@ enum filetype file_name_detect_type(const char *filename)
>  
>  	type = file_detect_type(buf);
>  
> +	/* Let's give FAT another chance */
> +	if (type == filetype_unknown) {

... then you could check for type == filetype_mbr here instead and
return filetype_mbr when the check for fat below fails.

> +		ret = is_fat_boot_sector((u8 *)buf, &bootsec);
> +		if (ret && bootsec != 0) {
> +			/* The first sector is an MBR, now check for
> +			 * FAT in the first partition */
> +			lseek(fd, (bootsec) * 512, SEEK_SET);

Please check the return value of lseek here. lseek will fail when the file
is on a tftp filesystem or when the file is not large enough.

Sascha

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