[PATCH 1/2] [v3] filetype: Improve FAT detection

[Franck Jullien]

2012/9/19 Sascha Hauer <s.hauer at pengutronix.de>:
> On Tue, Sep 18, 2012 at 11:51:35PM +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_or_mbr to check if a buffer is a FAT boot
>> or MBR sector
>>
>> Signed-off-by: Franck Jullien <franck.jullien at gmail.com>
>> ---
>>  common/filetype.c  |   63 ++++++++++++++++++++++++++++++++++++++++++----------
>>  include/filetype.h |    2 +
>>  2 files changed, 53 insertions(+), 12 deletions(-)
>>
>> diff --git a/common/filetype.c b/common/filetype.c
>> index e736d43..cb4db5c 100644
>> --- a/common/filetype.c
>> +++ b/common/filetype.c
>> @@ -26,6 +26,8 @@
>>  #include <fcntl.h>
>>  #include <fs.h>
>>  #include <malloc.h>
>> +#include <fat.h>
>> +#include <errno.h>
>>
>>  static const char *filetype_str[] = {
>>       [filetype_unknown] = "unknown",
>> @@ -42,6 +44,7 @@ static const char *filetype_str[] = {
>>       [filetype_sh] = "Bourne Shell",
>>       [filetype_mips_barebox] = "MIPS barebox image",
>>       [filetype_fat] = "FAT filesytem",
>> +     [filetype_mbr] = "MBR sector",
>>  };
>>
>>  const char *file_type_to_string(enum filetype f)
>> @@ -52,26 +55,42 @@ const char *file_type_to_string(enum filetype f)
>>       return NULL;
>>  }
>>
>> -static int is_fat(u8 *buf)
>> +#define MBR_StartSector              8       /* MBR: Offset of Starting Sector in Partition Table Entry */
>> +#define BS_55AA                      510     /* Boot sector signature (2) */
>> +#define MBR_Table            446     /* MBR: Partition table offset (2) */
>> +#define BS_FilSysType32              82      /* File system type (1) */
>> +#define BS_FilSysType                54      /* File system type (1) */
>> +
>> +enum filetype is_fat_or_mbr(unsigned char *sector, unsigned long *bootsec)
>
> Could you add a comment describing the meaning of the bootsec argument?
> Also, please make the sector argument 'const'. (I'll send a patch
> making the other functions take a const argument where appropriate)
>
>>  {
>> -     if (get_unaligned_le16(&buf[510]) != 0xAA55)
>> -             return 0;
>> +     if(bootsec)
>
> whitespace after 'if' please.
>
>> +             *bootsec = 0;
>> +
>> +     /* Check record signature (always placed at offset 510 even if the sector size is>512) */
>> +     if (get_unaligned_le16(&sector[BS_55AA]) != 0xAA55)
>> +             return filetype_unknown;
>> +
>> +     /* Check "FAT" string */
>> +     if ((get_unaligned_le32(&sector[BS_FilSysType]) & 0xFFFFFF) == 0x544146)
>> +             return filetype_fat;
>>
>> -     /* FAT */
>> -     if ((get_unaligned_le32(&buf[54]) & 0xFFFFFF) == 0x544146)
>> -             return 1;
>> +     if ((get_unaligned_le32(&sector[BS_FilSysType32]) & 0xFFFFFF) == 0x544146)
>> +             return filetype_fat;
>>
>> -     /* FAT32 */
>> -     if ((get_unaligned_le32(&buf[82]) & 0xFFFFFF) == 0x544146)
>> -             return 1;
>> +     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 = get_unaligned_le16(&sector[MBR_Table + MBR_StartSector]);
>>
>> -     return 0;
>> +     return filetype_mbr;
>>  }
>>
>>  enum filetype file_detect_type(void *_buf)
>>  {
>>       u32 *buf = _buf;
>>       u8 *buf8 = _buf;
>> +     enum filetype type;
>>
>>       if (strncmp(buf8, "#!/bin/sh", 9) == 0)
>>               return filetype_sh;
>> @@ -99,8 +118,10 @@ enum filetype file_detect_type(void *_buf)
>>               return filetype_aimage;
>>       if (strncmp(buf8 + 0x10, "barebox", 7) == 0)
>>               return filetype_mips_barebox;
>> -     if (is_fat(buf8))
>> -             return filetype_fat;
>> +
>> +     type = is_fat_or_mbr(buf8, NULL);
>> +     if (type != filetype_unknown)
>> +             return type;
>>
>>       return filetype_unknown;
>>  }
>> @@ -110,6 +131,7 @@ 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 +145,23 @@ enum filetype file_name_detect_type(const char *filename)
>>
>>       type = file_detect_type(buf);
>>
>> +     if (type == filetype_mbr) {
>> +             /* Get the first partition start sector
>> +              * and check for FAT in it */
>> +             is_fat_or_mbr((u8 *)buf, &bootsec);
>
> Unnecessary cast.
>
>> +             ret = lseek(fd, (bootsec) * 512, SEEK_SET);
>> +             if (ret < 0) {
>> +                     type = filetype_unknown;
>
> It's still an mbr in this case.
>
>> +                     goto err_out;
>> +             }
>> +             ret = read(fd, buf, 512);
>> +             if (ret < 0) {
>> +                     type = filetype_unknown;
>
> ditto.
>
> Sascha
>
>
> --
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I shouldn't write code late at night. Hopefully you are reading my
code with attention :)