Unable to find MBR type

Question

I have this code which is part of a project source. This code finds the MBR type: GRUB or LILO, and accordingly sets a flag.

Surprisingly in SLES 10-SP1 (SUSE Linux Enterprise Server), it is unable to determine. /dev/sda1 is my swap. /dev/sda2 is where the whole / is there, including the MBR.

Same code works for SLES11 and others. Here MBR_SIZE is #defined to 0x1be .

int lnxfsGetBootType(int pNumber)
{
    int                 i, retval = -1, ccode;
    PartInfo            *p = &cpuParts[pNumber];
    char                buffer[SECTOR_SIZE];
    var64               offset = 0;

    isdLogFileOut(ZISD_LOG_DEVELOPER,"[lnxGBT]\n");
    if (getenv("ZENDEVICE") || gUtilPart == 1) {
        offset = p->pOffset;        // look at the partition BPB
    }

    //Now try to find the installed boot loader...
    lseek64(p->handle, (var64)offset, SEEK_SET);    // either MBR or BPB
    ccode = read(p->handle, buffer, SECTOR_SIZE);

    for (i=0; i<MBR_SIZE-4;i++) {
        if (strncmp(&buffer[i], "LILO", 4) == 0) {
            if (offset == 0){
                retval = FLAG_LNXFS_LILO;
                isdLogFileOut(ZISD_LOG_WARNING,"\tLILO MBR found on %s\n",p->header.deviceName);
            } else {
                retval = FLAG_LNXFS_LILO; // 10.31.06 _BPB;
                isdLogFileOut(ZISD_LOG_WARNING,"\tLILO BPB found on %s\n",p->header.deviceName);
            }
        }
        if (strncmp(&buffer[i], "GRUB", 4) == 0) {
            if (offset == 0){
                retval = FLAG_LNXFS_GRUB;
                isdLogFileOut(ZISD_LOG_WARNING,"\tGRUB MBR found on %s\n",p->header.deviceName);
            } else {
                retval = FLAG_LNXFS_GRUB; // 10.31.06 _BPB;
                isdLogFileOut(ZISD_LOG_WARNING,"\tGRUB BPB found on %s\n",p->header.deviceName);
            }
        }
    }
    if (retval == -1)  {
        isdLogFileOut(ZISD_LOG_WARNING,"\tLILO or GRUB mbr/bpb not found on %s\n",p->header.deviceName);
    }

    return retval;
}  // lnxfsGetBootType  

Here partinfo, is a struct of partition type:
//Data structure used internally by the image engine to store information about the 
//partitions.  It encapsulates the PartHeader struct, whcih is used to store partition
//information in image archives
typedef struct _PartInfo
{
    PartHeader  header;
    int         handle;         //file handle for reading/writing physical device
    var32       flags;          //Various flags as needed.  Defined above.
    var64       pOffset;        //offset to partition from start of physical device
    int         deviceNumber;   //index into 'devices' where this partition's 
                                // physical device is located
    int         archIndex;      //for restoring only.  Index into imgParts of the
                                // archive partition this physical partition is
                                // mapped to
    int         bytesWritten;   //track number of sectors written so the device-level
                                // cache can be flushed
    void        *info;          //partition-type-specific info struct

/* snip */

The testing is being done with different virtual disk images under VMWare. I've confirmed the disks are formatted with MBR and not GPT.

Answer 1

I'm not sure what you mean when you say it doesn't work. If your point is that your code returns -1, could you show us a copy of the MBR? You can use this command to capture it:

sudo dd if=/dev/sda bs=512 count=1 | xxd

You mention that your MBR is on /dev/sda2 . That is very unusual indeed. If you mean that that is where the boot code is installed, that's a totally different thing. The MBR is always held on the first sector of the disk (assuming it is a DOS-format MBR).

I suppose it's possible that the problem in some of the failure cases is a seek failure or a short read. I've made some tweaks to add error handling and simplify a bit.

#define MBR_SIZE 0x1be

int lnxfsGetBootType(int pNumber)
{
    int                 retval = -1, ccode;
    PartInfo            *p = &cpuParts[pNumber];
    char                buffer[SECTOR_SIZE];
    off64_t  offset = 0;
    void *plilo, *pgrub;
    const char *what = "MBR";

    isdLogFileOut(ZISD_LOG_DEVELOPER,"[lnxGBT]\n");
    if (getenv("ZENDEVICE") || gUtilPart == 1) {
        offset = p->pOffset;        // look at the partition BPB
        what = "BPB";
    }

    // Now try to find the installed boot loader...
    if (lseek64(p->handle, offset, SEEK_SET) == -1) {
        isdLogFileOut(ZISD_LOG_ERROR,"\tFailed to seek to %s: %s\n", what, strerror(errno));
        return -1;
    }
    ccode = read(p->handle, buffer, SECTOR_SIZE);
    if (ccode != SECTOR_SIZE) {
            isdLogFileOut(ZISD_LOG_ERROR,"\tFailed to read BPB/MBR: %s\n",
                                strerror(errno));
            return -1;
    }
    plilo = memmem(buffer, ccode, "LILO", 4);
    pgrub = memmem(buffer, ccode, "GRUB", 4);
    if (plilo) {
        retval = FLAG_LNXFS_LILO;
        if (pgrub && pgrub < plilo)
            retval = FLAG_LNXFS_GRUB;
        }
    } else if (pgrub) {
        retval = FLAG_LNXFS_GRUB;
    }
    if (-1 == retval) {
        isdLogFileOut(ZISD_LOG_WARNING,"\tLILO or GRUB %s not found on %s\n", what, p->header.deviceName);
    } else {
        isdLogFileOut(ZISD_LOG_WARNING,"\t%s %s not found on %s\n", 
              (retval == FLAG_LNXFS_GRUB ? "GRUB" : "LILO"),
              what, p->header.deviceName);
    }
    return retval;
}  // lnxfsGetBootType