c - 找不到 MBR 类型

Question

我有这段代码，它是项目源代码的一部分。此代码查找 MBR 类型:GRUB 或 LILO，并相应地设置一个标志。

奇怪的是在SLES 10-SP1(SUSE Linux Enterprise Server)中，无法确定。/dev/sda1 是我的交换区。/dev/sda2 是整个 / 所在的位置，包括 MBR。

相同的代码适用于 SLES11 和其他版本。这里 MBR_SIZE 是 #defined 到 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 */

测试是在 VMWare 下使用不同的虚拟磁盘镜像进行的。我已确认磁盘格式化为 MBR 而不是 GPT。

Answer 1

当您说它不起作用时，我不确定您的意思。如果你的意思是你的代码返回 -1，你能给我们看一份 MBR 的副本吗？您可以使用此命令捕获它:

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

您提到您的 MBR 在 /dev/sda2 上。这确实很不寻常。如果您的意思是那是安装引导代码的地方，那是完全不同的事情。 MBR 始终位于磁盘的第一个扇区(假设它是 DOS 格式的 MBR)。

我想在某些失败案例中，问题可能是寻道失败或读取时间过短。我进行了一些调整以添加错误处理并进行一些简化。

#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