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关于SQL SERVER数据页checksum校验算法整理;北亚数据恢复中心

最新动态来源:点击数:236更新时间:2019/8/7

在SQL SERVER2005以上版本中,数据页默认开启checksum,标识为m_flagBits & 0x200 == True,其值m_tornBits位于页头0x3C,4字节。
其算法概述如下:


读8KB 进BUF
将BUF头部 CHECKSUM的4字节值清0
uint32 checksum = 0 //初始checksumfor i in range(0,15):
         //每扇区的初始checksum
         overall = 0;
        for ii in range(0,127):
                 //对当前扇区的每个4字节做累加异或
                overall = overall ^ BUF[i][ii];
                //对每扇区的checksum进行移位,方法为向左移位15-i位,
                //左边移出的15-i位补到最低位。
                checksum = checksum ^ rol(overall, 15- i);
return checksum; //Gets checksum


c源码如下:

//***CODE***//#include <stdio.h>#include <stdlib.h>
#define seed 15 //Initial seed(for first sector)#define CHAR_BIT 8
//***PROTOTYPES***//unsigned int page_checksum(int page_id, unsigned int *ondisk);unsigned int rol(unsigned int value, unsigned int rotation);
int main(int argc, char *argv[]) {

    unsigned int computed_checksum; //Var to retrieve calculated checksum
    unsigned int ondisk_checksum; //Var to retrieve checksum on disk

        computed_checksum = page_checksum(152, &ondisk_checksum); //page_checksum call to retrieve stored and calculated checksum for page 152

        //***PRINTS***//
        printf("Calculated checksum: 0x%08x", computed_checksum);
        printf("On disk checksum: 0x%08x", ondisk_checksum);

}
unsigned int page_checksum(int page_id, unsigned int *ondisk){

    FILE *fileptr;
    unsigned int i;
    unsigned int j;
    unsigned int checksum;
    unsigned int overall;
    unsigned int *pagebuf[16][128]; //A pointer to describe 2d array [sector][element]

    fileptr = fopen("C:\\Users\\andre\\Desktop\\teste.mdf", "r+b"); //Open dummy data file for binary read

    fseek(fileptr, page_id * 8192, SEEK_SET); //Calculate page address on data file and points to it

    fread(pagebuf, 4, 2048, fileptr); //Read page buffer

    fclose(fileptr);

    checksum = 0;
    overall = 0;

    *ondisk = pagebuf[0][15]; //This means that torn bits is stored on first sector in 15th element, Internals researches understand this

    pagebuf[0][15] = 0x00000000; //Fill checksum field with zeroes (this field will be discarded in algorithm)

    for (i = 0; i < 16; i++) //Loop through sectors
    {

        overall = 0; //Reset overall sum for sectors

        for (j = 0; j < 128; j++) //Loop through elements in sector i
        {
            overall = overall ^ (unsigned int)pagebuf[i][j]; //XOR operation between sector i elements
        }

        checksum = checksum ^ rol(overall, seed - i); //Current checksum is overall for sector i circular shifted by seed (15 - i)
    }

    return checksum; //Gets checksum

}
unsigned int rol(unsigned int value, unsigned int rotation){
    return (value) << (rotation) | (value) >> (sizeof(int) * CHAR_BIT - rotation) & ( (1 << rotation) -1);
}
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