使用/dev/kmem读取内核变量的值

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调试嵌入式Linux内核时，可能需要查看某个内核变量的值。/dev/kmem正好提供了访问内核虚拟内存的途径。现在的内核大都默认禁用了/dev/kmem，打开的方法是在 make menuconfig中选中 device drivers --> Character devices -->/dev/kmem virtual device support.

用 cat /proc/kallsyms | grep *** 找到要查看的变量的地址，这个地址作为本程序的输入参数。

int snd_write_bypass = 0;
EXPORT_SYMBOL_GPL(snd_write_bypass);

/dav/test # cat /proc/kallsyms | grep snd_write_bypass
8051e26c r __ksymtab_snd_write_bypass
80530219 r __kstrtab_snd_write_bypass

app code

#include <stdio.h>

#include <stdlib.h>

#include <fcntl.h>

#include <string.h>

#include <errno.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <sys/mman.h>

#define DEVKMEM         "/dev/kmem"

#define PAGE_SIZE       0x1000

#define PAGE_MASK       (~(PAGE_SIZE-1))

intmain(intargc, char* argv[])

{

    intfd;

    char*mbase;

    charread_buf[10];

    unsignedintregAddr;

    varAddr = strtoul(argv[1], 0, 16);

    unsignedintptr = varAddr & ~(PAGE_MASK);

    fd = open(DEVKMEM, O_RDONLY);

    if(fd == -1) {

        perror("open");

        exit(-1);

}

    mbase = mmap(0,PAGE_SIZE,PROT_READ,MAP_SHARED,fd, (varAddr & PAGE_MASK));

    if(mbase == MAP_FAILED) {

        printf("map failed %s\n",strerror(errno));

}

    printf("varAddr = 0x%X \n", varAddr);

    printf("mapbase = 0x%X \n", (unsigned int)mbase);

    printf("value   = 0x%X \n",*(unsignedint*)(mbase+ptr));

    close(fd);

    munmap(mbase,PAGE_SIZE);

    return0;

}

/dav/test # ./devkmem __kstrtab_snd_write_bypass
map failed Input/output error
varAddr = 0x0
mapbase = 0xFFFFFFFF
Segmentation fault

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