c - 如何使用/dev/kmem？

Question

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我得到了以下程序。它在 /dev/kmem 和 /dev/mem 上运行。

我想我可以从代码中学到一些东西。但是当我在我的 Beagle Board 上运行它时，给出了以下结果:

case 1: ( if(1) )
    root@omap:/home/ubuntu/tom# ./kmem_mem /boot/System.map-3.0.4-x3
    found jiffies at (0xc0870080) c0870080
    /dev/kmem read buf = 319317
    jiffies=319317 (read from virtual memory)

    /dev/mem: the offset is 870080
    the page size = 4096
    mmap: Invalid argument



case 2: ( if(0) )
    root@omap:/home/ubuntu/tom# ./kmem_mem /boot/System.map-3.0.4-x3 
    found jiffies at (0xc0870080) c0870080
    /dev/kmem read buf = 333631
    jiffies=333631 (read from virtual memory)

    /dev/mem: the offset is 870080
    /dev/mem read failed: Bad address
    jiffies=0 (read from physical memory)

我使用了下面的命令，这样 mmap 就可以使用 NULL 作为它的第一个参数。

root@omap:/home/ubuntu/tom# echo 0 > /proc/sys/vm/mmap_min_addr
root@omap:/home/ubuntu/tom# cat /proc/sys/vm/mmap_min_addr
0

如您所见，read_kmem() 工作正常，但 read_mem() 不起作用，似乎传递给它的“偏移量”是错误的。但是内核地址 - PAGE_OFFSET(0xC0000000) = physical address，是不是错了？

我的问题是:(1) 为什么在案例 1 中出现“mmap: Invalid argument”？(2) 为什么mmap只映射PAGE_SIZE长度的空间？(3)read_mem有什么问题？

有人可以帮忙吗？谢谢!

/*
* getjiff.c
*
* this toolkit shows how to get jiffies value from user space:
* 1. find jiffies's address from kernel image.
* 2. access virtual address space to get jiffies value.
* 3. access physical address sapce to get jiffies value.
*
* demostrate following techniques:
* o get ELF object symbol address by calling nlist()
* o access virtual memory space from /dev/kmem
* o access virtual memory space from /dev/mem
*/

#include <stdio.h>
#include <stdlib.h>         //exit
#include <linux/a.out.h>    //nlist
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <memory.h>

#define LONG *(volatile unsigned long*)

/* read from virtual memory */
int read_kmem(off_t offset, void* buf, size_t count)
{
    int fd;
    int n;

    fd = open("/dev/kmem", O_RDONLY);
    if (fd < 0)
    {
        perror("open /dev/kmem failed");
        return -1;
    }

    lseek(fd, offset, SEEK_SET);
    n = read(fd, buf, count);
    if (n != count)
        perror("/dev/kmem read failed");
    else
        printf("/dev/kmem read buf = %ld\n", *(unsigned long *)buf);

    close(fd);
    return n;
}

/* read from physical memory */
int read_mem(off_t offset, void* buf, size_t count)
{
    int fd;
    int n;
    int page_size;
    void *map_base; 
    unsigned long value;

    printf("/dev/mem: the offset is %lx\n", offset);

    fd = open("/dev/mem", O_RDONLY);
    if (fd < 0)
    {
        perror("open /dev/mem failed");
        return -1;
    }

    if(1){
        page_size = getpagesize();
        printf("the page size = %d\n", page_size);
        map_base = mmap(0,page_size,PROT_READ,MAP_SHARED,fd,offset);
        if (map_base == MAP_FAILED){
            perror("mmap");
            exit(1);
        }
        value = LONG(map_base);
        printf("/dev/mem: the value is %ld\n", value);
        buf = (unsigned long *)map_base;
    }

    if(0){
        lseek(fd, offset, SEEK_SET);
        n = read(fd, buf, count);
        if (n != count)
            perror("/dev/mem read failed");
        else
            printf("/dev/mem read buf = %ld\n", *(unsigned long *)buf);
    }

    close(fd);
    return n;
}

int main(int argc, char **argv)
{
    FILE *fp;
    char addr_str[11]="0x";
    char var[51];
    unsigned long addr;
    unsigned long jiffies;
    char ch;
    int r;

    if (argc != 2) {
            fprintf(stderr,"usage: %s System.map\n",argv[0]);
            exit(-1);
    }

    if ((fp = fopen(argv[1],"r")) == NULL) {
            perror("fopen");
            exit(-1);
    }

    do {
            r = fscanf(fp,"%8s %c %50s\n",&addr_str[2],&ch,var);    // format of System.map
            if (strcmp(var,"jiffies")==0)
                    break;
    } while(r > 0);
    if (r < 0) {
            printf("could not find jiffies\n");
            exit(-1);
    }

    addr = strtoul(addr_str,NULL,16);                               //Convert string to unsigned long integer
    printf("found jiffies at (%s) %08lx\n",addr_str,addr);

    read_kmem(addr, &jiffies, sizeof(jiffies));
    printf("jiffies=%ld (read from virtual memory)\n\n", jiffies);

    jiffies = 0;                                                    //reinit for checking read_mem() below

    read_mem(addr-0xC0000000, &jiffies, sizeof(jiffies));
    printf("jiffies=%ld (read from physical memory)\n", jiffies);

    return 0;
} 

Answer 1

我已经为 dd 尝试了 combinations or offset 和 bs 并找到了这个解决方案:

在 PC 上，在构建目录中我找到了 jiffies 的位置。

grep -w jiffies System.map
c04660c0 D jiffies

在 PandaBoard 上:

在/proc/iomem中你可以看到:

80000000-9c7fffff : 系统内存
80008000-80435263 : 内核代码
80464000-804d0d97 : 内核数据
a0000000-bfefffff : 系统 RAM

RAM从物理80000000开始，Kernel数据从80464000开始。看起来像jiffies的地址。

然后从虚拟地址转换为物理地址:virt - 0xC000000 + 0x8000000。

dd if=/dev/mem skip=$((0x804660c)) bs=$((0x10)) count=1 2>/dev/null |十六进制转储
0000000 02b9 0002 0001 0000 0000 0000 0000 0000
0000010

多试几次，看看值是如何递增的。

<我>总结:/dev/mem使用phys地址，RAM从phys地址0x8000000开始