Devmem工具使用详解

夏天Aileft

已于 2024-10-24 09:52:20 修改

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分类专栏： Linux 文章标签： 1024程序员节

于 2024-10-24 09:50:27 首次发布

本文链接：https://blog.youkuaiyun.com/weixin_41238626/article/details/143200601

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✨前言：

devmem2 是一个非常简单而有效的工具，它允许用户直接读取和写入物理内存地址，这通常用于内存映射 I/O（MMIO）。在嵌入式系统和硬件开发中，它常常被用作调试和系统配置的手段。

✨逻辑流程

🌟参数解析：

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调用程序时，解析命令行参数，至少要提供一个目标地址。
选择访问类型（读、写）和写入的数据（如果指定的话）。

🌟打开 /dev/mem：

以读写权限打开 /dev/mem。这是一个在 Linux 系统中用于访问物理内存的特殊设备文件。
映射物理内存页：
使用 mmap() 将物理内存页映射到进程的虚拟地址空间。映射的大小为 4096 字节（一个页面）。
计算目标地址与页大小的掩码，并确保目标地址在映射的页内。

🌟访问内存：

根据访问类型（字节、半字、字、长字）选择相应的数据大小进行内存访问：
l:读取的是8byte
w:读取的是4byte
h:读取的是2byte
b:读取的是1byte
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读取指定地址的数据
打印读取到的数据

🌟写入数据：

如果提供了数据，程序将尝试将其写入指定的物理内存地址。
再次进行数据读取以确认写入操作是否成功，并打印出相关信息。
解除映射并关闭文件描述符：
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通过 munmap() 解除之前的内存映射。
关闭 /dev/mem 的文件描述符以释放资源。

🌟源码：

/*
 * devmem2.c: Simple program to read/write from/to any location in memory.
 *
 *  Copyright (C) 2000, Jan-Derk Bakker (jdb@lartmaker.nl)
 *
 *
 * This software has been developed for the LART computing board
 * (http://www.lart.tudelft.nl/). The development has been sponsored by
 * the Mobile MultiMedia Communications (http://www.mmc.tudelft.nl/)
 * and Ubiquitous Communications (http://www.ubicom.tudelft.nl/)
 * projects.
 *
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <fcntl.h>
#include <ctype.h>
#include <termios.h>
#include <sys/types.h>
#include <sys/mman.h>

#define FATAL do { fprintf(stderr, "Error at line %d, file %s (%d) [%s]\n", \
  __LINE__, __FILE__, errno, strerror(errno)); exit(1); } while(0)

#define MAP_SIZE 4096UL
#define MAP_MASK (MAP_SIZE - 1)

static inline void *fixup_addr(void *addr, size_t size);

int main(int argc, char **argv) {
    int fd;
    void *map_base, *virt_addr;
    unsigned long read_result, write_val;
    off_t target;
    int access_type = 'w';
    char fmt_str[128];
    size_t data_size;

    if(argc < 2) {
        fprintf(stderr, "\nUsage:\t%s { address } [ type [ data ] ]\n"
            "\taddress : memory address to act upon\n"
            "\ttype    : access operation type : [b]yte, [h]alfword, [w]ord, [l]ong\n"
            "\tdata    : data to be written\n\n",
            argv[0]);
        exit(1);
    }
    target = strtoul(argv[1], 0, 0);

    if(argc > 2)
        access_type = tolower(argv[2][0]);


    if((fd = open("/dev/mem", O_RDWR | O_SYNC)) == -1) FATAL;
    printf("/dev/mem opened.\n");
    fflush(stdout);

    /* Map one page */
    map_base = mmap(0, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, target & ~MAP_MASK);
    if(map_base == (void *) -1) FATAL;
    printf("Memory mapped at address %p.\n", map_base);
    fflush(stdout);

    virt_addr = map_base + (target & MAP_MASK);
    switch(access_type) {
        case 'b':
            data_size = sizeof(unsigned char);
            virt_addr = fixup_addr(virt_addr, data_size);
            read_result = *((unsigned char *) virt_addr);
            break;
        case 'h':
            data_size = sizeof(unsigned short);
            virt_addr = fixup_addr(virt_addr, data_size);
            read_result = *((unsigned short *) virt_addr);
            break;
        case 'w':
            data_size = sizeof(uint32_t);
            virt_addr = fixup_addr(virt_addr, data_size);
            read_result = *((uint32_t *) virt_addr);
            break;
        case 'l':
            data_size = sizeof(uint64_t);
            virt_addr = fixup_addr(virt_addr, data_size);
            read_result = *((uint64_t *) virt_addr);
            break;
        default:
            fprintf(stderr, "Illegal data type '%c'.\n", access_type);
            exit(2);
    }
    sprintf(fmt_str, "Read at address  0x%%08lX (%%p): 0x%%0%dlX\n", 2*data_size);
    printf(fmt_str, (unsigned long)target, virt_addr, read_result);
    fflush(stdout);

    if(argc > 3) {
        write_val = strtoul(argv[3], 0, 0);
        switch(access_type) {
            case 'b':
                virt_addr = fixup_addr(virt_addr, sizeof(unsigned char));
                *((unsigned char *) virt_addr) = write_val;
                read_result = *((unsigned char *) virt_addr);
                break;
            case 'h':
                virt_addr = fixup_addr(virt_addr, sizeof(unsigned short));
                *((unsigned short *) virt_addr) = write_val;
                read_result = *((unsigned short *) virt_addr);
                break;
            case 'w':
                virt_addr = fixup_addr(virt_addr, sizeof(uint32_t));
                *((uint32_t *) virt_addr) = write_val;
                read_result = *((uint32_t *) virt_addr);
                break;
            case 'l':
                virt_addr = fixup_addr(virt_addr, sizeof(uint64_t));
                *((uint64_t *) virt_addr) = write_val;
                read_result = *((uint64_t *) virt_addr);
                break;
        }
        sprintf(fmt_str, "Write at address 0x%%08lX (%%p): 0x%%0%dlX, "
                "readback 0x%%0%dlX\n", 2*data_size, 2*data_size);
        printf(fmt_str, (unsigned long)target, virt_addr,
               write_val, read_result);
        fflush(stdout);
    }

    if(munmap(map_base, MAP_SIZE) == -1) FATAL;
    close(fd);
    return 0;
}

static inline void *fixup_addr(void *addr, size_t size)
{
#ifdef FORCE_STRICT_ALIGNMENT
       unsigned long aligned_addr = (unsigned long)addr;
       aligned_addr &= ~(size - 1);
       addr = (void *)aligned_addr;
#endif
       return addr;
}