linuxmemoryarmmemory-addressmemory-mapping

How to access physical addresses from user space in Linux?


On a ARM based system running Linux, I have a device that's memory mapped to a physical address. From a user space program where all addresses are virtual, how can I read content from this address?


Solution

  • You can map a device file to a user process memory using mmap(2) system call. Usually, device files are mappings of physical memory to the file system. Otherwise, you have to write a kernel module which creates such a file or provides a way to map the needed memory to a user process.

    Another way is remapping parts of /dev/mem to a user memory.

    Edit: Example of mmaping /dev/mem (this program must have access to /dev/mem, e.g. have root rights):

    #include <stdio.h>
    #include <stdlib.h>
    #include <fcntl.h>
    #include <sys/mman.h>
    #include <unistd.h>
    
    int main(int argc, char *argv[]) {
        if (argc < 3) {
            printf("Usage: %s <phys_addr> <offset>\n", argv[0]);
            return 0;
        }
    
        off_t offset = strtoul(argv[1], NULL, 0);
        size_t len = strtoul(argv[2], NULL, 0);
    
        // Truncate offset to a multiple of the page size, or mmap will fail.
        size_t pagesize = sysconf(_SC_PAGE_SIZE);
        off_t page_base = (offset / pagesize) * pagesize;
        off_t page_offset = offset - page_base;
    
        int fd = open("/dev/mem", O_SYNC);
        unsigned char *mem = mmap(NULL, page_offset + len, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, page_base);
        if (mem == MAP_FAILED) {
            perror("Can't map memory");
            return -1;
        }
    
        size_t i;
        for (i = 0; i < len; ++i)
            printf("%02x ", (int)mem[page_offset + i]);
    
        return 0;
    }