Kernel-state space address directly mapped to user-state space access

The "abstract" kernel-to-user-space address mapping in Linux allows the user-level application to directly access the kernel address, which is the Mmap method. The application can directly access the device's I/O storage or DMA buffers through memory mapping. A Memory map associates a segment of the user space with the device's memory, and the program reads or writes within the mapped address range, which is actually access to the device.

struct file_operations simple_fops = {      .owner =    THIS_MODULE,      .open =     simple_open,      .mmap =    simple_mmap,// mmap接口      .release =  simple_release,  };  

Allocate memory at initialization time:

buffer = Kmalloc (4096 , Gfp_kernel); PRINTK ( "mmap buffer =%p\n" , buffer); Buffer_area= (int  *) (((unsigned  long ) buffer + page_size-1 ) & Page_mask); for  (Virt_addr= (unsigned  long ) Buffer_area; virt_ addr< (unsigned  long ) Buffer_area+4096 ; Virt_addr+=page_size) {setpagereserved (Virt_to_page (VIRT_ADDR)); /* Configure the page to hold */} memset  (Buffer, ' C ' , 100 ); //initialized to ' C '  

The Simple_mmap function implements the Mmap file interface:

staticint simple_mmap(structstruct vm_area_struct *vma)  {      int ret;      ret = remap_pfn_range(vma,             vma->vm_start,             virt_to_phys((void*)((unsignedlong)kmalloc_area)) >> PAGE_SHIFT,             vma->vm_end-vma->vm_start,             PAGE_SHARED);      if0) {          return -EAGAIN;      }      return0;  }  

The test program reference code is as follows:

intMain (void) {intFd Char*addr=null; FD =Open("/dev/mmap", O_RDWR);if(FD <0) {perror ("Open");Exit(1); } addr = Mmap (NULL,4096, prot_read| Prot_write, map_shared, FD,0);if(addr = = map_failed) {Perror ("Mmap");Exit(1); }printf('%s\ n ', addr); memset (addr,' F ', -); addr[0]=' P ';printf('%s\ n ', addr); Munmap (addr,4096); Addr=null;Close(FD); FD =Open("/dev/mmap", O_RDWR);//Re-open for verificationif(FD <0) {perror ("Open");Exit(1); } addr = Mmap (NULL,4096, prot_read| Prot_write, map_shared, FD,0);if(addr = = map_failed) {Perror ("Mmap");Exit(1); }printf('%s\ n ', addr); Munmap (addr,4096);Close(FD);return(0); }

The results of the operation are as follows:

[[email protected]/home]#insmod demo.ko    mmap buffer = c3c82000 [[email protected]/home]#mknod /dev/mmap c 224 0  [[email protected]/home]#./read  CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC  pfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff  pfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff  

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Kernel-state space address directly mapped to user-state space access