mmap driver implementation

• mmap_kernel.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/fs.h>
#include <linux/debugfs.h>
#include <linux/slab.h>
#include <linux/version.h>
#include <linux/mm.h>  /* mmap related stuff */


/* Memory mapping: provides user programs with direct access to device memory
 * Mapped area must be multiple of PAGE_SIZE, and starting address aligned to
 * PAGE_SIZE
 *
 * syscall: mmap(caddr_t addr, size_t len, int ptro, int flags, int fd, off_t off)
 * file operation: int (*mmap)(struct file *f, struct vm_area_struct *vma)
 *
 * Driver needs to: build page tables for address range, and replace vma->vm_ops
 * Building page tables:
 *    - all at once: remap_page_range
 *    - one page at a time: nopage method. Finds correct page for address, and
 *      increments its reference cout. Must be implemented if driver supports
 *      mremap syscall
 *
 * fields in struct vm_area_struct:
 *     unsigned long vm_start, vm_end: virtual address range covered
 *                   by VMA
 *     struct file *vm_file: file associated with VMA
 *     struct vm_operations_struct *vm_ops: functions that kernel
 *                   will invoke to operate in VMA
 *     void *vm_private_data: used by driver to store its own information
 *
 * VMA operations:
 *     void (*open)(struct vm_area_struct *vma): invoked when a new reference
 *                   to the VMA is made, except when the VMA is first created,
 *                   when mmap is called
 *     struct page *(*nopage)(struct vm_area_struct *area,
 *                            unsigned long address, int write_access):
 *                   invoked by page fault handler when process tries to access
 *                   valid page in VMA, but not currently in memory
 */

#define DEV_NAME "mmap_example"
#define PRINTFUNC() printk(KERN_ALERT DEV_NAME ": %s called\n", __func__)
#define PRINT(a) printk(KERN_ALERT DEV_NAME ": %s: %s\n", __func__, a)

struct dentry*  file1;

struct mmap_info {
    char* data; /* the data */
    int reference;       /* how many times it is mmapped */
};



/* keep track of how many times it is mmapped */
void mmap_open(struct vm_area_struct* vma)
{
    struct mmap_info* info = (struct mmap_info*)vma->vm_private_data;
    info->reference++;
}

/* decrement reference cout */
void mmap_close(struct vm_area_struct* vma)
{
    struct mmap_info* info = (struct mmap_info*)vma->vm_private_data;
    info->reference--;
}

/* nopage is called the first time a memory area is accessed which is not in memory,
 * it does the actual mapping between kernel and user space memory
 */
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)
struct page* mmap_nopage(struct vm_area_struct* vma, unsigned long address,
                         int* type)
#else
int mmap_fault(struct vm_area_struct* vma, struct vm_fault* vmf)
#endif
{
    struct page* page;
    struct mmap_info* info;
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,6,0)
    unsigned long address = (unsigned long)vmf->virtual_address;
#endif
    PRINTFUNC();

    /* is the address valid? */
    if (address > vma->vm_end) {
        PRINT("invalid address");
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)
        return NOPAGE_SIGBUS;
#else
        return VM_FAULT_SIGBUS;
#endif
    }

    /* the data is in vma->vm_private_data */
    info = (struct mmap_info*)vma->vm_private_data;

    if (!info->data) {
        PRINT("no data");
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)
        return NOPAGE_OOM;
#else
        return VM_FAULT_SIGBUS;
#endif
    }

    /* get the page */
    page = virt_to_page(info->data);

    /* increment the reference count of this page */
    get_page(page);
    /* type is the page fault type */
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)

    if (type) {
        *type = VM_FAULT_MINOR;
    }

    return page;
#else
    vmf->page = page;
    return 0;
#endif
}

struct vm_operations_struct mmap_vm_ops = {
    .open =     mmap_open,
    .close =    mmap_close,
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,6,0)
    .nopage =   mmap_nopage,
#else
    .fault =    mmap_fault,
#endif
};

int my_mmap(struct file* filp, struct vm_area_struct* vma)
{
    PRINTFUNC();
    vma->vm_ops = &mmap_vm_ops;
    vma->vm_flags |= VM_RESERVED;
    /* assign the file private data to the vm private data */
    vma->vm_private_data = filp->private_data;
    mmap_open(vma);
    return 0;
}

int my_close(struct inode* inode, struct file* filp)
{
    struct mmap_info* info = filp->private_data;
    PRINTFUNC();
    free_page((unsigned long)info->data);
    kfree(info);
    filp->private_data = NULL;
    return 0;
}

int my_open(struct inode* inode, struct file* filp)
{
    struct mmap_info* info;
    PRINTFUNC();

    info = kmalloc(sizeof(struct mmap_info), GFP_KERNEL);
    /* obtain new memory */
    info->data = (char*)get_zeroed_page(GFP_KERNEL);
    /* writing something to it */
    memcpy(info->data, "hello from kernel this is file: ", 32);
    memcpy(info->data + 32, filp->f_dentry->d_name.name,
           strlen(filp->f_dentry->d_name.name));
    /* assign this info struct to the file */
    filp->private_data = info;
    return 0;
}

static const struct file_operations my_fops = {
    .open = my_open,
    .release = my_close,
    .mmap = my_mmap,
};

static int __init mmapexample_module_init(void)
{
    PRINTFUNC();
    file1 = debugfs_create_file(DEV_NAME, 0644, NULL, NULL, &my_fops);
    return 0;
}

static void __exit mmapexample_module_exit(void)
{
    PRINTFUNC();
    debugfs_remove(file1);

}

module_init(mmapexample_module_init);
module_exit(mmapexample_module_exit);
MODULE_AUTHOR("Jose Medina");
MODULE_LICENSE("GPL");

• Makefile

obj-m   := mmap_kernel.o
KDIR    := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd) 
  
all: 
	$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

• mmap_user.c

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <sys/mman.h>

#define PAGE_SIZE 4096

int main(int argc, char** argv)
{
    int configfd;
    configfd = open("/sys/kernel/debug/mmap_example", O_RDWR);

    if (configfd < 0) {
        perror("open");
        return -1;
    }

    char* address = NULL;
    address = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, configfd,
                   0);

    if (address == MAP_FAILED) {
        perror("mmap");
        return -1;
    }

    printf("initial message: %s\n", address);
    memcpy(address + 11 , "*user*", 6);
    printf("changed message: %s\n", address);
    close(configfd);
    return 0;
}

• Testing

root@debian:/home/user/Documents/mmap# make
make -C /lib/modules/3.2.0-4-686-pae/build SUBDIRS=/home/user/Documents/mmap  modules
make[1]: Entering directory `/usr/src/linux-headers-3.2.0-4-686-pae'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory `/usr/src/linux-headers-3.2.0-4-686-pae'
 
root@debian:/home/user/Documents/mmap# insmod mmap_kernel.ko
 
root@debian:/home/user/Documents/mmap# gcc mmap_user.c
 
root@debian:/home/user/Documents/mmap# mount -t debugfs nodev /sys/kernel/debug/
 
root@debian:/home/user/Documents/mmap# ./a.out 
initial message: hello from kernel this is file: mmap_example
changed message: hello from *user* this is file: mmap_example

• kernel log

Apr 14 13:29:03 debian kernel: [14012.052550] mmap_example: mmapexample_module_init called
Apr 14 13:35:29 debian kernel: [14396.556063] mmap_example: my_open called
Apr 14 13:35:29 debian kernel: [14396.556414] mmap_example: my_mmap called
Apr 14 13:35:29 debian kernel: [14396.556997] mmap_example: mmap_fault called
Apr 14 13:35:29 debian kernel: [14396.575186] mmap_example: my_close called