linux-kernel-module-cheat/kernel_module/user/mmap.c

#include <assert.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h> /* sysconf */

enum { BUFFER_SIZE = 4 };

int main(int argc, char **argv)
{
	int fd;
	long page_size;
	char *address1, *address2;
	char buf[BUFFER_SIZE];

	if (argc < 2) {
		printf("Usage: %s <mmap_file>\n", argv[0]);
		return EXIT_FAILURE;
	}
	page_size = sysconf(_SC_PAGE_SIZE);
	printf("open pathname = %s\n", argv[1]);
	fd = open(argv[1], O_RDWR | O_SYNC);
	if (fd < 0) {
		perror("open");
		assert(0);
	}
	printf("fd = %d\n", fd);

    /* mmap twice for double fun. */
	puts("mmap 1");
	address1 = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (address1 == MAP_FAILED) {
		perror("mmap");
		assert(0);
	}
	puts("mmap 2");
	address2 = mmap(NULL, page_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
	if (address2 == MAP_FAILED) {
		perror("mmap");
		return EXIT_FAILURE;
	}
	assert(address1 != address2);

    /* Read and modify memory. */
	puts("access 1");
	assert(!strcmp(address1, "asdf"));
	/* vm_fault */
	puts("access 2");
	assert(!strcmp(address2, "asdf"));
	/* vm_fault */
	strcpy(address1, "qwer");
	/* Also modified. So both virtual addresses point to the same physical address. */
	assert(!strcmp(address2, "qwer"));

    /* Check that modifications made from userland are also visible from the kernel. */
	read(fd, buf, BUFFER_SIZE);
	assert(!memcmp(buf, "qwer", BUFFER_SIZE));

	/* Modify the data from the kernel, and check that the change is visible from userland. */
	write(fd, "zxcv", 4);
	assert(!strcmp(address1, "zxcv"));
	assert(!strcmp(address2, "zxcv"));

    /* Cleanup. */
    puts("munmap 1");
	if (munmap(address1, page_size)) {
		perror("munmap");
		assert(0);
	}
    puts("munmap 2");
	if (munmap(address2, page_size)) {
		perror("munmap");
		assert(0);
	}
    puts("close");
	close(fd);
	return EXIT_SUCCESS;
}