mirror of
https://github.com/cirosantilli/linux-kernel-module-cheat.git
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119 lines
2.9 KiB
C
119 lines
2.9 KiB
C
/* https://cirosantilli.com/linux-kernel-module-cheat#mmap
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*
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* Example of mmap on files.
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*
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* Create a file, mmap to it, write to maped memory, close.
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*
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* Then read the file and confirm it was written to.
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*
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* Implemented in Linux by the mmap syscall.
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*/
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#define _XOPEN_SOURCE 700
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#include <assert.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <sys/mman.h>
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#include <sys/stat.h>
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#include <sys/types.h>
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#include <unistd.h>
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#include <lkmc.h>
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int main(void) {
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char *filepath = LKMC_TMP_FILE;
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enum Constexpr { NUMINTS = 4 };
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size_t filesize = NUMINTS * sizeof(int);
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int i;
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int fd;
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int result;
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/* mmapped array of int's */
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int *map;
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/* Write to file with mmap. */
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{
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/* `O_WRONLY` is not sufficient when mmaping, need `O_RDWR`.*/
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fd = open(filepath, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
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if (fd == -1) {
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perror("open");
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exit(EXIT_FAILURE);
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}
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/* Set fd position and write to it to strech the file. */
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result = lseek(fd, filesize - 1, SEEK_SET);
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if (result == -1) {
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close(fd);
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perror("lseek");
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exit(EXIT_FAILURE);
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}
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/* Write something to the file to actually strech it. */
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result = write(fd, "", 1);
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if (result != 1) {
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close(fd);
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perror("write");
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exit(EXIT_FAILURE);
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}
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/* Do the actual mapping call. */
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map = mmap(NULL, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
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if (map == MAP_FAILED) {
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close(fd);
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perror("mmap");
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exit(EXIT_FAILURE);
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}
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/* Write int's to the file as if it were memory because MAP_SHARED was used. */
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for (i = 0; i < NUMINTS; ++i) {
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map[i] = i;
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}
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/* Free mmapped memory. */
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if (munmap(map, filesize) == -1) {
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perror("munmap");
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exit(EXIT_FAILURE);
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}
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/* Un-mmaping doesn't close the file, so we still need to do that. */
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if (close(fd) == -1) {
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perror("close");
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exit(EXIT_FAILURE);
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}
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}
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/* Read result back in. */
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{
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fd = open(filepath, O_RDONLY, 0);
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if (fd == -1) {
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perror("open");
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exit(EXIT_FAILURE);
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}
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map = mmap(0, filesize, PROT_READ, MAP_SHARED, fd, 0);
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if (map == MAP_FAILED) {
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close(fd);
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perror("mmap");
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exit(EXIT_FAILURE);
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}
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assert(map[1] == 1);
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/* Segmentation fault because no `PROT_WRITE`: */
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/*map[1] = 2;*/
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if (munmap(map, filesize) == -1) {
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perror("munmap");
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exit(EXIT_FAILURE);
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}
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if (close(fd) == -1) {
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perror("close");
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exit(EXIT_FAILURE);
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}
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}
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return EXIT_SUCCESS;
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}
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