readme

readme.c

#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/fs.h>
#include <linux/fiemap.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

#define SECTOR_OFFSET 4

int main(int argc, char **argv)
{
    struct fiemap *fm;
    int file;
    int disk;
    char obuf[512];
    char ibuf[1024];
    char expected[512];
    unsigned char *ibufp = (unsigned char *) ((uint64_t) (ibuf + 512) & ~511ULL);

    if (argc != 3) {
        fprintf(stderr, "Usage: %s file block-device\n", argv[0]);
        return 1;
    }

    fm = calloc(sizeof(struct fiemap) + sizeof(struct fiemap_extent), 1);
    fm->fm_start = SECTOR_OFFSET;
    fm->fm_length = 1;
    fm->fm_extent_count = 1;

    memset(obuf, 0xff, sizeof(obuf));
    memset(expected, 0, sizeof(expected));
    memset(expected, 0xff, SECTOR_OFFSET);

    file = open(argv[1], O_CREAT | O_TRUNC | O_WRONLY, 0600);
    if (file == -1) {
        perror("Opening file");
        return 1;
    }
    disk = open(argv[2], O_RDONLY | O_DIRECT);
    if (disk == -1) {
        perror("Opening disk");
        return 1;
    }
    write(file, obuf, SECTOR_OFFSET);
    while (1) {
        uint64_t offset;

        if (write(file, obuf, 512) != 512) {
            fprintf(stderr, "Short write\n");
            return 1;
        }
        fsync(file);

        fm->fm_start += 512;
        fm->fm_flags = 0;
        if (ioctl(file, FS_IOC_FIEMAP, fm)) {
            perror("ioctl failed");
            return 1;
        }
        offset = fm->fm_extents[0].fe_physical + fm->fm_start -
                fm->fm_extents[0].fe_logical;
        //printf("%llu %llu\n", fm->fm_start, offset);

        if (pread(disk, ibufp, 512, offset & ~511ULL) != 512) {
            fprintf(stderr, "Short read\n");
            return 1;
        }
        if (memcmp(ibufp, expected, sizeof(expected))) {
            int i;

            printf("fail %llu %llu\n", fm->fm_start, offset);
            for (i = 0; i < 512; i++) {
                printf("%02x ", ibufp[i]);
                if (i % 16 == 15) {
                    printf("\n");
                }
            }
            return 1;
        }
    }
}