corrupt_gdt_free_blocks.c revision 2633ce5ca5024d5565c9068c29ec39a8c3ed10e9 
1#include <sys/types.h>
2#include <sys/stat.h>
3#include <fcntl.h>
4#include <stdio.h>
5#include <stdlib.h>
6
7#include "ext4.h"
8#include "ext4_utils.h"
9
10#define SB_OFFSET 1024
11
12int main(int argc, char *argv[])
13{
14    char me[] = "corrupt_gdt_free_blocks";
15    int fd;
16    int block_size;
17    int num_bgs;
18    int i;
19    struct ext4_super_block sb;
20    struct ext2_group_desc gd;
21
22    if (argc != 2) {
23        fprintf(stderr, "%s: Usage: %s <ext4_block_device>\n", me, me);
24        exit(1);
25    }
26
27    fd = open(argv[1], O_RDWR);
28
29    if (fd < 0) {
30        fprintf(stderr, "%s: Cannot open block device %s\n", me, argv[1]);
31        exit(1);
32    }
33
34    if (lseek(fd, SB_OFFSET, SEEK_SET) == -1) {
35        fprintf(stderr, "%s: Cannot lseek to superblock to read\n", me);
36        exit(1);
37    }
38
39    if (read(fd, &sb, sizeof(sb)) != sizeof(sb)) {
40        fprintf(stderr, "%s: Cannot read superblock\n", me);
41        exit(1);
42    }
43
44    if (sb.s_magic != 0xEF53) {
45        fprintf(stderr, "%s: invalid superblock magic\n", me);
46        exit(1);
47    }
48
49    /* Make sure the block size is 2K or 4K */
50    if ((sb.s_log_block_size != 1) && (sb.s_log_block_size != 2)) {
51        fprintf(stderr, "%s: block size not 2K or 4K\n", me);
52        exit(1);
53    }
54
55    block_size = 1 << (10 + sb.s_log_block_size);
56    num_bgs = DIV_ROUND_UP(sb.s_blocks_count_lo, sb.s_blocks_per_group);
57
58    if (sb.s_desc_size != sizeof(struct ext2_group_desc)) {
59        fprintf(stderr, "%s: Can't handle block group descriptor size of %d\n",
60                me, sb.s_desc_size);
61        exit(1);
62    }
63
64    /* read first block group descriptor, decrement free block count, and
65     * write it back out
66     */
67    if (lseek(fd, block_size, SEEK_SET) == -1) {
68        fprintf(stderr, "%s: Cannot lseek to block group descriptor table to read\n", me);
69        exit(1);
70    }
71
72    /* Read in block group descriptors till we read one that has at least one free block */
73
74    for (i=0; i < num_bgs; i++) {
75        if (read(fd, &gd, sizeof(gd)) != sizeof(gd)) {
76            fprintf(stderr, "%s: Cannot read group descriptor %d\n", me, i);
77            exit(1);
78        }
79        if (gd.bg_free_blocks_count) {
80            break;
81        }
82    }
83
84    gd.bg_free_blocks_count--;
85
86    if (lseek(fd, -sizeof(gd), SEEK_CUR) == -1) {
87        fprintf(stderr, "%s: Cannot lseek to block group descriptor table to write\n", me);
88        exit(1);
89    }
90
91    if (write(fd, &gd, sizeof(gd)) != sizeof(gd)) {
92        fprintf(stderr, "%s: Cannot write modified group descriptor\n", me);
93        exit(1);
94    }
95
96    close(fd);
97
98    return 0;
99}
100
101