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
main(int argc,char * argv[])12 int 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