I've been working on a small program to find the first valid block of a
swapfile. If I understand it correctly we need the first contiguos cluster
of blocks (not counting the header) which is at least PAGE_SIZE big and
the starting block must be page aligned.
The block number must then be converted in an offset from the start of
the device and the number must be expressed in multiple of PAGE_SIZE.
This is what I've come up with ;)
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/fs.h>
#include <errno.h>
#define SWAP_SIG "SWAPSPACE2"
int main(int argc, char **argv) {
int block, last_block, first_block;
int size, blk_size;
int fd;
int i;
struct stat stat;
unsigned char buf[10];
int err = 0;
int const page_size = getpagesize();
if (argc < 2)
return EINVAL;
fd = open(argv[1], O_RDONLY);
if (fd < 0) {
perror("open()");
return ENOENT;
}
/* Check swap signature */
lseek(fd, page_size - 10, SEEK_SET);
read(fd, buf, 10);
if (memcmp(buf, SWAP_SIG, 10)) {
fprintf(stderr, "Swap signature not found\n");
err = EINVAL;
goto out;
}
fstat(fd, &stat);
ioctl(fd, FIGETBSZ, &blk_size);
/* Skip swap header (1 page) */
i = page_size / blk_size;
size = 0;
last_block = 0;
first_block = 0;
for (; i < stat.st_blocks; i++) {
block = i;
if (ioctl(fd, FIBMAP, &block)) {
perror("ioctl()");
err = EIO;
goto out;
}
if (block != last_block + 1) {
/* Swap space must be page aligned */
if (block * blk_size % page_size)
continue;
first_block = block;
size = 0;
}
size += blk_size;
last_block = block;
if (size >= page_size)
break;
}
if (size < page_size) {
fprintf(stderr, "Invalid swapfile\n");
err = EINVAL;
} else
printf("%d\n", first_block * blk_size / page_size);
out:
close(fd);
return err;
}
How does it look?
Luca
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