fsstress-induced corruption reproduced

Theodore Ts'o
2009-12-31T15:50:32Z

One of the things which has been annoying me for a while now is a
hard-to-reproduce xfsqa failure in test #13 (fsstress), which causes the
a test failure because the file system found to be inconsistent:

Inode NNN, i_blocks is X, should be Y.

I finally reproduced it; the problem happens when we fallocate() a
region of the file which we had recently written, and which is still in
the page cache marked as delayed allocation blocks. When we finally
write those blocks out, since they are marked BH_Delay,
ext4_get_blocks() calls ext4_da_update_reserve_space(), which ends up
bumping i_blocks a second time and charging the blocks against the
user's quota a second time. Oops.

Fortunately the fsck problem is one that will be fixed with a preen (and
if quota is enabled, a quotacheck), so it's not super serious, but we
should fix it when we have a chance. If anyone has time to look at it,
please let me know. Otherwise, I'll put it on my todo list. I don't
consider seriously urgent since the case is highly unlikely to occur in
real life, and it doesn't have any security implications; the worst an
attacker could do is end up charging excesss quota to herself.

I've included a simple reproduction case below; if you run this program,
it will create a file "test-file" in the current working directory which
will appear to be 32k, even though it is really only 16k long, and if
you then unmount the test file system and run e2fsck -p on it, you will get
the error message:

Inode XXX, i_blocks is 64, should be 32. FIXED.

- Ted

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <fcntl.h>
#include <fcntl.h>

#define BUFSIZE 1024

int main(int argc, char **argv)
{
int i, fd, ret;
char buf[BUFSIZE];

fd = open("test-file", O_RDWR|O_CREAT|O_TRUNC, 0644);
if (fd < 0) {
perror("open");
exit(1);
}
memset(&buf, 0, BUFSIZE);
for (i=0; i < 16; i++) {
ret = write(fd, &buf, BUFSIZE);
if (ret < 0) {
perror("write");
exit(1);
}
if (ret != BUFSIZE) {
fprintf(stderr, "Write return expected %d, got %d\n",
BUFSIZE, ret);
exit(1);
}
}
ret = fallocate(fd, 0, 0, 16384);
if (ret < 0) {
perror("fallocate");
exit(1);
}
ret = fsync(fd);
if (ret < 0) {
perror("fsync");
exit(1);
}
ret = close(fd);
if (ret < 0) {
perror("close");
exit(1);
}
exit(0);
}