BUG: BTRFS and O_DIRECT could lead to wrong checksum and wrong data

[Goffredo Baroncelli <kreijack@libero.it>]

Hi all,

I discovered two bugs when O_DIRECT is used...

1) a corrupted file doesn't return -EIO when O_DIRECT is used

Normally BTRFS prevents to access the contents of a corrupted file; however I was able read the content of a corrupted file simply using O_DIRECT

# in a new btrfs filesystem, create a file
$ sudo mkfs.btrfs -f /dev/sdd5
$ mount /dev/sdd5 t
$ (while true; do echo -n "abcefg" ; done )| sudo dd of=t/abcd bs=$((16*1024)) iflag=fullblock count=1024

# corrupt the file
$ sudo filefrag -v t/abcd 
Filesystem type is: 9123683e
File size of t/abcd is 16777216 (4096 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    3475:      70656..     74131:   3476:            
   1:     3476..    4095:      74212..     74831:    620:      74132: last,eof
t/abcd: 2 extents found
$ sudo umount t
$ sudo ~/btrfs/btrfs-progs/btrfs-corrupt-block -l $((70656*4096)) -b 10 /dev/sdd5
mirror 1 logical 289406976 physical 289406976 device /dev/sdd5
corrupting 289406976 copy 1

# try to access the file; expected result: -EIO
$ sudo mount /dev/sdd5 t
$ dd if=t/abcd | hexdump -c | head
dd: error reading 't/abcd': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.000477413 s, 0.0 kB/s


# try to access the file using O_DIRECT; expected result: -EIO, instead the file is accessible
$ dd if=t/abcd iflag=direct bs=4096 | hexdump -c | head
0000000 001 001 001 001 001 001 001 001 001 001 001 001 001 001 001 001
*
0001000   f   g   a   b   c   e   f   g   a   b   c   e   f   g   a   b
0001010   c   e   f   g   a   b   c   e   f   g   a   b   c   e   f   g
0001020   a   b   c   e   f   g   a   b   c   e   f   g   a   b   c   e
0001030   f   g   a   b   c   e   f   g   a   b   c   e   f   g   a   b
0001040   c   e   f   g   a   b   c   e   f   g   a   b   c   e   f   g
0001050   a   b   c   e   f   g   a   b   c   e   f   g   a   b   c   e
0001060   f   g   a   b   c   e   f   g   a   b   c   e   f   g   a   b
0001070   c   e   f   g   a   b   c   e   f   g   a   b   c   e   f   g

(dmesg report the checksum mismatch)
[13265.085645] BTRFS warning (device sdd5): csum failed root 5 ino 257 off 0 csum 0x98f94189 expected csum 0x0ab6be80 mirror 1

Note the first 4k filled by 0x01 !!!!!

Conclusion: even if the file is corrupted and normally BTRFS prevent to access it, using O_DIRECT
a) no error is returned to the caller
b) instead of the page stored on the disk, it is returned a page filled with 0x01 (according also with the function __readpage_endio_check())


2) The second bug, is a more severe bug. If during a writing of a buffer with O_DIRECT, the buffer is updated at the same time by a second process, the checksum may be incorrect.

At the end of the email there is the code which shows the problem: two process share the same memory: the first write it to the disk, the second update the buffer continuously. A third process try to read the file, but it got time to time -EIO

If you ran my code in a btrfs filesystem you got a lot of 

ERROR: read thread; r = 8192, expected = 16384
ERROR: read thread; r = 8192, expected = 16384
ERROR: read thread; e = 5 - Input/output error
ERROR: read thread; e = 5 - Input/output error

The firsts lines are related to a shorter read (which may happens). The lasts lines are related to a checksum mismatch. The dmesg is filled by lines like
[...]
[14873.573547] BTRFS warning (device sdd5): csum failed root 5 ino 259 off 4096 csum 0x0683c6df expected csum 0x55eb85e6 mirror 1
[...]

This is definitely a bug. 

I think that using O_DIRECT and updating a page at the same time could happen in a VM. In BTRFS this  could lead to a wrong checksum. The problem is that if BTRFS detects a checksum error during a reading:
a) if O_DIRECT is not used in the read
	* -EIO is returned
Definitely BAD

b) if O_DIRECT is used in the read
	* it doesn't return the error to the caller
	* it returns a page filled by 0x01 instead of the data from the disk
Even worse than a)	

Note1: even using O_DIRECT with O_SYNC, the problem still persist.
Note2: the man page of open(2) is filled by a lot of notes about O_DIRECT, but also it stated that using O_DIRECT+fork()+mmap(... MAP_SHARED) is legally.
Note3: even "ZFS on linux" has its trouble with O_DIRECT: if fact ZFS doesn't support it; see https://github.com/zfsonlinux/zfs/issues/224

BR
G.Baroncelli

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#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <assert.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <unistd.h>

#define FILESIZE	(4096*4)

int fd;
char *buffer = NULL;

void read_thread(const char *nf) {
	
	void *data = mmap(NULL,  FILESIZE,
				PROT_READ|PROT_WRITE, 
				MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
	
	assert(data);
	fprintf(stderr, "read_thread:  data = %p\n", data);
	int rfd;
	rfd = open(nf, O_RDONLY);
	
	for(;;) {
		ssize_t r = pread(rfd, data, FILESIZE, 0);
		if (r < 0) {
			int e = errno;
			fprintf(stderr, "ERROR: read thread; e = %d - %s\n", 
			       e, strerror(e));

		} else if (r != FILESIZE) {
			fprintf(stderr, "ERROR: read thread; r = %ld, expected = %d\n", 
			       r, FILESIZE);
		}
	}
}

void write_thread(void) {

	for(;;) {
		ssize_t r = pwrite(fd, buffer, FILESIZE, 0);
		assert(r == FILESIZE);
	}
}

void update_thread(void) {

	for(;;) {
		int i;
		for (i = 0 ; i < FILESIZE ; i++)
			buffer[i] += i+10;
	}
}


int main(int argc, char **argv) {
	
	if (argc < 2) {
		fprintf(stderr, "usage: %s <fname>\n", argv[0]);
		exit(100);
	}
	
	
	buffer = mmap(NULL,  FILESIZE,
				PROT_READ|PROT_WRITE, 
				MAP_SHARED|MAP_ANONYMOUS, -1, 0);
	
	assert(buffer);
	fprintf(stderr, "main:  data = %p\n", buffer);
	
	fd = open(argv[1], O_RDWR|O_DIRECT|O_CREAT, 0660);
	assert(fd>=0);
	
	ssize_t r = pwrite(fd, buffer, FILESIZE, 0);
	assert(r == FILESIZE);
	
	pid_t child;
	
	child = fork();
	assert(child >= 0);
	if (child == 0)
		write_thread();
	fprintf(stderr, "write_thread pid = %d\n", child);
	
	child = fork();
	assert(child >= 0);
	if (child == 0)
		read_thread(argv[1]);
	fprintf(stderr, "read_thread pid = %d\n", child);
	
	child = fork();
	assert(child >= 0);
	if (child == 0)
		update_thread();
	fprintf(stderr, "update_thread pid = %d\n", child);
	
	for(;;)
		sleep(100*100*100);

	
	return 0;
}

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-- 
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D  17B2 0EDA 9B37 8B82 E0B5