[CHECKER] ext3 bug in ftruncate() with O_SYNC?

[CHECKER] ext3 bug in ftruncate() with O_SYNC?

[Ben Pfaff]

Hi.  We're doing some checking on Linux file systems and found
what appears to be a bug in the Linux 2.6.11 implementation of
ext3: when ftruncate shrinks a file, using a file descriptor
opened with O_SYNC, the file size is not updated synchronously.
I've appended a test program that illustrates the problem.  After
this program runs, the file system shows a file with length 1031,
not 4 as would be expected if the ftruncate completed
synchronously.  (If I insert an fsync before closing the file,
the file length is correct.)

Does this look like a bug to you guys?

Thanks,

Ben.

----------------------------------------------------------------------

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <assert.h>

#define CHECK(ret) if(ret < 0) {perror(0); assert(0);}

int systemf(const char *fmt, ...)
{
	static char cmd[1024];

	va_list ap;
	va_start(ap, fmt);
	vsprintf(cmd, fmt, ap);
	va_end(ap);
	
	fprintf(stderr, "running cmd \"%s\"\n", cmd);
	return system(cmd);
}

main(int argc, char *argv[])
{
	int ret, fd;
	systemf("umount /dev/sbd0");
	systemf("sbin/mkfs.ext3 -F -j -b 1024 /dev/sbd0 2048");
	systemf("sbin/e2fsck.shared -y /dev/sbd0");
	systemf("mount -t ext3 /dev/sbd0 /mnt/sbd0 -o commit=65535");
	systemf("umount /dev/sbd0");
	systemf("mount -t ext3 /dev/sbd0 /mnt/sbd0 -o commit=65535");
	
	fd = open("/mnt/sbd0/0001", O_CREAT | O_RDWR | O_SYNC, 0700);
	CHECK(fd);
	ret = write(fd, "foobar", 6);
	CHECK(ret);
	ret = ftruncate(fd, 1031);
	CHECK(ret);
	ret = pwrite(fd, "bazzle", 6, 1031 - 6);
	CHECK(ret);
	ret = ftruncate(fd, 4);
	CHECK(ret);
        ret = close(fd);
        CHECK(ret);

#if 0
	{
		#include "../sbd/sbd.h"
		int sbd_fd = open("/dev/sbd0", O_RDONLY);
		ret = ioctl(sbd_fd, SBD_COPY_DISK, 1);
		CHECK(ret);
		close(sbd_fd);
	}
#else
	systemf("reboot -f -n");
#endif
	return 0;
}



-- 
Ben Pfaff 
email: blp@cs.stanford.edu
web: http://benpfaff.org

Re: [CHECKER] ext3 bug in ftruncate() with O_SYNC?

[Andrew Morton]

Ben Pfaff <blp@cs.stanford.edu> wrote:
>
> Hi.  We're doing some checking on Linux file systems and found
> what appears to be a bug in the Linux 2.6.11 implementation of
> ext3: when ftruncate shrinks a file, using a file descriptor
> opened with O_SYNC, the file size is not updated synchronously.
> I've appended a test program that illustrates the problem.  After
> this program runs, the file system shows a file with length 1031,
> not 4 as would be expected if the ftruncate completed
> synchronously.  (If I insert an fsync before closing the file,
> the file length is correct.)
> 
> Does this look like a bug to you guys?
> 

The spec says "Write I/O operations on the file descriptor shall complete
as defined by synchronized I/O file integrity completion".

Is ftruncate a "write I/O operation"?  No.  Should ftruncate() honour
O_SYNC?  I'd say so, yes.  After all, an extending ftruncate is a
sort-of-write.

Unfortunately Linux doesn't pass the file* down to the
filesytem's ->truncate() handler, so the fs doesn't know that the
ftruncated fd was opened O_SYNC.  I don't think _any_ Linux filesystems get
this right.


> Ben.
> 
> ----------------------------------------------------------------------
> 
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <stdarg.h>
> #include <assert.h>
> 
> #define CHECK(ret) if(ret < 0) {perror(0); assert(0);}
> 
> int systemf(const char *fmt, ...)
> {
> 	static char cmd[1024];
> 
> 	va_list ap;
> 	va_start(ap, fmt);
> 	vsprintf(cmd, fmt, ap);
> 	va_end(ap);
> 	
> 	fprintf(stderr, "running cmd \"%s\"\n", cmd);
> 	return system(cmd);
> }
> 
> main(int argc, char *argv[])
> {
> 	int ret, fd;
> 	systemf("umount /dev/sbd0");
> 	systemf("sbin/mkfs.ext3 -F -j -b 1024 /dev/sbd0 2048");
> 	systemf("sbin/e2fsck.shared -y /dev/sbd0");
> 	systemf("mount -t ext3 /dev/sbd0 /mnt/sbd0 -o commit=65535");
> 	systemf("umount /dev/sbd0");
> 	systemf("mount -t ext3 /dev/sbd0 /mnt/sbd0 -o commit=65535");
> 	
> 	fd = open("/mnt/sbd0/0001", O_CREAT | O_RDWR | O_SYNC, 0700);
> 	CHECK(fd);
> 	ret = write(fd, "foobar", 6);
> 	CHECK(ret);
> 	ret = ftruncate(fd, 1031);
> 	CHECK(ret);
> 	ret = pwrite(fd, "bazzle", 6, 1031 - 6);
> 	CHECK(ret);
> 	ret = ftruncate(fd, 4);
> 	CHECK(ret);
>         ret = close(fd);
>         CHECK(ret);
> 
> #if 0
> 	{
> 		#include "../sbd/sbd.h"
> 		int sbd_fd = open("/dev/sbd0", O_RDONLY);
> 		ret = ioctl(sbd_fd, SBD_COPY_DISK, 1);
> 		CHECK(ret);
> 		close(sbd_fd);
> 	}
> #else
> 	systemf("reboot -f -n");
> #endif
> 	return 0;
> }
> 
> 
> 
> -- 
> Ben Pfaff 
> email: blp@cs.stanford.edu
> web: http://benpfaff.org
> -
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Re: [CHECKER] ext3 bug in ftruncate() with O_SYNC?

[Stephen C. Tweedie]

Hi,

On Tue, 2005-03-22 at 03:51, Andrew Morton wrote:

> The spec says "Write I/O operations on the file descriptor shall complete
> as defined by synchronized I/O file integrity completion".
> 
> Is ftruncate a "write I/O operation"?  No.

SUS seems to be pretty clear on this.  The syscall descriptions for
write(2) and pwrite(2) explicitly describe O_SYNC as requiring
synchronized I/O file integrity completion.  ftruncate() has no such
requirement.

It would certainly be a reasonable thing to do, but I don't think it
strictly counts as a bug that we're not honouring O_SYNC here.

--Stephen