Re: Data Corruption bug with Samba's vfs_iouring and Linux 5.6.7/5.7rc3

[Jeremy Allison <jra@samba.org>]

On Thu, May 07, 2020 at 10:50:40AM -0600, Jens Axboe wrote:
> On 5/7/20 10:48 AM, Jeremy Allison wrote:
> > On Thu, May 07, 2020 at 10:43:17AM -0600, Jens Axboe wrote:
> >>
> >> Just like for regular system calls, applications must be able to deal
> >> with short IO.
> > 
> > Thanks, that's a helpful definitive reply. Of course, the SMB3
> > protocol is designed to deal with short IO replies as well, and
> > the Samba and linux kernel clients are well-enough written that
> > they do so. MacOS and Windows however..
> 
> I'm honestly surprised that such broken clients exists! Even being
> a somewhat old timer cynic...
> 
> > Unfortunately they're the most popular clients on the planet,
> > so we'll probably have to fix Samba to never return short IOs.
> 
> That does sound like the best way forward, short IOs is possible
> with regular system calls as well, but will definitely be a lot
> more frequent with io_uring depending on the access patterns,
> page cache, number of threads, and so on.

OK, I just want to be *REALLY CLEAR* what you're telling me
(I've already written the pread/pwrite wrappers for Samba
that deal with short IO but want to ensure I understand
fully before making any changes to Samba).

You're saying that on a bog-standard ext4 disk file:

ret = pread(fd, buf, count, offset);

can return *less* than count bytes if there's no IO
error and the file size is greater than offset+count
and no one else is in the middle of a truncate etc. ?

And:

ret = pwrite(fd, buf, count, offset);

can return less* than count bytes if there's no IO
error and there's ample space on disk ?

I have to say I've *never* seen that happen, and
Samba is widely enough used that IO corruption from
short reads/writes from MacOSX and Windows clients
would have been widely reported by now.

Look at how quickly someone spotted disk corruption
because of the change in userspace-visible behavior
of the io_uring interface. We only shipped that code
03 March 2020 and someone *already* found it.

Jeremy.