[PATCH 2/2] netfs: Fix missing zero-length check in unbuffered write

[Christian Brauner]

On Mon, Feb 19, 2024 at 09:38:33AM +0100, Linux regression tracking (Thorsten Leemhuis) wrote:
> On 29.01.24 10:49, David Howells wrote:
> > Fix netfs_unbuffered_write_iter() to return immediately if
> > generic_write_checks() returns 0, indicating there's nothing to write.
> > Note that netfs_file_write_iter() already does this.
> > 
> > Also, whilst we're at it, put in checks for the size being zero before we
> > even take the locks.  Note that generic_write_checks() can still reduce the
> > size to zero, so we still need that check.
> > 
> > Without this, a warning similar to the following is logged to dmesg:
> > 
> > 	netfs: Zero-sized write [R=1b6da]
> > 
> > and the syscall fails with EIO, e.g.:
> > 
> > 	/sbin/ldconfig.real: Writing of cache extension data failed: Input/output error
> > 
> > This can be reproduced on 9p by:
> > 
> > 	xfs_io -f -c 'pwrite 0 0' /xfstest.test/foo
> > 
> > Fixes: 153a9961b551 ("netfs: Implement unbuffered/DIO write support")
> > Reported-by: Eric Van Hensbergen <ericvh at kernel.org>
> > Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
> 
> David, thx for fixing Eric's regression, which I'm tracking.
> 
> Christian, just wondering: that patch afaics is sitting in vfs.netfs for
> about three weeks now -- is that intentional or did it maybe fell
> through the cracks somehow?

I've moved it to vfs.fixes now and will send later this week. Thanks for
the reminder!