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

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@kernel.org>
Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Jeff Layton <jlayton@kernel.org>
cc: v9fs@lists.linux.dev
cc: linux_oss@crudebyte.com
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
 fs/netfs/buffered_write.c | 3 +++
 fs/netfs/direct_write.c   | 5 ++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

On Mon, 2024-01-29 at 09:49 +0000, 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@kernel.org>
> Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Dominique Martinet <asmadeus@codewreck.org>
> cc: Jeff Layton <jlayton@kernel.org>
> cc: v9fs@lists.linux.dev
> cc: linux_oss@crudebyte.com
> cc: netfs@lists.linux.dev
> cc: linux-fsdevel@vger.kernel.org
> ---
>  fs/netfs/buffered_write.c | 3 +++
>  fs/netfs/direct_write.c   | 5 ++++-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
> index a3059b3168fd..9a0d32e4b422 100644
> --- a/fs/netfs/buffered_write.c
> +++ b/fs/netfs/buffered_write.c
> @@ -477,6 +477,9 @@ ssize_t netfs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  
>  	_enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from), i_size_read(inode));
>  
> +	if (!iov_iter_count(from))
> +		return 0;
> +
>  	if ((iocb->ki_flags & IOCB_DIRECT) ||
>  	    test_bit(NETFS_ICTX_UNBUFFERED, &ictx->flags))
>  		return netfs_unbuffered_write_iter(iocb, from);
> diff --git a/fs/netfs/direct_write.c b/fs/netfs/direct_write.c
> index 60a40d293c87..bee047e20f5d 100644
> --- a/fs/netfs/direct_write.c
> +++ b/fs/netfs/direct_write.c
> @@ -139,6 +139,9 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  
>  	_enter("%llx,%zx,%llx", iocb->ki_pos, iov_iter_count(from), i_size_read(inode));
>  
> +	if (!iov_iter_count(from))
> +		return 0;
> +
>  	trace_netfs_write_iter(iocb, from);
>  	netfs_stat(&netfs_n_rh_dio_write);
>  
> @@ -146,7 +149,7 @@ ssize_t netfs_unbuffered_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  	if (ret < 0)
>  		return ret;
>  	ret = generic_write_checks(iocb, from);
> -	if (ret < 0)
> +	if (ret <= 0)
>  		goto out;
>  	ret = file_remove_privs(file);
>  	if (ret < 0)
> 

Reviewed-by: Jeff Layton <jlayton@kernel.org>

David Howells wrote on Mon, Jan 29, 2024 at 09:49:19AM +0000:
> 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@kernel.org>
> Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
> Signed-off-by: David Howells <dhowells@redhat.com>
> cc: Dominique Martinet <asmadeus@codewreck.org>

Thanks!

Tested-by: Dominique Martinet <asmadeus@codewreck.org>

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@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?

> [...]

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
Everything you wanna know about Linux kernel regression tracking:
https://linux-regtracking.leemhuis.info/about/#tldr
If I did something stupid, please tell me, as explained on that page.

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@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!