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

[Jeff Layton]

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@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/ZbQUU6QKmIftKsmo@FV7GG9FTHL/
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> cc: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> cc: Jeff Layton <jlayton@xxxxxxxxxx>
> cc: v9fs@xxxxxxxxxxxxxxx
> cc: linux_oss@xxxxxxxxxxxxx
> cc: netfs@xxxxxxxxxxxxxxx
> cc: linux-fsdevel@xxxxxxxxxxxxxxx
> ---
>  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@xxxxxxxxxx>