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: 153a9961b5 ("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>
Link: https://lore.kernel.org/r/20240129094924.1221977-3-dhowells@redhat.com
Tested-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
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>
Signed-off-by: Christian Brauner <brauner@kernel.org>

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);

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)