linux-kernel - Re: [GIT PULL] tcm_vhost: Initial merge of vhost level target fabric driver

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Date:	Sat, 18 Aug 2012 23:04:32 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	target-devel <target-devel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>,
	Stefan Hajnoczi <stefanha@...il.com>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Zhi Yong Wu <wuzhy@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	lf-virt <virtualization@...ts.linux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [GIT PULL] tcm_vhost: Initial merge of vhost level target fabric
 driver

Hi Nicholas,
I just noticed this problem in the interface:

+#include <linux/vhost.h>
+
+/*
+ * Used by QEMU userspace to ensure a consistent vhost-scsi ABI.
+ *
+ * ABI Rev 0: July 2012 version starting point for v3.6-rc merge
candidate +
+ *            RFC-v2 vhost-scsi userspace.  Add GET_ABI_VERSION ioctl
usage
+ */
+
+#define VHOST_SCSI_ABI_VERSION 0
+
+struct vhost_scsi_target {
+       int abi_version;
+       unsigned char vhost_wwpn[TRANSPORT_IQN_LEN];
+       unsigned short vhost_tpgt;
+};
+

Here TRANSPORT_IQN_LEN is 224, which is a multiple of 4.
Since vhost_tpgt is 2 bytes and abi_version is 4, the total size would
be 230.  But gcc needs struct size be aligned to first field size, which
is 4 bytes, so it pads the structure by extra 2 bytes to the total of
232.

This padding is very undesirable in an ABI:
- it can not be initialized easily
- it can not be checked easily
- it can leak information between kernel and userspace

Simplest solution is probably just to make the padding
explicit:

+struct vhost_scsi_target {
+       int abi_version;
+       unsigned char vhost_wwpn[TRANSPORT_IQN_LEN];
+       unsigned short vhost_tpgt;
+       unsigned short reserved;
+};
+

I think we should fix this buglet before it goes out to users.

-- 
MST
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