[PATCH v2] vhost: fix external message handlers

[David Marchand]

Following a rework, external message handlers were receiving a pointer
to a vhost_user message (as stated in the API), but lost the ability to
interact with fds attached to the message.
Restore the original layout and put a build check and reminders.

Bugzilla ID: 953
Fixes: 5e0099dc709e ("vhost: remove payload size limitation")

Reported-by: Fan Zhang <roy.fan.zhang at intel.com>
Signed-off-by: David Marchand <david.marchand at redhat.com>
---
Changes since v1:
- fixed build with clang,

---
 lib/vhost/vhost_user.c | 8 ++++----
 lib/vhost/vhost_user.h | 7 +++++--
 2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/lib/vhost/vhost_user.c b/lib/vhost/vhost_user.c
index 723c6890c3..589b950458 100644
--- a/lib/vhost/vhost_user.c
+++ b/lib/vhost/vhost_user.c
@@ -3023,8 +3023,8 @@ vhost_user_msg_handler(int vid, int fd)
 
 	handled = false;
 	if (dev->extern_ops.pre_msg_handle) {
-		ret = (*dev->extern_ops.pre_msg_handle)(dev->vid,
-				(void *)&ctx.msg);
+		RTE_BUILD_BUG_ON(offsetof(struct vhu_msg_context, msg) != 0);
+		ret = (*dev->extern_ops.pre_msg_handle)(dev->vid, &ctx);
 		switch (ret) {
 		case RTE_VHOST_MSG_RESULT_REPLY:
 			send_vhost_reply(dev, fd, &ctx);
@@ -3069,8 +3069,8 @@ vhost_user_msg_handler(int vid, int fd)
 skip_to_post_handle:
 	if (ret != RTE_VHOST_MSG_RESULT_ERR &&
 			dev->extern_ops.post_msg_handle) {
-		ret = (*dev->extern_ops.post_msg_handle)(dev->vid,
-				(void *)&ctx.msg);
+		RTE_BUILD_BUG_ON(offsetof(struct vhu_msg_context, msg) != 0);
+		ret = (*dev->extern_ops.post_msg_handle)(dev->vid, &ctx);
 		switch (ret) {
 		case RTE_VHOST_MSG_RESULT_REPLY:
 			send_vhost_reply(dev, fd, &ctx);
diff --git a/lib/vhost/vhost_user.h b/lib/vhost/vhost_user.h
index be53669f3b..c946cc2ef4 100644
--- a/lib/vhost/vhost_user.h
+++ b/lib/vhost/vhost_user.h
@@ -152,10 +152,13 @@ typedef struct VhostUserMsg {
 	/* Nothing should be added after the payload */
 } __rte_packed VhostUserMsg;
 
-struct vhu_msg_context {
+/* Note: this structure and VhostUserMsg can't be changed carelessly as
+ * external message handlers rely on them.
+ */
+struct __rte_packed vhu_msg_context {
+	VhostUserMsg msg;
 	int fds[VHOST_MEMORY_MAX_NREGIONS];
 	int fd_num;
-	VhostUserMsg msg;
 };
 
 #define VHOST_USER_HDR_SIZE offsetof(VhostUserMsg, payload.u64)
-- 
2.23.0