[PATCH] vhost: fix external message handlers

[Maxime Coquelin]

Hi,

On 3/7/22 19:11, David Marchand wrote:
> 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")
> 
> Signed-off-by: David Marchand <david.marchand at redhat.com>
> ---
> This patch is untested, but sending quickly to get feedback from the
> reporter and comments from author and maintainers.
> 
> 
> ---
>   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..555f89c97a 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.
> + */
> +__rte_packed struct 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)

We should revisit the callbacks prototype when we'll be allowed to break
API, passing the message pointer as void * is definitely not a good
idea.

In the mean time, I agree with your fix:

Reviewed-by: Maxime Coquelin <maxime.coquelin at redhat.com>

We may also add Reported-by tag when applying.

Thanks!
Maxime