Re: [PATCH] vhost: fix external message handlers

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 3/8/22 09:37, David Marchand wrote:
> On Tue, Mar 8, 2022 at 9:34 AM Maxime Coquelin
> <maxime.coquelin@redhat.com> wrote:
>> 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@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.
> 
> Indeed.
> 
>>
>> In the mean time, I agree with your fix:
>>
>> Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
>>
>> We may also add Reported-by tag when applying.
> 
> There is an issue with clang, I'll send a v2.
> 

Ok.
Fan, can you have a try and confirm it fixes the issue on your side?

Thanks,
Maxime