Re: [PATCH] vhost: fix external message handlers

[David Marchand <david.marchand@redhat.com>]

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.


-- 
David Marchand