On Fri, Nov 7, 2025 at 3:58 PM Albert Esteve <[email protected]> wrote:
>
> On Fri, Nov 7, 2025 at 2:26 PM Michael S. Tsirkin <[email protected]> wrote:
> >
> > On Fri, Oct 17, 2025 at 09:20:11AM +0200, Albert Esteve wrote:
> > > Refactor backend_read() function and add a reply_ack variable
> > > to have the option for handlers to force tweak whether they should
> > > send a reply or not without depending on VHOST_USER_NEED_REPLY_MASK
> > > flag.
> > >
> > > This fixes an issue with
> > > vhost_user_backend_handle_shared_object_lookup() logic, as the
> > > error path was not closing the backend channel correctly. So,
> > > we can remove the reply call from within the handler, make
> > > sure it returns early on errors as other handlers do and
> > > set the reply_ack variable on backend_read() to true to ensure
> > > that it will send a response, thus keeping the original intent.
> > >
> > > Fixes: 160947666276c5b7f6bca4d746bcac2966635d79
> >
> >
> > I fixed this, should include the subject.
> >
> > > Cc: [email protected]
> > > Signed-off-by: Albert Esteve <[email protected]>
> >
> >
> > So I picked this for now, but I worry that we are opening ourselves
> > up to buggy backends which forget to set the flag and expect
> > frontend to behave properly.
> >
> > I think a better fix would be instead of forcing, check reply_ack,
> > and fail if not set correctly.
>
> Fair point. I will send a follow-up PATCH with this check and fail early.
So I was going to do this follow-up patch but then reading the spec in
vhost-user.rst I saw:
```For the message types that already solicit a reply from the back-end,
the presence of ``VHOST_USER_PROTOCOL_F_REPLY_ACK`` or need_reply bit
being set brings no behavioural change.```
Since this message type (SHARED_OBJECT_LOOKUP) explicitly solicit a
reply, I understand that the correct behaviour is what we did in this
patch: forcing it to true.
>
> >
> > > ---
> > > hw/virtio/vhost-user.c | 42 +++++++++++++++---------------------------
> > > 1 file changed, 15 insertions(+), 27 deletions(-)
> > >
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index 890be55937..762d7218d3 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -1696,14 +1696,6 @@ static bool vhost_user_send_resp(QIOChannel *ioc,
> > > VhostUserHeader *hdr,
> > > return !qio_channel_writev_all(ioc, iov, ARRAY_SIZE(iov), errp);
> > > }
> > >
> > > -static bool
> > > -vhost_user_backend_send_dmabuf_fd(QIOChannel *ioc, VhostUserHeader *hdr,
> > > - VhostUserPayload *payload, Error
> > > **errp)
> > > -{
> > > - hdr->size = sizeof(payload->u64);
> > > - return vhost_user_send_resp(ioc, hdr, payload, errp);
> > > -}
> > > -
> > > int vhost_user_get_shared_object(struct vhost_dev *dev, unsigned char
> > > *uuid,
> > > int *dmabuf_fd)
> > > {
> > > @@ -1744,19 +1736,15 @@ int vhost_user_get_shared_object(struct vhost_dev
> > > *dev, unsigned char *uuid,
> > >
> > > static int
> > > vhost_user_backend_handle_shared_object_lookup(struct vhost_user *u,
> > > - QIOChannel *ioc,
> > > - VhostUserHeader *hdr,
> > > - VhostUserPayload *payload)
> > > + VhostUserShared *object)
> > > {
> > > QemuUUID uuid;
> > > CharBackend *chr = u->user->chr;
> > > - Error *local_err = NULL;
> > > int dmabuf_fd = -1;
> > > int fd_num = 0;
> > >
> > > - memcpy(uuid.data, payload->object.uuid,
> > > sizeof(payload->object.uuid));
> > > + memcpy(uuid.data, object->uuid, sizeof(object->uuid));
> > >
> > > - payload->u64 = 0;
> > > switch (virtio_object_type(&uuid)) {
> > > case TYPE_DMABUF:
> > > dmabuf_fd = virtio_lookup_dmabuf(&uuid);
> > > @@ -1765,18 +1753,16 @@
> > > vhost_user_backend_handle_shared_object_lookup(struct vhost_user *u,
> > > {
> > > struct vhost_dev *dev = virtio_lookup_vhost_device(&uuid);
> > > if (dev == NULL) {
> > > - payload->u64 = -EINVAL;
> > > - break;
> > > + return -EINVAL;
> > > }
> > > int ret = vhost_user_get_shared_object(dev, uuid.data,
> > > &dmabuf_fd);
> > > if (ret < 0) {
> > > - payload->u64 = ret;
> > > + return ret;
> > > }
> > > break;
> > > }
> > > case TYPE_INVALID:
> > > - payload->u64 = -EINVAL;
> > > - break;
> > > + return -EINVAL;
> > > }
> > >
> > > if (dmabuf_fd != -1) {
> > > @@ -1785,11 +1771,6 @@
> > > vhost_user_backend_handle_shared_object_lookup(struct vhost_user *u,
> > >
> > > if (qemu_chr_fe_set_msgfds(chr, &dmabuf_fd, fd_num) < 0) {
> > > error_report("Failed to set msg fds.");
> > > - payload->u64 = -EINVAL;
> > > - }
> > > -
> > > - if (!vhost_user_backend_send_dmabuf_fd(ioc, hdr, payload,
> > > &local_err)) {
> > > - error_report_err(local_err);
> > > return -EINVAL;
> > > }
> > >
> > > @@ -2008,6 +1989,7 @@ static gboolean backend_read(QIOChannel *ioc,
> > > GIOCondition condition,
> > > struct iovec iov;
> > > g_autofree int *fd = NULL;
> > > size_t fdsize = 0;
> > > + bool reply_ack;
> > > int i;
> > >
> > > /* Read header */
> > > @@ -2026,6 +2008,8 @@ static gboolean backend_read(QIOChannel *ioc,
> > > GIOCondition condition,
> > > goto err;
> > > }
> > >
> > > + reply_ack = hdr.flags & VHOST_USER_NEED_REPLY_MASK;
> > > +
> > > /* Read payload */
> > > if (qio_channel_read_all(ioc, (char *) &payload, hdr.size,
> > > &local_err)) {
> > > error_report_err(local_err);
> > > @@ -2051,11 +2035,14 @@ static gboolean backend_read(QIOChannel *ioc,
> > > GIOCondition condition,
> > >
> > > &payload.object);
> > > break;
> > > case VHOST_USER_BACKEND_SHARED_OBJECT_LOOKUP:
> > > - ret =
> > > vhost_user_backend_handle_shared_object_lookup(dev->opaque, ioc,
> > > - &hdr,
> > > &payload);
> > > + /* The backend always expects a response */
> > > + reply_ack = true;
> > > + ret = vhost_user_backend_handle_shared_object_lookup(dev->opaque,
> > > +
> > > &payload.object);
> > > break;
> > > case VHOST_USER_BACKEND_SHMEM_MAP:
> > > /* Handler manages its own response, check error and close
> > > connection */
> > > + reply_ack = false;
> > > if (vhost_user_backend_handle_shmem_map(dev, ioc, &hdr, &payload,
> > > fd ? fd[0] : -1) < 0) {
> > > goto err;
> > > @@ -2063,6 +2050,7 @@ static gboolean backend_read(QIOChannel *ioc,
> > > GIOCondition condition,
> > > break;
> > > case VHOST_USER_BACKEND_SHMEM_UNMAP:
> > > /* Handler manages its own response, check error and close
> > > connection */
> > > + reply_ack = false;
> > > if (vhost_user_backend_handle_shmem_unmap(dev, ioc, &hdr,
> > > &payload) < 0) {
> > > goto err;
> > > }
> > > @@ -2076,7 +2064,7 @@ static gboolean backend_read(QIOChannel *ioc,
> > > GIOCondition condition,
> > > * REPLY_ACK feature handling. Other reply types has to be managed
> > > * directly in their request handlers.
> > > */
> > > - if (hdr.flags & VHOST_USER_NEED_REPLY_MASK) {
> > > + if (reply_ack) {
> > > payload.u64 = !!ret;
> > > hdr.size = sizeof(payload.u64);
> > >
> > > --
> > > 2.49.0
> >
