Re: [PATCH 3/3] vhost, kcov: collect coverage from vhost_worker

[Andrey Konovalov]

On Wed, Oct 23, 2019 at 3:50 PM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>
> On Wed, Oct 23, 2019 at 3:35 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> >
> > On Wed, Oct 23, 2019 at 10:36 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Oct 22, 2019 at 6:46 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> wrote:
> > > >
> > > > This patch adds kcov_remote_start()/kcov_remote_stop() annotations to the
> > > > vhost_worker() function, which is responsible for processing vhost works.
> > > > Since vhost_worker() threads are spawned per vhost device instance
> > > > the common kcov handle is used for kcov_remote_start()/stop() annotations
> > > > (see Documentation/dev-tools/kcov.rst for details). As the result kcov can
> > > > now be used to collect coverage from vhost worker threads.
> > > >
> > > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > > ---
> > > >  drivers/vhost/vhost.c | 6 ++++++
> > > >  drivers/vhost/vhost.h | 1 +
> > > >  2 files changed, 7 insertions(+)
> > > >
> > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > > index 36ca2cf419bf..a5a557c4b67f 100644
> > > > --- a/drivers/vhost/vhost.c
> > > > +++ b/drivers/vhost/vhost.c
> > > > @@ -30,6 +30,7 @@
> > > >  #include <linux/sched/signal.h>
> > > >  #include <linux/interval_tree_generic.h>
> > > >  #include <linux/nospec.h>
> > > > +#include <linux/kcov.h>
> > > >
> > > >  #include "vhost.h"
> > > >
> > > > @@ -357,7 +358,9 @@ static int vhost_worker(void *data)
> > > >                 llist_for_each_entry_safe(work, work_next, node, node) {
> > > >                         clear_bit(VHOST_WORK_QUEUED, &work->flags);
> > > >                         __set_current_state(TASK_RUNNING);
> > > > +                       kcov_remote_start(dev->kcov_handle);
> > > >                         work->fn(work);
> > > > +                       kcov_remote_stop();
> > > >                         if (need_resched())
> > > >                                 schedule();
> > > >                 }
> > > > @@ -546,6 +549,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
> > > >
> > > >         /* No owner, become one */
> > > >         dev->mm = get_task_mm(current);
> > > > +       dev->kcov_handle = current->kcov_handle;
> > >
> > > kcov_handle is not present in task_struct if !CONFIG_KCOV
> > >
> > > Also this does not use KCOV_SUBSYSTEM_COMMON.
> > > We discussed something along the following lines:
> > >
> > > u64 kcov_remote_handle(u64 subsys, u64 id)
> > > {
> > >   WARN_ON(subsys or id has wrong bits set).
> >
> > Hm, we can't have warnings in kcov_remote_handle() that is exposed in
> > uapi headers. What we can do is return 0 (invalid handle) if subsys/id
> > have incorrect bits set. And then we can either have another
> > kcov_remote_handle() internally (with a different name though) that
> > has a warning, or have warning in kcov_remote_start(). WDYT?
>
> I would probably add the warning to kcov_remote_start(). This avoids
> the need for another function and will catch a wrong ID if caller
> generated it by some other means.
> And then ioctls should also detect bad handles passed in and return
> EINVAL. Then we will cover errors for both kernel and user programs.

OK, will do in v2.

>
> >
> > >   return ...;
> > > }
> > >
> > > kcov_remote_handle(KCOV_SUBSYSTEM_USB, bus);
> > > kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, current->kcov_handle);

I'll add internal kcov_remote_handle_common() and
kcov_remote_handle_usb() helpers to simplify kcov hooks in usb/vhost
code though.

> > >
> > >
> > > >         worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
> > > >         if (IS_ERR(worker)) {
> > > >                 err = PTR_ERR(worker);
> > > > @@ -571,6 +575,7 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
> > > >         if (dev->mm)
> > > >                 mmput(dev->mm);
> > > >         dev->mm = NULL;
> > > > +       dev->kcov_handle = 0;
> > > >  err_mm:
> > > >         return err;
> > > >  }
> > > > @@ -682,6 +687,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> > > >         if (dev->worker) {
> > > >                 kthread_stop(dev->worker);
> > > >                 dev->worker = NULL;
> > > > +               dev->kcov_handle = 0;
> > > >         }
> > > >         if (dev->mm)
> > > >                 mmput(dev->mm);
> > > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > > index e9ed2722b633..a123fd70847e 100644
> > > > --- a/drivers/vhost/vhost.h
> > > > +++ b/drivers/vhost/vhost.h
> > > > @@ -173,6 +173,7 @@ struct vhost_dev {
> > > >         int iov_limit;
> > > >         int weight;
> > > >         int byte_weight;
> > > > +       u64 kcov_handle;
> > > >  };
> > > >
> > > >  bool vhost_exceeds_weight(struct vhost_virtqueue *vq, int pkts, int total_len);
> > > > --
> > > > 2.23.0.866.gb869b98d4c-goog
> > > >