Re: [PATCH RFC 2/3] usb, kcov: collect coverage from hub_event

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Oct 17, 2019 at 09:06:56PM +0200, Andrey Konovalov wrote:
> On Thu, Oct 17, 2019 at 8:19 PM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Thu, Oct 17, 2019 at 07:44:14PM +0200, Andrey Konovalov wrote:
> > > This patch adds kcov_remote_start/kcov_remote_stop annotations to the
> > > hub_event function, which is responsible for processing events on USB
> > > buses, in particular events that happen during USB device enumeration.
> > > Each USB bus gets a unique id, which can be used to attach a kcov device
> > > to a particular USB bus for coverage collection.
> > >
> > > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> > > ---
> > >  drivers/usb/core/hub.c    | 4 ++++
> > >  include/linux/kcov.h      | 1 +
> > >  include/uapi/linux/kcov.h | 7 +++++++
> > >  3 files changed, 12 insertions(+)
> > >
> > > diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> > > index 236313f41f4a..03a40e41b099 100644
> > > --- a/drivers/usb/core/hub.c
> > > +++ b/drivers/usb/core/hub.c
> > > @@ -5374,6 +5374,8 @@ static void hub_event(struct work_struct *work)
> > >       hub_dev = hub->intfdev;
> > >       intf = to_usb_interface(hub_dev);
> > >
> > > +     kcov_remote_start(kcov_remote_handle_usb(hdev->bus->busnum));
> > > +
> > >       dev_dbg(hub_dev, "state %d ports %d chg %04x evt %04x\n",
> > >                       hdev->state, hdev->maxchild,
> > >                       /* NOTE: expects max 15 ports... */
> > > @@ -5480,6 +5482,8 @@ static void hub_event(struct work_struct *work)
> > >       /* Balance the stuff in kick_hub_wq() and allow autosuspend */
> > >       usb_autopm_put_interface(intf);
> > >       kref_put(&hub->kref, hub_release);
> > > +
> > > +     kcov_remote_stop();
> > >  }
> > >
> > >  static const struct usb_device_id hub_id_table[] = {
> > > diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> > > index 702672d98d35..38a47e0b67c2 100644
> > > --- a/include/linux/kcov.h
> > > +++ b/include/linux/kcov.h
> > > @@ -30,6 +30,7 @@ void kcov_task_exit(struct task_struct *t);
> > >  /*
> > >   * Reserved handle ranges:
> > >   * 0000000000000000 - 0000ffffffffffff : common handles
> > > + * 0001000000000000 - 0001ffffffffffff : USB subsystem handles
> >
> > So how many bits are you going to have for any in-kernel tasks?  Aren't
> > you going to run out quickly?
> 
> With these patches we only collect coverage from hub_event threads,
> and we need one ID per USB bus, the number of which is quite limited.
> But then we might want to collect coverage from other parts of the USB
> subsystem, so we might need more IDs. I don't expect the number of
> different subsystem from which we want to collect coverage to be
> large, so the idea here is to use 2 bytes of an ID to denote the
> subsystem, and the other 6 to denote different coverage collection
> sections within it.
> 
> But overall, which encoding scheme to use here is a good question.
> Ideas are welcome.
> 
> > >   */
> > >  void kcov_remote_start(u64 handle);
> > >  void kcov_remote_stop(void);
> > > diff --git a/include/uapi/linux/kcov.h b/include/uapi/linux/kcov.h
> > > index 46f78f716ca9..45c9ae59cebc 100644
> > > --- a/include/uapi/linux/kcov.h
> > > +++ b/include/uapi/linux/kcov.h
> > > @@ -43,4 +43,11 @@ enum {
> > >  #define KCOV_CMP_SIZE(n)        ((n) << 1)
> > >  #define KCOV_CMP_MASK           KCOV_CMP_SIZE(3)
> > >
> > > +#define KCOV_REMOTE_HANDLE_USB  0x0001000000000000ull
> > > +
> > > +static inline __u64 kcov_remote_handle_usb(unsigned int bus)
> > > +{
> > > +     return KCOV_REMOTE_HANDLE_USB + (__u64)bus;
> > > +}
> >
> > Why is this function in a uapi .h file?  What userspace code would call
> > this?
> 
> A userspace process that wants to collect coverage from USB bus # N
> needs to pass kcov_remote_handle_usb(N) into KCOV_REMOTE_ENABLE ioctl.

Ugh, ok.  Then you should make "unsigned int bus" a __u64 so that this
actually will work on all kernels properly.

thanks,

greg k-h