Re: [PATCH] kcov, usb, vhost: specify contexts for remote coverage sections

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Oct 7, 2020 at 7:30 PM Andrey Konovalov <andreyknvl@google.com> wrote:
>
> Currently there's a KCOV remote coverage collection section in
> __usb_hcd_giveback_urb(). Initially that section was added based on the
> assumption that usb_hcd_giveback_urb() can only be called in interrupt
> context as indicated by a comment before it.
>
> As it turns out, it's actually valid to call usb_hcd_giveback_urb() in task
> context, provided that the caller turned off the interrupts; USB/IP actually
> does that. This can lead to a nested KCOV remote coverage collection
> sections both trying to collect coverage in task context. This isn't
> supported by KCOV, and leads to a WARNING.
>
> The approach this patch takes is to annotate every call of kcov_remote_*()
> callbacks with the context those callbacks are supposed to be executed in.
> If the current context doesn't match the mask provided to a callback,
> that callback is ignored. KCOV currently only supports collecting remote
> coverage in two contexts: task and softirq.
>
> As the result, the coverage from USB/IP related usb_hcd_giveback_urb() calls
> won't be collected, but the WARNING is fixed.
>
> A potential future improvement would be to support nested remote coverage
> collection sections, but this patch doesn't address that.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kcov.rst |  5 +++++
>  drivers/usb/core/hcd.c           |  4 ++--
>  drivers/usb/core/hub.c           |  4 ++--
>  drivers/vhost/vhost.c            |  5 +++--
>  include/linux/kcov.h             | 24 ++++++++++++++----------
>  kernel/kcov.c                    | 16 ++++++++++++----
>  6 files changed, 38 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 8548b0b04e43..99fda94a34c5 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -235,6 +235,11 @@ saved to the kcov_handle field in the current task_struct and needs to be
>  passed to the newly spawned threads via custom annotations. Those threads
>  should in turn be annotated with kcov_remote_start()/kcov_remote_stop().
>
> +Besides a handle, kcov_remote_start()/kcov_remote_stop() annotations accept
> +a context mask. This mask describes the contexts in which these annotations
> +should be applied. E.g. specifying KCOV_CONTEXT_SOFTIRQ will result in the
> +corresponding annotations being ignored in any context other than softirq.
> +
>  Internally kcov stores handles as u64 integers. The top byte of a handle
>  is used to denote the id of a subsystem that this handle belongs to, and
>  the lower 4 bytes are used to denote the id of a thread instance within
> diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
> index a33b849e8beb..1b090e3218a8 100644
> --- a/drivers/usb/core/hcd.c
> +++ b/drivers/usb/core/hcd.c
> @@ -1646,9 +1646,9 @@ static void __usb_hcd_giveback_urb(struct urb *urb)
>
>         /* pass ownership to the completion handler */
>         urb->status = status;
> -       kcov_remote_start_usb((u64)urb->dev->bus->busnum);
> +       kcov_remote_start_usb((u64)urb->dev->bus->busnum, KCOV_CONTEXT_SOFTIRQ);
>         urb->complete(urb);
> -       kcov_remote_stop();
> +       kcov_remote_stop(KCOV_CONTEXT_SOFTIRQ);
>
>         usb_anchor_resume_wakeups(anchor);
>         atomic_dec(&urb->use_count);
> diff --git a/drivers/usb/core/hub.c b/drivers/usb/core/hub.c
> index 5b768b80d1ee..d17db72dd020 100644
> --- a/drivers/usb/core/hub.c
> +++ b/drivers/usb/core/hub.c
> @@ -5509,7 +5509,7 @@ static void hub_event(struct work_struct *work)
>         hub_dev = hub->intfdev;
>         intf = to_usb_interface(hub_dev);
>
> -       kcov_remote_start_usb((u64)hdev->bus->busnum);
> +       kcov_remote_start_usb((u64)hdev->bus->busnum, KCOV_CONTEXT_TASK);
>
>         dev_dbg(hub_dev, "state %d ports %d chg %04x evt %04x\n",
>                         hdev->state, hdev->maxchild,
> @@ -5618,7 +5618,7 @@ static void hub_event(struct work_struct *work)
>         usb_autopm_put_interface(intf);
>         kref_put(&hub->kref, hub_release);
>
> -       kcov_remote_stop();
> +       kcov_remote_stop(KCOV_CONTEXT_TASK);
>  }
>
>  static const struct usb_device_id hub_id_table[] = {
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index b45519ca66a7..8913de414e89 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -363,9 +363,10 @@ 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_common(dev->kcov_handle);
> +                       kcov_remote_start_common(dev->kcov_handle,
> +                                                       KCOV_CONTEXT_TASK);
>                         work->fn(work);
> -                       kcov_remote_stop();
> +                       kcov_remote_stop(KCOV_CONTEXT_TASK);
>                         if (need_resched())
>                                 schedule();
>                 }
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index a10e84707d82..507003038918 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -22,6 +22,10 @@ enum kcov_mode {
>         KCOV_MODE_TRACE_CMP = 3,
>  };
>
> +#define KCOV_CONTEXT_TASK      (1u << 0)
> +#define KCOV_CONTEXT_SOFTIRQ   (1u << 1)
> +#define KCOV_CONTEXT_MASK      (KCOV_CONTEXT_TASK | KCOV_CONTEXT_SOFTIRQ)
> +
>  #define KCOV_IN_CTXSW  (1 << 30)
>
>  void kcov_task_init(struct task_struct *t);
> @@ -38,18 +42,18 @@ do {                                                \
>  } while (0)
>
>  /* See Documentation/dev-tools/kcov.rst for usage details. */
> -void kcov_remote_start(u64 handle);
> -void kcov_remote_stop(void);
> +void kcov_remote_start(u64 handle, unsigned int context);
> +void kcov_remote_stop(unsigned int context);
>  u64 kcov_common_handle(void);
>
> -static inline void kcov_remote_start_common(u64 id)
> +static inline void kcov_remote_start_common(u64 id, unsigned int context)
>  {
> -       kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id));
> +       kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id), context);
>  }
>
> -static inline void kcov_remote_start_usb(u64 id)
> +static inline void kcov_remote_start_usb(u64 id, unsigned int context)
>  {
> -       kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id));
> +       kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id), context);
>  }
>
>  #else
> @@ -58,14 +62,14 @@ static inline void kcov_task_init(struct task_struct *t) {}
>  static inline void kcov_task_exit(struct task_struct *t) {}
>  static inline void kcov_prepare_switch(struct task_struct *t) {}
>  static inline void kcov_finish_switch(struct task_struct *t) {}
> -static inline void kcov_remote_start(u64 handle) {}
> -static inline void kcov_remote_stop(void) {}
> +static inline void kcov_remote_start(u64 handle, unsigned int context) {}
> +static inline void kcov_remote_stop(unsigned int context) {}
>  static inline u64 kcov_common_handle(void)
>  {
>         return 0;
>  }
> -static inline void kcov_remote_start_common(u64 id) {}
> -static inline void kcov_remote_start_usb(u64 id) {}
> +static inline void kcov_remote_start_common(u64 id, unsigned int context) {}
> +static inline void kcov_remote_start_usb(u64 id, unsigned int context) {}
>
>  #endif /* CONFIG_KCOV */
>  #endif /* _LINUX_KCOV_H */
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 6b8368be89c8..911bece5242e 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -808,7 +808,7 @@ static void kcov_remote_softirq_stop(struct task_struct *t)
>         }
>  }
>
> -void kcov_remote_start(u64 handle)
> +void kcov_remote_start(u64 handle, unsigned int context)
>  {
>         struct task_struct *t = current;
>         struct kcov_remote *remote;
> @@ -821,7 +821,11 @@ void kcov_remote_start(u64 handle)
>
>         if (WARN_ON(!kcov_check_handle(handle, true, true, true)))
>                 return;
> -       if (!in_task() && !in_serving_softirq())
> +       if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context))
> +               return;
> +       if ((context & KCOV_CONTEXT_TASK) && !in_task())
> +               return;

This actually has to be:

if (in_task() && !(context & KCOV_CONTEXT_TASK))

And similarly below.

Will fix in v2.

> +       if ((context & KCOV_CONTEXT_SOFTIRQ) && !in_softirq())
>                 return;
>
>         local_irq_save(flags);
> @@ -952,7 +956,7 @@ static void kcov_move_area(enum kcov_mode mode, void *dst_area,
>  }
>
>  /* See the comment before kcov_remote_start() for usage details. */
> -void kcov_remote_stop(void)
> +void kcov_remote_stop(unsigned int context)
>  {
>         struct task_struct *t = current;
>         struct kcov *kcov;
> @@ -962,7 +966,11 @@ void kcov_remote_stop(void)
>         int sequence;
>         unsigned long flags;
>
> -       if (!in_task() && !in_serving_softirq())
> +       if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context))
> +               return;
> +       if ((context & KCOV_CONTEXT_TASK) && !in_task())
> +               return;
> +       if ((context & KCOV_CONTEXT_SOFTIRQ) && !in_softirq())
>                 return;
>
>         local_irq_save(flags);
> --
> 2.28.0.806.g8561365e88-goog
>