Re: [PATCH v4] kcov, usb: specify contexts for remote coverage sections

[Dmitry Vyukov]

On Mon, Oct 12, 2020 at 7:17 PM Andrey Konovalov <andreyknvl@xxxxxxxxxx> 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. This is what happens when
> syzkaller is fuzzing the USB stack via the dummy_hcd driver.
>
> 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 does
> exactly 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.

How does this recursion happen? There is literal recursion in the task
context? A function starts a remote coverage section and calls another
function that also starts a remote coverage section?

Or is there recursion between task context and softirq context? But
this should not happen if softirq's disabled around
usb_hcd_giveback_urb call in task context...

We do want to collect coverage from usb_hcd_giveback_urb in the task
context eventually, right?
Is this API supposed to be final? Or it only puts down fire re the warning?

I don't understand how this API can be used in other contexts.
Let's say there is recursion in task context and we want to collect
coverage in task context (the function is only called in task
context). This API won't help.
Let's say a function is called from both task and softirq context and
these can recurse (softirq arrive while in remote task section). This
API won't help. It will force to choose either task or softirq, but
let's say you can't make that choice because they are equally
important.
The API helps to work around the unimplemented recursion in KCOV, but
it's also specific to this particular case. It's not necessary that
recursion is specific to one context only and it's not necessary that
a user can choose to sacrifice one of the contexts.
Also, if we support recursion in one way or another, we will never
want to use this API, right?



> The approach this patch takes is to add another set of kcov_remote_*()
> callbacks that specify the context they 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. This patch constraints KCOV to
> only collect coverage from __usb_hcd_giveback_urb() when it's executed in
> softirq context.
>
> 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@xxxxxxxxxx>
> Acked-by: Marco Elver <elver@xxxxxxxxxx>
> ---
>
> Changes v3->v4:
> - Drop unnecessary returns from kcov callbacks.
>
> ---
>  Documentation/dev-tools/kcov.rst |  6 ++++++
>  drivers/usb/core/hcd.c           |  4 ++--
>  include/linux/kcov.h             | 31 +++++++++++++++++++++++++++++--
>  kernel/kcov.c                    | 26 +++++++++++++++++++-------
>  4 files changed, 56 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
> index 8548b0b04e43..2c0f58988512 100644
> --- a/Documentation/dev-tools/kcov.rst
> +++ b/Documentation/dev-tools/kcov.rst
> @@ -235,6 +235,12 @@ 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 the annotations that only accept a handle, there are also
> +kcov_remote_start_context()/kcov_remote_stop_context() that 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..ea93d9ebcb2e 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_softirq((u64)urb->dev->bus->busnum);
>         urb->complete(urb);
> -       kcov_remote_stop();
> +       kcov_remote_stop_softirq();
>
>         usb_anchor_resume_wakeups(anchor);
>         atomic_dec(&urb->use_count);
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index a10e84707d82..a9c025c3e1df 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);
> @@ -38,10 +42,21 @@ 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_context(u64 handle, unsigned int context);
> +void kcov_remote_stop_context(unsigned int context);
>  u64 kcov_common_handle(void);
>
> +static inline void kcov_remote_start(u64 handle)
> +{
> +       kcov_remote_start_context(handle, KCOV_CONTEXT_MASK);
> +}
> +
> +static inline void kcov_remote_stop(void)
> +{
> +       kcov_remote_stop_context(KCOV_CONTEXT_MASK);
> +}
> +
>  static inline void kcov_remote_start_common(u64 id)
>  {
>         kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id));
> @@ -52,6 +67,16 @@ static inline void kcov_remote_start_usb(u64 id)
>         kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id));
>  }
>
> +static inline void kcov_remote_start_usb_softirq(u64 id)
> +{
> +       kcov_remote_start_context(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id), KCOV_CONTEXT_SOFTIRQ);
> +}
> +
> +static inline void kcov_remote_stop_softirq(void)
> +{
> +       kcov_remote_stop_context(KCOV_CONTEXT_SOFTIRQ);
> +}
> +
>  #else
>
>  static inline void kcov_task_init(struct task_struct *t) {}
> @@ -66,6 +91,8 @@ static inline u64 kcov_common_handle(void)
>  }
>  static inline void kcov_remote_start_common(u64 id) {}
>  static inline void kcov_remote_start_usb(u64 id) {}
> +static inline void kcov_remote_start_usb_softirq(u64 id) {}
> +static inline void kcov_remote_stop_softirq(void) {}
>
>  #endif /* CONFIG_KCOV */
>  #endif /* _LINUX_KCOV_H */
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 6b8368be89c8..3ccdbe060f47 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -808,7 +808,8 @@ static void kcov_remote_softirq_stop(struct task_struct *t)
>         }
>  }
>
> -void kcov_remote_start(u64 handle)
> +/* Also see kcov_remote_start() defined in include/linux/kcov.h. */
> +void kcov_remote_start_context(u64 handle, unsigned int context)
>  {
>         struct task_struct *t = current;
>         struct kcov_remote *remote;
> @@ -821,7 +822,11 @@ 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 (in_task() && !(context & KCOV_CONTEXT_TASK))
> +               return;
> +       if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ))
>                 return;
>
>         local_irq_save(flags);
> @@ -894,7 +899,7 @@ void kcov_remote_start(u64 handle)
>         local_irq_restore(flags);
>
>  }
> -EXPORT_SYMBOL(kcov_remote_start);
> +EXPORT_SYMBOL(kcov_remote_start_context);
>
>  static void kcov_move_area(enum kcov_mode mode, void *dst_area,
>                                 unsigned int dst_area_size, void *src_area)
> @@ -951,8 +956,11 @@ 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)
> +/*
> + * Also see kcov_remote_stop() defined in include/linux/kcov.h.
> + * See the comment before kcov_remote_start_context() for usage details.
> + */
> +void kcov_remote_stop_context(unsigned int context)
>  {
>         struct task_struct *t = current;
>         struct kcov *kcov;
> @@ -962,7 +970,11 @@ 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 (in_task() && !(context & KCOV_CONTEXT_TASK))
> +               return;
> +       if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ))
>                 return;
>
>         local_irq_save(flags);
> @@ -1018,7 +1030,7 @@ void kcov_remote_stop(void)
>         /* Get in kcov_remote_start(). */
>         kcov_put(kcov);
>  }
> -EXPORT_SYMBOL(kcov_remote_stop);
> +EXPORT_SYMBOL(kcov_remote_stop_context);
>
>  /* See the comment before kcov_remote_start() for usage details. */
>  u64 kcov_common_handle(void)
> --
> 2.28.0.1011.ga647a8990f-goog
>