kcov, usb: only collect coverage from __usb_hcd_giveback_urb in softirq

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.

Change __usb_hcd_giveback_urb() to only call kcov_remote_*() callbacks
when it's being executed in a 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.
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Link: https://lore.kernel.org/r/f3a7a153f0719cb53ec385b16e912798bd3e4cf9.1602856358.git.andreyknvl@google.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hcd.c

@@ -1647,9 +1647,16 @@ 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);
+	/*
+	 * This function can be called in task context inside another remote
+	 * coverage collection section, but KCOV doesn't support that kind of
+	 * recursion yet. Only collect coverage in softirq context for now.
+	 */
+	if (in_serving_softirq())
+		kcov_remote_start_usb((u64)urb->dev->bus->busnum);
 	urb->complete(urb);
-	kcov_remote_stop();
+	if (in_serving_softirq())
+		kcov_remote_stop();
 
 	usb_anchor_resume_wakeups(anchor);
 	atomic_dec(&urb->use_count);