[PATCH v4] kcov, usb: only collect coverage from __usb_hcd_giveback_urb in softirq

[Andrey Konovalov]

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.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Acked-by: Marco Elver <elver@xxxxxxxxxx>
---

Changes v3->v4:
- Don't make any kcov changes, do a softirq context check in usb code
  instead.

---
 drivers/usb/core/hcd.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c
index a33b849e8beb..2f6a39e09dc6 100644
--- a/drivers/usb/core/hcd.c
+++ b/drivers/usb/core/hcd.c
@@ -1646,9 +1646,16 @@ 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);
-- 
2.29.0.rc1.297.gfa9743e501-goog