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

On Wed, Oct 7, 2020 at 9:29 PM Alan Stern [off-list ref] wrote:

On Wed, Oct 07, 2020 at 07:30:51PM +0200, Andrey Konovalov wrote:

quoted

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 <redacted>
---

quoted

--- 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);

This isn't right.  __usb_hcd_giveback_urb() can execute in pretty much
any context; its constraint is that interrupts must be disabled.

You're right, but here we constraint kcov to only collect coverage in
case __usb_hcd_giveback_urb() is  executed in softirq context. This is
what happens when we're fuzzing USB with the dummy driver, which is
the case we currently take care of. Whenever someone has a desire to
collect coverage in other contexts, it will need to be implemented
separately.