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Date:   Mon, 28 Dec 2020 15:52:50 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
        kasan-dev@googlegroups.com, Dmitry Vyukov <dvyukov@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Nazime Hande Harputluoglu <handeharput@gmail.com>
Subject: Re: [PATCH v5] kcov, usb: only collect coverage from
 __usb_hcd_giveback_urb in softirq
Message-ID: <X+nxQo7q2n4dGzoy@kroah.com>
References: <d7035335fdfe7493067fbf7d677db57807a42d5d.1606175031.git.andreyknvl@google.com>
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Precedence: bulk

On Tue, Nov 24, 2020 at 12:47:25AM +0100, Andrey Konovalov 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.
> 
> Change __usb_hcd_giveback_urb() to only call kcov_remote_*() callbacks
> when it's being executed in a softirq. To avoid calling
> in_serving_softirq() directly in the driver code, add a couple of new kcov
> wrappers.
> 
> As the result of this change, 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>
> Acked-by: Marco Elver <elver@google.com>
> ---
> 
> Changes in v5:
> - Don't call in_serving_softirq() in USB driver code directly, do that
>   via kcov wrappers.

Does not apply to 5.11-rc1 :(