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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Fri, 16 Oct 2020 16:33:19 +0200
Message-ID: <CACT4Y+a4RZvZf=XjvE8eUTma8n5y-pqSxG=f37LRUjPwaVVm2w@mail.gmail.com>
Subject: Re: [PATCH v4] kcov, usb: only collect coverage from
 __usb_hcd_giveback_urb in softirq
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        USB list <linux-usb@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Shuah Khan <shuah@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>,
        Aleksandr Nogikh <nogikh@google.com>,
        Nazime Hande Harputluoglu <handeharput@gmail.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Oct 16, 2020 at 3:57 PM Andrey Konovalov <andreyknvl@google.com> 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. 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@google.com>
> Acked-by: Marco Elver <elver@google.com>
> ---
>
> 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();

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

Looks simpler :)

>         usb_anchor_resume_wakeups(anchor);
>         atomic_dec(&urb->use_count);
> --
> 2.29.0.rc1.297.gfa9743e501-goog
>