Re: [PATCH V6 9/9] virtio: use WARN_ON() to warning illegal status value

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Peter Oberparleiter <oberpar@linux.ibm.com>,
	lulu@redhat.com, paulmck@kernel.org, linux-s390@vger.kernel.org,
	peterz@infradead.org, maz@kernel.org, cohuck@redhat.com,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, pasic@linux.ibm.com,
	eperezma@redhat.com, Vineeth Vijayan <vneethv@linux.ibm.com>,
	tglx@linutronix.de
Subject: Re: [PATCH V6 9/9] virtio: use WARN_ON() to warning illegal status value
Date: Fri, 27 May 2022 06:50:00 -0400	[thread overview]
Message-ID: <20220527064118-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20220527060120.20964-10-jasowang@redhat.com>

At a minimum, I don't see why it's part of the series. Host can always
crash the guest if it wants to ...
The point of BUG_ON is device or driver is already corrupted so we
should not try to drive it.  If you still want this in pls come up with
a better commit log explaining the why.

On Fri, May 27, 2022 at 02:01:20PM +0800, Jason Wang wrote:
> We used to use BUG_ON() in virtio_device_ready() to detect illegal

not really, BUG_ON just crashes the kernel.  we detect by checking
status.

> status value, this seems sub-optimal since the value is under the
> control of the device. Switch to use WARN_ON() instead.

some people use crash on warn so ...

> 
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Halil Pasic <pasic@linux.ibm.com>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Cc: Vineeth Vijayan <vneethv@linux.ibm.com>
> Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
> Cc: linux-s390@vger.kernel.org
> Signed-off-by: Jason Wang <jasowang@redhat.com>

> ---
>  include/linux/virtio_config.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> index d4edfd7d91bb..9a36051ceb76 100644
> --- a/include/linux/virtio_config.h
> +++ b/include/linux/virtio_config.h
> @@ -255,7 +255,7 @@ void virtio_device_ready(struct virtio_device *dev)
>  {
>  	unsigned status = dev->config->get_status(dev);
>  
> -	BUG_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
> +	WARN_ON(status & VIRTIO_CONFIG_S_DRIVER_OK);
>  

we lose debuggability as guest will try to continue.
if we are doing this let us print a helpful message and dump a lot of
state right here.

>  	/*
>  	 * The virtio_synchronize_cbs() makes sure vring_interrupt()
> -- 
> 2.25.1

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