linux-kernel - Re: [RFC PATCH 1/1] s390/cio: make ccw_device_dma

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Date:   Mon, 11 Oct 2021 20:48:37 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>,
        Vineeth Vijayan <vneethv@...ux.ibm.com>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Michael Mueller <mimu@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, bfu@...hat.com,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [RFC PATCH 1/1] s390/cio: make ccw_device_dma_* more robust

On Mon, 11 Oct 2021 16:33:45 +0200
Cornelia Huck <cohuck@...hat.com> wrote:

> On Mon, Oct 11 2021, Pierre Morel <pmorel@...ux.ibm.com> wrote:
> 
> > On 10/11/21 1:59 PM, Halil Pasic wrote:  
> >> diff --git a/drivers/s390/cio/device_ops.c b/drivers/s390/cio/device_ops.c
> >> index 0fe7b2f2e7f5..c533d1dadc6b 100644
> >> --- a/drivers/s390/cio/device_ops.c
> >> +++ b/drivers/s390/cio/device_ops.c
> >> @@ -825,13 +825,23 @@ EXPORT_SYMBOL_GPL(ccw_device_get_chid);
> >>    */
> >>   void *ccw_device_dma_zalloc(struct ccw_device *cdev, size_t size)
> >>   {
> >> -	return cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size);
> >> +	void *addr;
> >> +
> >> +	if (!get_device(&cdev->dev))
> >> +		return NULL;
> >> +	addr = cio_gp_dma_zalloc(cdev->private->dma_pool, &cdev->dev, size);
> >> +	if (IS_ERR_OR_NULL(addr))  
> >
> > I can be wrong but it seems that only dma_alloc_coherent() used in 
> > cio_gp_dma_zalloc() report an error but the error is ignored and used as 
> > a valid pointer.  
> 
> Hm, I thought dma_alloc_coherent() returned either NULL or a valid
> address?

Yes, that is what is documented.

> 
> >
> > So shouldn't we modify this function and just test for a NULL address here?  
> 
> If I read cio_gp_dma_zalloc() correctly, we either get NULL or a valid
> address, so yes.
> 

I don't think the extra care will hurt us too badly. I prefer to keep
the IS_ERR_OR_NULL() check because it needs less domain specific
knowledge to be understood, and because it is more robust.

Regards,
Halil