linux-kernel - Re: [PATCH v1 16/16] dma-mapping: Disallow .map_sg operations from returning zero on error

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Message-ID: <21b7f2f1-ccac-4567-6933-a258dcad7099@arm.com>
Date:   Fri, 16 Jul 2021 13:19:23 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     linux-s390@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-parisc@...r.kernel.org,
        Martin Oliveira <martin.oliveira@...eticom.com>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Stephen Bates <sbates@...thlin.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v1 16/16] dma-mapping: Disallow .map_sg operations from
 returning zero on error

On 2021-07-16 07:33, Christoph Hellwig wrote:
> On Thu, Jul 15, 2021 at 10:45:44AM -0600, Logan Gunthorpe wrote:
>> @@ -194,6 +194,8 @@ static int __dma_map_sg_attrs(struct device *dev, struct scatterlist *sg,
>>   	else
>>   		ents = ops->map_sg(dev, sg, nents, dir, attrs);
>>   
>> +	WARN_ON_ONCE(ents == 0);
> 
> Turns this into a negative error code while we're at it, just to keep
> the callers sane?

Right, by this point returning the 0 would pass through 
dma_map_sg_attrs() OK, but AFAICS dma_map_sgtable() would now get 
confused and return success but with sgt->nents = 0. Coercing it to an 
error code (which dma_map_sg_attrs() would then just change right back) 
seems sensible for the sake of easy robustness.

Robin.