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Message-ID: <0e9356ee-9ae0-fd6d-b4ba-b1f05d8ba144@arm.com>
Date:   Tue, 17 May 2022 11:18:54 +0100
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Subject: Re: [PATCH] iommu/dma: Fix check for error return from
 iommu_map_sg_atomic()
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To:     Niklas Schnelle <schnelle@linux.ibm.com>,
        Christoph Hellwig <hch@lst.de>
Cc:     Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org
References: <20220513153948.310119-1-schnelle@linux.ibm.com>
 <20220517083657.GA16377@lst.de>
 <27ae8b9f8e61dce4b31a37622e98b1c57b21b104.camel@linux.ibm.com>
From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <27ae8b9f8e61dce4b31a37622e98b1c57b21b104.camel@linux.ibm.com>
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On 2022-05-17 11:17, Niklas Schnelle wrote:
> On Tue, 2022-05-17 at 10:36 +0200, Christoph Hellwig wrote:
>> On Fri, May 13, 2022 at 05:39:48PM +0200, Niklas Schnelle wrote:
>>> In __iommu_dma_alloc_noncontiguous() the value returned by
>>> iommu_map_sg_atomic() is checked for being smaller than size. Before
>>> commit ad8f36e4b6b1 ("iommu: return full error code from
>>> iommu_map_sg[_atomic]()") this simply checked if the requested size was
>>> successfully mapped.
>>>
>>> After that commit iommu_map_sg_atomic() may also return a negative
>>> error value. In principle this too would be covered by the existing
>>> check. There is one problem however, as size is of type size_t while the
>>> return type of iommu_map_sg_atomic() is now of type ssize_t the latter gets
>>> converted to size_t and negative error values end up as very large
>>> positive values making the check succeed. Fix this by making the return
>>> type visible with a local variable and add an explicit cast to ssize_t.
>>>
>>> Fixes: ad8f36e4b6b1 ("iommu: return full error code from iommu_map_sg[_atomic]()")
>>> Cc: stable@vger.kernel.org
>>> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
>>
>> I don't see what the point of the newly added local variable is here.
>> Just casting size should be all that is needed as far as I can tell.
> 
> No technical reason just found it easier to read and more descriptive.
> I'll sent a v2 with just the cast, it does simplify the commit message.

Note that this is already fixed upstream, though:

https://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git/commit/?h=core&id=a3884774d731f03d3a3dd4fb70ec2d9341ceb39d

Robin.