linux-kernel - Re: [PATCH] iommu/dma: Fix check for error return from iommu_map_sg

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Message-ID: <0e9356ee-9ae0-fd6d-b4ba-b1f05d8ba144@arm.com>
Date:   Tue, 17 May 2022 11:18:54 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH] iommu/dma: Fix check for error return from
 iommu_map_sg_atomic()

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@...r.kernel.org
>>> Signed-off-by: Niklas Schnelle <schnelle@...ux.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.