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

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Message-ID: <27ae8b9f8e61dce4b31a37622e98b1c57b21b104.camel@linux.ibm.com>
Date:   Tue, 17 May 2022 12:17:29 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Robin Murphy <robin.murphy@....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 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.