Re: [PATCH] iommu/dma: Fix check for error return from iommu_map_sg_atomic()

[Robin Murphy]

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@xxxxxxxxxxxxxxx
> > > Signed-off-by: Niklas Schnelle <schnelle@xxxxxxxxxxxxx>
> >
> > 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.