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

[Christoph Hellwig]

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.