Potentially invalid memory accesses in file drivers/iommu/intel-iommu.c

Potentially invalid memory accesses in file drivers/iommu/intel-iommu.c

[Shaobo]

Hi there,

My name is Shaobo He and I am a graduate student at University of Utah. 
I am using a static analysis tool to search for null pointer 
dereferences and came across a couple of potentially invalid memory 
accesses in the file drivers/iommu/intel-iommu.c. Function 
`domain_get_iommu` can return a NULL pointer. There are four uses of 
this function in the file drivers/iommu/intel-iommu.c:

1. In function `__intel_map_single:
The return value is not NULL-checked.

2. In function `add_unmap`:
The return value is not NULL-checked.

3. In function `intel_unmap`:
The return value is not NULL-checked in its callees.

4. In function `intel_map_sg`:
The return value is not NULL-checked.

So I was wondering if the NULL condition is actually infeasible 
otherwise there seems to exist NULL pointer dereferences.

Best,
Shaobo