pool_queue: Add static assert for management_array offset

[fangyi-zhou]

In order for the GNU extension of static initialisation of flexible
array member to work. Care must be taken to check whether the offset of
management_array is the size of the structure, so that accessing the
management_array will be to the allocated static memory following the
pool struct. If this assertion fails, accessing management_array might
end of accessing the struct padding inserted by the compiler.

Additional explanation:

Current struct layout

byte	byte	byte	byte
magic	magic	magic	magic
array	array	array	array
stride	stride	stride	stride
num	num_allocated	first_free	spinlock
->

-> = location management_array points to (outside the struct)

If future modifications are to be added to this data structure, say adding/removing an u8 field. A possible layout will be

byte	byte	byte	byte
magic	magic	magic	magic
array	array	array	array
stride	stride	stride	stride
num	num_allocated	first_free	spinlock
new_field	->padding	padding	padding

-> = location management_array points to (inside the struct)

In this case, management_array points to the struct padding due to the layout of uvisor_pool_entry_t being u8. However, the actual management array is located after the paddings (due to struct alignments).

The C standard does not make clear requirements on struct paddings, but it would be nice to avoid the grey area and be certain.

See also: http://www.cl.cam.ac.uk/~pes20/cerberus/notes30.pdf Section 3.3

[Patater]

Please use UVISOR_STATIC_ASSERT. We have it implemented now.

[fangyi-zhou]

Changed