cxl/acpi: Annotate struct cxl_cxims_data with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct cxl_cxims_data.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Davidlohr Bueso <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Alison Schofield <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Reviewed-by: Vishal Verma <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

drivers/cxl/acpi.c

@@ -14,7 +14,7 @@
 
 struct cxl_cxims_data {
  int nr_maps;
- u64 xormaps[];
+ u64 xormaps[] __counted_by(nr_maps);
 };
 
 /*
@@ -112,9 +112,9 @@ static int cxl_parse_cxims(union acpi_subtable_headers *header, void *arg,
  GFP_KERNEL);
  if (!cximsd)
  return -ENOMEM;
+ cximsd->nr_maps = nr_maps;
  memcpy(cximsd->xormaps, cxims->xormap_list,
  nr_maps * sizeof(*cximsd->xormaps));
- cximsd->nr_maps = nr_maps;
  cxlrd->platform_data = cximsd;
 
  return 0;