In the Linux kernel, the following vulnerability has been resolved:

bpf: Use array_map_meta_equal for percpu array inner map replacement

percpu_array_map_ops.map_meta_equal points to the generic
bpf_map_meta_equal(), which does not compare max_entries.  When a
percpu array serves as an inner map, replacing it with one that has
fewer max_entries bypasses the check.  Since percpu_array_map_gen_lookup()
inlines the original template's index_mask as a JIT immediate, a lookup
on the replacement map can access pptrs[] out of bounds.

Point percpu_array_map_ops.map_meta_equal to array_map_meta_equal(),
which already enforces the max_entries equality check.

Add a selftest to verify that replacing a percpu array inner map with
a differently-sized one is rejected.