BTF verifier has a size resolution bug which in some circumstances leads to
invalid size resolution for, e.g., TYPEDEF modifier.  This happens if we have
[1] PTR -> [2] TYPEDEF -> [3] ARRAY, in which case due to being in pointer
context ARRAY size won't be resolved (because for pointer it doesn't matter, so
it's a sink in pointer context), but it will be permanently remembered as zero
for TYPEDEF and TYPEDEF will be marked as RESOLVED. Eventually ARRAY size will
be resolved correctly, but TYPEDEF resolved_size won't be updated anymore.
This, subsequently, will lead to erroneous map creation failure, if that
TYPEDEF is specified as either key or value, as key_size/value_size won't
correspond to resolved size of TYPEDEF (kernel will believe it's zero).

Note, that if BTF was ordered as [1] ARRAY <- [2] TYPEDEF <- [3] PTR, this
won't be a problem, as by the time we get to TYPEDEF, ARRAY's size is already
calculated and stored.

This bug manifests itself in rejecting BTF-defined maps that use array
typedef as a value type:

typedef int array_t[16];

struct {
    __uint(type, BPF_MAP_TYPE_ARRAY);
    __type(value, array_t); /* i.e., array_t *value; */
} test_map SEC(".maps");

The fix consists on not relying on modifier's resolved_size and instead using
modifier's resolved_id (type ID for "concrete" type to which modifier
eventually resolves) and doing size determination for that resolved type. This
allow to preserve existing "early DFS termination" logic for PTR or
STRUCT_OR_ARRAY contexts, but still do correct size determination for modifier
types.

Fixes: eb3f595d ("bpf: btf: Validate type reference")
Cc: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>