libbpf: Add identical pointer detection to btf_dedup_is_equiv()

Recently as a side-effect of

commit ac05394 ("compiler.h: introduce TYPEOF_UNQUAL() macro")

issues were observed in deduplication between modules and kernel BTF
such that a large number of kernel types were not deduplicated so
were found in module BTF (task_struct, bpf_prog etc).  The root cause
appeared to be a failure to dedup struct types, specifically those
with members that were pointers with __percpu annotations.

The issue in dedup is at the point that we are deduplicating structures,
we have not yet deduplicated reference types like pointers.  If multiple
copies of a pointer point at the same (deduplicated) integer as in this
case, we do not see them as identical.  Special handling already exists
to deal with structures and arrays, so add pointer handling here too.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250429161042.2069678-1-alan.maguire@oracle.com

Author: alan-maguire

tools/lib/bpf/btf.c

@@ -4396,6 +4396,19 @@ static bool btf_dedup_identical_structs(struct btf_dedup *d, __u32 id1, __u32 id
 	return true;
 }
 
+static bool btf_dedup_identical_ptrs(struct btf_dedup *d, __u32 id1, __u32 id2)
+{
+	struct btf_type *t1, *t2;
+
+	t1 = btf_type_by_id(d->btf, id1);
+	t2 = btf_type_by_id(d->btf, id2);
+
+	if (!btf_is_ptr(t1) || !btf_is_ptr(t2))
+		return false;
+
+	return t1->type == t2->type;
+}
+
 /*
  * Check equivalence of BTF type graph formed by candidate struct/union (we'll
  * call it "candidate graph" in this description for brevity) to a type graph
@@ -4528,6 +4541,9 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id,
 		 */
 		if (btf_dedup_identical_structs(d, hypot_type_id, cand_id))
 			return 1;
+		/* A similar case is again observed for PTRs. */
+		if (btf_dedup_identical_ptrs(d, hypot_type_id, cand_id))
+			return 1;
 		return 0;
 	}