When checking available canonical candidates for struct/union algorithm
utilizes btf_dedup_is_equiv to determine if candidate is suitable. This
check is not enough when candidate is corresponding FWD for that
struct/union, because according to equivalence logic they are
equivalent. When it so happens that FWD and STRUCT/UNION end in hashing
to the same bucket, it's possible to create remapping loop from FWD to
STRUCT and STRUCT to same FWD, which will cause btf_dedup() to loop
forever.

This patch fixes the issue by additionally checking that type and
canonical candidate are strictly equal (utilizing btf_equal_struct).

Fixes: d5caef5b5655 ("btf: add BTF types deduplication algorithm")
Reported-by: Arnaldo Carvalho de Melo <redacted>
Signed-off-by: Andrii Nakryiko <redacted>
---
 tools/lib/bpf/btf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 6bbb710216e6..53db26d158c9 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c

@@ -2255,7 +2255,7 @@ static void btf_dedup_merge_hypot_map(struct btf_dedup *d)
 static int btf_dedup_struct_type(struct btf_dedup *d, __u32 type_id)
 {
 	struct btf_dedup_node *cand_node;
-	struct btf_type *t;
+	struct btf_type *cand_type, *t;
 	/* if we don't find equivalent type, then we are canonical */
 	__u32 new_id = type_id;
 	__u16 kind;

@@ -2275,6 +2275,10 @@ static int btf_dedup_struct_type(struct btf_dedup *d, __u32 type_id)
 	for_each_dedup_cand(d, h, cand_node) {
 		int eq;
 
+		cand_type = d->btf->types[cand_node->type_id];
+		if (!btf_equal_struct(t, cand_type))
+			continue;
+
 		btf_dedup_clear_hypot_map(d);
 		eq = btf_dedup_is_equiv(d, type_id, cand_node->type_id);
 		if (eq < 0)

-- 
2.17.1