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 <a...@redhat.com>
Signed-off-by: Andrii Nakryiko <andr...@fb.com>
Acked-by: Song Liu <songliubrav...@fb.com>
Acked-by: Yonghong Song <y...@fb.com>
---
tools/lib/bpf/btf.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 0f83f99211c6..c0b575c6ebb0 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -1663,7 +1663,7 @@ static __u32 btf_hash_struct(struct btf_type *t)
* IDs. This check is performed during type graph equivalence check and
* referenced types equivalence is checked separately.
*/
-static bool btf_equal_struct(struct btf_type *t1, struct btf_type *t2)
+static bool btf_shallow_equal_struct(struct btf_type *t1, struct btf_type *t2)
{
struct btf_member *m1, *m2;
__u16 vlen;
@@ -2124,7 +2124,7 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32
cand_id,
struct btf_member *cand_m, *canon_m;
__u16 vlen;
- if (!btf_equal_struct(cand_type, canon_type))
+ if (!btf_shallow_equal_struct(cand_type, canon_type))
return 0;
vlen = BTF_INFO_VLEN(cand_type->info);
cand_m = (struct btf_member *)(cand_type + 1);
@@ -2265,7 +2265,7 @@ 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;
@@ -2285,6 +2285,20 @@ static int btf_dedup_struct_type(struct btf_dedup *d,
__u32 type_id)
for_each_dedup_cand(d, h, cand_node) {
int eq;
+ /*
+ * Even though btf_dedup_is_equiv() checks for
+ * btf_shallow_equal_struct() internally when checking two
+ * structs (unions) for equivalence, we need to guard here
+ * from picking matching FWD type as a dedup candidate.
+ * This can happen due to hash collision. In such case just
+ * relying on btf_dedup_is_equiv() would lead to potentially
+ * creating a loop (FWD -> STRUCT and STRUCT -> FWD), because
+ * FWD and compatible STRUCT/UNION are considered equivalent.
+ */
+ cand_type = d->btf->types[cand_node->type_id];
+ if (!btf_shallow_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
