Commit 81f88f6a authored by Alan Maguire's avatar Alan Maguire Committed by Andrii Nakryiko
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libbpf: Add debug messaging in dedup equivalence/identity matching 

We have seen a number of issues like [1]; failures to deduplicate
key kernel data structures like task_struct.  These are often hard
to debug from pahole even with verbose output, especially when
identity/equivalence checks fail deep in a nested struct comparison.

Here we add debug messages of the form

libbpf: STRUCT 'task_struct' size=2560 vlen=194 cand_id[54222] canon_id[102820] shallow-equal but not equiv for field#23 'sched_class': 0

These will be emitted during dedup from pahole when --verbose/-V
is specified.  This greatly helps identify exactly where dedup
failures are experienced.

[1] https://lore.kernel.org/bpf/b8e8b560-bce5-414b-846d-0da6d22a9983@oracle.com/



Changes since v1:

- updated debug messages to refer to shallow-equal, added ids (Andrii)

Signed-off-by: default avatarAlan Maguire <alan.maguire@oracle.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20251203191507.55565-1-alan.maguire@oracle.com
parent 835a5075

tools/lib/bpf/btf.c

+24 −5
Original line number Diff line number Diff line
@@ -4431,11 +4431,14 @@ static bool btf_dedup_identical_types(struct btf_dedup *d, __u32 id1, __u32 id2, 
	struct btf_type *t1, *t2;

	int k1, k2;

recur:

	if (depth <= 0)

		return false;



	t1 = btf_type_by_id(d->btf, id1);

	t2 = btf_type_by_id(d->btf, id2);

	if (depth <= 0) {

		pr_debug("Reached depth limit for identical type comparison for '%s'/'%s'\n",

			 btf__name_by_offset(d->btf, t1->name_off),

			 btf__name_by_offset(d->btf, t2->name_off));

		return false;

	}



	k1 = btf_kind(t1);

	k2 = btf_kind(t2);
@@ -4497,9 +4500,17 @@ static bool btf_dedup_identical_types(struct btf_dedup *d, __u32 id1, __u32 id2, 
		for (i = 0, n = btf_vlen(t1); i < n; i++, m1++, m2++) {

			if (m1->type == m2->type)

				continue;

			if (!btf_dedup_identical_types(d, m1->type, m2->type, depth - 1))

			if (!btf_dedup_identical_types(d, m1->type, m2->type, depth - 1)) {

				if (t1->name_off) {

					pr_debug("%s '%s' size=%d vlen=%d id1[%u] id2[%u] shallow-equal but not identical for field#%d '%s'\n",

						 k1 == BTF_KIND_STRUCT ? "STRUCT" : "UNION",

						 btf__name_by_offset(d->btf, t1->name_off),

						 t1->size, btf_vlen(t1), id1, id2, i,

						 btf__name_by_offset(d->btf, m1->name_off));

				}

				return false;

			}

		}

		return true;

	}

	case BTF_KIND_FUNC_PROTO: {
@@ -4739,8 +4750,16 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id, 
		canon_m = btf_members(canon_type);

		for (i = 0; i < vlen; i++) {

			eq = btf_dedup_is_equiv(d, cand_m->type, canon_m->type);

			if (eq <= 0)

			if (eq <= 0) {

				if (cand_type->name_off) {

					pr_debug("%s '%s' size=%d vlen=%d cand_id[%u] canon_id[%u] shallow-equal but not equiv for field#%d '%s': %d\n",

						 cand_kind == BTF_KIND_STRUCT ? "STRUCT" : "UNION",

						 btf__name_by_offset(d->btf, cand_type->name_off),

						 cand_type->size, vlen, cand_id, canon_id, i,

						 btf__name_by_offset(d->btf, cand_m->name_off), eq);

				}

				return eq;

			}

			cand_m++;

			canon_m++;

		}