[PATCH v2 bpf-next 5/5] selftests/bpf: add btf_dedup test of FWD/STRUCT resolution

[Andrii Nakryiko <andriin@fb.com>]

This patch adds a btf_dedup test exercising logic of STRUCT<->FWD
resolution and validating that STRUCT is not resolved to a FWD. It also
forces hash collisions, forcing both FWD and STRUCT to be candidates for
each other. Previously this condition caused infinite loop due to FWD
pointing to STRUCT and STRUCT pointing to its FWD.

Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
---
 tools/testing/selftests/bpf/test_btf.c | 45 ++++++++++++++++++++++++++
 1 file changed, 45 insertions(+)

diff --git a/tools/testing/selftests/bpf/test_btf.c b/tools/testing/selftests/bpf/test_btf.c
index 1426c0a905c8..38797aa627a7 100644
--- a/tools/testing/selftests/bpf/test_btf.c
+++ b/tools/testing/selftests/bpf/test_btf.c
@@ -5731,6 +5731,51 @@ const struct btf_dedup_test dedup_tests[] = {
 		.dont_resolve_fwds = false,
 	},
 },
+{
+	.descr = "dedup: struct <-> fwd resolution w/ hash collision",
+	/*
+	 * // CU 1:
+	 * struct x;
+	 * struct s {
+	 *	struct x *x;
+	 * };
+	 * // CU 2:
+	 * struct x {};
+	 * struct s {
+	 *	struct x *x;
+	 * };
+	 */
+	.input = {
+		.raw_types = {
+			/* CU 1 */
+			BTF_FWD_ENC(NAME_TBD, 0 /* struct fwd */),	/* [1] fwd x      */
+			BTF_PTR_ENC(1),					/* [2] ptr -> [1] */
+			BTF_STRUCT_ENC(NAME_TBD, 1, 8),			/* [3] struct s   */
+				BTF_MEMBER_ENC(NAME_TBD, 2, 0),
+			/* CU 2 */
+			BTF_STRUCT_ENC(NAME_TBD, 0, 0),			/* [4] struct x   */
+			BTF_PTR_ENC(4),					/* [5] ptr -> [4] */
+			BTF_STRUCT_ENC(NAME_TBD, 1, 8),			/* [6] struct s   */
+				BTF_MEMBER_ENC(NAME_TBD, 5, 0),
+			BTF_END_RAW,
+		},
+		BTF_STR_SEC("\0x\0s\0x\0x\0s\0x\0"),
+	},
+	.expect = {
+		.raw_types = {
+			BTF_PTR_ENC(3),					/* [1] ptr -> [3] */
+			BTF_STRUCT_ENC(NAME_TBD, 1, 8),			/* [2] struct s   */
+				BTF_MEMBER_ENC(NAME_TBD, 1, 0),
+			BTF_STRUCT_ENC(NAME_NTH(2), 0, 0),		/* [3] struct x   */
+			BTF_END_RAW,
+		},
+		BTF_STR_SEC("\0s\0x"),
+	},
+	.opts = {
+		.dont_resolve_fwds = false,
+		.dedup_table_size = 1, /* force hash collisions */
+	},
+},
 {
 	.descr = "dedup: all possible kinds (no duplicates)",
 	.input = {
-- 
2.17.1