[PATCH bpf-next] selftests/bpf: fix btf_dump __int128 test failure with clang build kernel

* [PATCH bpf-next] selftests/bpf: fix btf_dump __int128 test failure with clang build kernel
@ 2021-09-24  2:58 Yonghong Song
  2021-09-24 18:12 ` Alan Maguire
  0 siblings, 1 reply; 3+ messages in thread
From: Yonghong Song @ 2021-09-24  2:58 UTC (permalink / raw)
  To: bpf; +Cc: Alexei Starovoitov, Andrii Nakryiko, Daniel Borkmann, kernel-team

With clang build kernel (adding LLVM=1 to kernel and selftests/bpf build
command line), I hit the following test failure:

  $ ./test_progs -t btf_dump
  ...
  btf_dump_data:PASS:ensure expected/actual match 0 nsec
  btf_dump_data:FAIL:find type id unexpected find type id: actual -2 < expected 0
  btf_dump_data:FAIL:find type id unexpected find type id: actual -2 < expected 0
  test_btf_dump_int_data:FAIL:dump __int128 unexpected error: -2 (errno 2)
  #15/9 btf_dump/btf_dump: int_data:FAIL

Further analysis showed gcc build kernel has type "__int128" in dwarf/BTF
and it doesn't exist in clang build kernel. Code searching for kernel code
found the following:
  arch/s390/include/asm/types.h:  unsigned __int128 pair;
  crypto/ecc.c:   unsigned __int128 m = (unsigned __int128)left * right;
  include/linux/math64.h: return (u64)(((unsigned __int128)a * mul) >> shift);
  include/linux/math64.h: return (u64)(((unsigned __int128)a * mul) >> shift);
  lib/ubsan.h:typedef __int128 s_max;
  lib/ubsan.h:typedef unsigned __int128 u_max;

In my case, CONFIG_UBSAN is not enabled. Even if we only have "unsigned __int128"
in the code, somehow gcc still put "__int128" in dwarf while clang didn't.
Hence current test works fine for gcc but not for clang.

Enabling CONFIG_UBSAN is an option to provide __int128 type into dwarf
reliably for both gcc and clang, but not everybody enables CONFIG_UBSAN
in their kernel build. So the best choice is to use "unsigned __int128" type
which is available in both clang and gcc build kernels. But clang and gcc
dwarf encoded names for "unsigned __int128" are different:

  [$ ~] cat t.c
  unsigned __int128 a;
  [$ ~] gcc -g -c t.c && llvm-dwarfdump t.o | grep __int128
                  DW_AT_type      (0x00000031 "__int128 unsigned")
                  DW_AT_name      ("__int128 unsigned")
  [$ ~] clang -g -c t.c && llvm-dwarfdump t.o | grep __int128
                  DW_AT_type      (0x00000033 "unsigned __int128")
                  DW_AT_name      ("unsigned __int128")

The test change in this patch tries to test type name before
doing actual test.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 .../selftests/bpf/prog_tests/btf_dump.c       | 27 ++++++++++++++-----
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
index 52ccf0cf35e1..87f9df653e4e 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
@@ -358,12 +358,27 @@ static void test_btf_dump_int_data(struct btf *btf, struct btf_dump *d,
 	TEST_BTF_DUMP_DATA_OVER(btf, d, NULL, str, int, sizeof(int)-1, "", 1);
 
 #ifdef __SIZEOF_INT128__
-	TEST_BTF_DUMP_DATA(btf, d, NULL, str, __int128, BTF_F_COMPACT,
-			   "(__int128)0xffffffffffffffff",
-			   0xffffffffffffffff);
-	ASSERT_OK(btf_dump_data(btf, d, "__int128", NULL, 0, &i, 16, str,
-				"(__int128)0xfffffffffffffffffffffffffffffffe"),
-		  "dump __int128");
+	/* gcc encode unsigned __int128 type with name "__int128 unsigned" in dwarf,
+	 * and clang encode it with name "unsigned __int128" in dwarf.
+	 * Do an availability test for either variant before doing actual test.
+	 */
+	if (btf__find_by_name(btf, "unsigned __int128") > 0) {
+		TEST_BTF_DUMP_DATA(btf, d, NULL, str, unsigned __int128, BTF_F_COMPACT,
+				   "(unsigned __int128)0xffffffffffffffff",
+				   0xffffffffffffffff);
+		ASSERT_OK(btf_dump_data(btf, d, "unsigned __int128", NULL, 0, &i, 16, str,
+					"(unsigned __int128)0xfffffffffffffffffffffffffffffffe"),
+			  "dump unsigned __int128");
+	} else if (btf__find_by_name(btf, "__int128 unsigned") > 0) {
+		TEST_BTF_DUMP_DATA(btf, d, NULL, str, __int128 unsigned, BTF_F_COMPACT,
+				   "(__int128 unsigned)0xffffffffffffffff",
+				   0xffffffffffffffff);
+		ASSERT_OK(btf_dump_data(btf, d, "__int128 unsigned", NULL, 0, &i, 16, str,
+					"(__int128 unsigned)0xfffffffffffffffffffffffffffffffe"),
+			  "dump unsigned __int128");
+	} else {
+		ASSERT_TRUE(false, "unsigned_int128_not_found");
+	}
 #endif
 }
 
-- 
2.30.2


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