selftests/bpf: Fix btf_dump __int128 test failure with clang build kernel

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 <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Reviewed-by: Alan Maguire <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/btf_dump.c

···
 	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
 }