Re: [PATCH bpf-next v1] libbpf: ensure new BTF objects inherit input endianness

[Eduard Zingerman <eddyz87@gmail.com>]

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On Fri, 2024-08-30 at 02:51 -0700, Tony Ambardar wrote:
> The pahole master branch recently added support for "distilled BTF" based
> on libbpf v1.5, but may add .BTF and .BTF.base sections with the wrong byte
> order (e.g. on s390x BPF CI), which then lead to kernel Oops when loaded.
> 
> Fix by updating libbpf's btf__distill_base() and btf_new_empty() to retain
> the byte order of any source BTF objects when creating new ones.
> 
> Reported-by: Song Liu <song@kernel.org>
> Reported-by: Eduard Zingerman <eddyz87@gmail.com>
> Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
> Link: https://lore.kernel.org/bpf/6358db36c5f68b07873a0a5be2d062b1af5ea5f8.camel@gmail.com/
> Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

But we also need a test for this. Like the one attached.
Or Alan can share his test, which is much shorter but skips round trip to bytes and back.

[...]

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From 30fd46e4c84ba90627b039b1e721449ca8b656ad Mon Sep 17 00:00:00 2001
From: Eduard Zingerman <eddyz87@gmail.com>
Date: Fri, 30 Aug 2024 04:08:44 -0700
Subject: [PATCH bpf-next] selftests/bpf: checl if distilled base inherits
 source endianness

Create a BTF with endianness different from host, make a distilled
base/split BTF pair from it, dump as raw bytes, import again and
verify that endianness is preserved.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/prog_tests/btf_distill.c    | 73 +++++++++++++++++++
 1 file changed, 73 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_distill.c b/tools/testing/selftests/bpf/prog_tests/btf_distill.c
index bfbe795823a2..810b2e434562 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_distill.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_distill.c
@@ -535,6 +535,77 @@ static void test_distilled_base_vmlinux(void)
 	btf__free(vmlinux_btf);
 }
 
+static bool is_host_big_endian(void)
+{
+	return htons(0x1234) == 0x1234;
+}
+
+/* Split and new base BTFs should inherit endianness from source BTF. */
+static void test_distilled_endianness(void)
+{
+	struct btf *base = NULL, *split = NULL, *new_base = NULL, *new_split = NULL;
+	struct btf *new_base1 = NULL, *new_split1 = NULL;
+	enum btf_endianness inverse_endianness;
+	const void *raw_data;
+	__u32 size;
+
+	printf("is_host_big_endian? %d\n", is_host_big_endian());
+	inverse_endianness = is_host_big_endian() ? BTF_LITTLE_ENDIAN : BTF_BIG_ENDIAN;
+	base = btf__new_empty();
+	btf__set_endianness(base, inverse_endianness);
+	if (!ASSERT_OK_PTR(base, "empty_main_btf"))
+		return;
+	btf__add_int(base, "int", 4, BTF_INT_SIGNED);   /* [1] int */
+	VALIDATE_RAW_BTF(
+		base,
+		"[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED");
+	split = btf__new_empty_split(base);
+	if (!ASSERT_OK_PTR(split, "empty_split_btf"))
+		goto cleanup;
+	btf__add_ptr(split, 1);
+	VALIDATE_RAW_BTF(
+		split,
+		"[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED",
+		"[2] PTR '(anon)' type_id=1");
+	if (!ASSERT_EQ(0, btf__distill_base(split, &new_base, &new_split),
+		       "distilled_base") ||
+	    !ASSERT_OK_PTR(new_base, "distilled_base") ||
+	    !ASSERT_OK_PTR(new_split, "distilled_split") ||
+	    !ASSERT_EQ(2, btf__type_cnt(new_base), "distilled_base_type_cnt"))
+		goto cleanup;
+	VALIDATE_RAW_BTF(
+		new_split,
+		"[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED",
+		"[2] PTR '(anon)' type_id=1");
+
+	raw_data = btf__raw_data(new_base, &size);
+	if (!ASSERT_OK_PTR(raw_data, "btf__raw_data #1"))
+		goto cleanup;
+	new_base1 = btf__new(raw_data, size);
+	if (!ASSERT_OK_PTR(new_base1, "new_base1 = btf__new()"))
+		goto cleanup;
+	raw_data = btf__raw_data(new_split, &size);
+	if (!ASSERT_OK_PTR(raw_data, "btf__raw_data #2"))
+		goto cleanup;
+	new_split1 = btf__new_split(raw_data, size, new_base1);
+	if (!ASSERT_OK_PTR(new_split1, "new_split1 = btf__new()"))
+		goto cleanup;
+
+	ASSERT_EQ(btf__endianness(new_base1), inverse_endianness, "new_base1 endianness");
+	ASSERT_EQ(btf__endianness(new_split1), inverse_endianness, "new_split1 endianness");
+	VALIDATE_RAW_BTF(
+		new_split1,
+		"[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED",
+		"[2] PTR '(anon)' type_id=1");
+cleanup:
+	btf__free(new_split1);
+	btf__free(new_base1);
+	btf__free(new_split);
+	btf__free(new_base);
+	btf__free(split);
+	btf__free(base);
+}
+
 void test_btf_distill(void)
 {
 	if (test__start_subtest("distilled_base"))
@@ -549,4 +620,6 @@ void test_btf_distill(void)
 		test_distilled_base_multi_err2();
 	if (test__start_subtest("distilled_base_vmlinux"))
 		test_distilled_base_vmlinux();
+	if (test__start_subtest("distilled_endianness"))
+		test_distilled_endianness();
 }
-- 
2.46.0