linux-kernel - [PATCH bpf-next 2/2] selftests: bpf: add a test for mmapable vmlinux BTF

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Message-Id: <20250501-vmlinux-mmap-v1-2-aa2724572598@isovalent.com>
Date: Thu, 01 May 2025 15:28:22 +0100
From: Lorenz Bauer <lmb@...valent.com>
To: Arnd Bergmann <arnd@...db.de>, Alexei Starovoitov <ast@...nel.org>, 
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
 Yonghong Song <yonghong.song@...ux.dev>, 
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, 
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
 Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, 
 Shuah Khan <shuah@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, 
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 Lorenz Bauer <lmb@...valent.com>
Subject: [PATCH bpf-next 2/2] selftests: bpf: add a test for mmapable
 vmlinux BTF

Add a basic test for the ability to mmap /sys/kernel/btf/vmlinux. Since
libbpf doesn't have an API to parse BTF from memory we do some basic
sanity checks ourselves.

Signed-off-by: Lorenz Bauer <lmb@...valent.com>
---
 tools/testing/selftests/bpf/prog_tests/btf_sysfs.c | 79 ++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c b/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c
new file mode 100644
index 0000000000000000000000000000000000000000..8dffed136b4757779028ec0971b56ff541f2218c
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/btf_sysfs.c
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
+/* Copyright (c) 2025 Isovalent */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#define BTF_MAGIC 0xeB9F
+
+static const char *btf_path = "/sys/kernel/btf/vmlinux";
+
+void test_btf_sysfs(void)
+{
+	struct stat st;
+	__u64 btf_size;
+	void *raw_data = NULL;
+	int fd = -1;
+	size_t trailing;
+	long page_size;
+
+	page_size = sysconf(_SC_PAGESIZE);
+	if (!ASSERT_GE(page_size, 0, "get_page_size"))
+		goto cleanup;
+
+	if (!ASSERT_OK(stat(btf_path, &st), "stat_btf"))
+		goto cleanup;
+
+	btf_size = st.st_size;
+	trailing = page_size - (btf_size % page_size) % page_size;
+
+	fd = open(btf_path, O_RDONLY);
+	if (!ASSERT_GE(fd, 0, "open_btf"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_writable"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size, PROT_READ, MAP_SHARED, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_shared"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size + trailing + 1, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_invalid_size"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, btf_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_NEQ(raw_data, MAP_FAILED, "mmap_btf"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_WRITE), -1,
+	    "mprotect_writable"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(mprotect(raw_data, btf_size, PROT_READ | PROT_EXEC), -1,
+	    "mprotect_executable"))
+		goto cleanup;
+
+	/* Check BTF magic value */
+	if (!ASSERT_EQ(*(__u16 *)raw_data, BTF_MAGIC, "btf_magic"))
+		goto cleanup;
+
+	/* Check padding is zeroed */
+	for (int i = 0; i < trailing; i++) {
+		if (((__u8 *)raw_data)[btf_size + i] != 0) {
+			PRINT_FAIL("tail of BTF is not zero at page offset %d\n", i);
+			goto cleanup;
+		}
+	}
+
+cleanup:
+	if (raw_data && raw_data != MAP_FAILED)
+		munmap(raw_data, btf_size);
+	if (fd >= 0)
+		close(fd);
+}

-- 
2.49.0