selftests: bpf: Add a test for mmapable vmlinux BTF

Add a basic test for the ability to mmap /sys/kernel/btf/vmlinux.
Ensure that the data is valid BTF and that it is padded with zero.

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

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

···
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-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>
+
+static void test_btf_mmap_sysfs(const char *path, struct btf *base)
+{
+	struct stat st;
+	__u64 btf_size, end;
+	void *raw_data = NULL;
+	int fd = -1;
+	long page_size;
+	struct btf *btf = NULL;
+
+	page_size = sysconf(_SC_PAGESIZE);
+	if (!ASSERT_GE(page_size, 0, "get_page_size"))
+		goto cleanup;
+
+	if (!ASSERT_OK(stat(path, &st), "stat_btf"))
+		goto cleanup;
+
+	btf_size = st.st_size;
+	end = (btf_size + page_size - 1) / page_size * page_size;
+
+	fd = open(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, end + 1, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_EQ(raw_data, MAP_FAILED, "mmap_btf_invalid_size"))
+		goto cleanup;
+
+	raw_data = mmap(NULL, end, PROT_READ, MAP_PRIVATE, fd, 0);
+	if (!ASSERT_OK_PTR(raw_data, "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 padding is zeroed */
+	for (int i = btf_size; i < end; i++) {
+		if (((__u8 *)raw_data)[i] != 0) {
+			PRINT_FAIL("tail of BTF is not zero at page offset %d\n", i);
+			goto cleanup;
+		}
+	}
+
+	btf = btf__new_split(raw_data, btf_size, base);
+	if (!ASSERT_OK_PTR(btf, "parse_btf"))
+		goto cleanup;
+
+cleanup:
+	btf__free(btf);
+	if (raw_data && raw_data != MAP_FAILED)
+		munmap(raw_data, btf_size);
+	if (fd >= 0)
+		close(fd);
+}
+
+void test_btf_sysfs(void)
+{
+	test_btf_mmap_sysfs("/sys/kernel/btf/vmlinux", NULL);
+}