selftests/bpf: Add bpffs preload test.

Add a test that mounts two bpffs instances and checks progs.debug
and maps.debug for sanity data.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200819042759.51280-5-alexei.starovoitov@gmail.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#define _GNU_SOURCE
+#include <sched.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <test_progs.h>
+
+#define TDIR "/sys/kernel/debug"
+
+static int read_iter(char *file)
+{
+	/* 1024 should be enough to get contiguous 4 "iter" letters at some point */
+	char buf[1024];
+	int fd, len;
+
+	fd = open(file, 0);
+	if (fd < 0)
+		return -1;
+	while ((len = read(fd, buf, sizeof(buf))) > 0)
+		if (strstr(buf, "iter")) {
+			close(fd);
+			return 0;
+		}
+	close(fd);
+	return -1;
+}
+
+static int fn(void)
+{
+	int err, duration = 0;
+
+	err = unshare(CLONE_NEWNS);
+	if (CHECK(err, "unshare", "failed: %d\n", errno))
+		goto out;
+
+	err = mount("", "/", "", MS_REC | MS_PRIVATE, NULL);
+	if (CHECK(err, "mount /", "failed: %d\n", errno))
+		goto out;
+
+	err = umount(TDIR);
+	if (CHECK(err, "umount " TDIR, "failed: %d\n", errno))
+		goto out;
+
+	err = mount("none", TDIR, "tmpfs", 0, NULL);
+	if (CHECK(err, "mount", "mount root failed: %d\n", errno))
+		goto out;
+
+	err = mkdir(TDIR "/fs1", 0777);
+	if (CHECK(err, "mkdir "TDIR"/fs1", "failed: %d\n", errno))
+		goto out;
+	err = mkdir(TDIR "/fs2", 0777);
+	if (CHECK(err, "mkdir "TDIR"/fs2", "failed: %d\n", errno))
+		goto out;
+
+	err = mount("bpf", TDIR "/fs1", "bpf", 0, NULL);
+	if (CHECK(err, "mount bpffs "TDIR"/fs1", "failed: %d\n", errno))
+		goto out;
+	err = mount("bpf", TDIR "/fs2", "bpf", 0, NULL);
+	if (CHECK(err, "mount bpffs " TDIR "/fs2", "failed: %d\n", errno))
+		goto out;
+
+	err = read_iter(TDIR "/fs1/maps.debug");
+	if (CHECK(err, "reading " TDIR "/fs1/maps.debug", "failed\n"))
+		goto out;
+	err = read_iter(TDIR "/fs2/progs.debug");
+	if (CHECK(err, "reading " TDIR "/fs2/progs.debug", "failed\n"))
+		goto out;
+out:
+	umount(TDIR "/fs1");
+	umount(TDIR "/fs2");
+	rmdir(TDIR "/fs1");
+	rmdir(TDIR "/fs2");
+	umount(TDIR);
+	exit(err);
+}
+
+void test_test_bpffs(void)
+{
+	int err, duration = 0, status = 0;
+	pid_t pid;
+
+	pid = fork();
+	if (CHECK(pid == -1, "clone", "clone failed %d", errno))
+		return;
+	if (pid == 0)
+		fn();
+	err = waitpid(pid, &status, 0);
+	if (CHECK(err == -1 && errno != ECHILD, "waitpid", "failed %d", errno))
+		return;
+	if (CHECK(WEXITSTATUS(status), "bpffs test ", "failed %d", WEXITSTATUS(status)))
+		return;
+}