selftests/bpf: Prevent out-of-bounds stack access in test_bpffs

mainline inclusion
from mainline-5.17-rc1
commit 57428298
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=57428298b5acf2ba2dd98359c532774f6eaeecb3

-------------------------------------------------

Buf can be not zero-terminated leading to strstr() to access data beyond
the intended buf[] array. Fix by forcing zero termination.
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-12-andrii@kernel.org
(cherry picked from commit 57428298)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

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

@@ -18,11 +18,13 @@ static int read_iter(char *file)
 	fd = open(file, 0);
 	if (fd < 0)
 		return -1;
-	while ((len = read(fd, buf, sizeof(buf))) > 0)
+	while ((len = read(fd, buf, sizeof(buf))) > 0) {
+		buf[sizeof(buf) - 1] = '\0';
 		if (strstr(buf, "iter")) {
 			close(fd);
 			return 0;
 		}
+	}
 	close(fd);
 	return -1;
 }