selftest/bpf: Add test for var-offset stack access

Add a higher-level test (C BPF program) for the new functionality -
variable access stack reads and writes.
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210207011027.676572-5-andreimatei1@gmail.com

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

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "test_stack_var_off.skel.h"
+
+/* Test read and writes to the stack performed with offsets that are not
+ * statically known.
+ */
+void test_stack_var_off(void)
+{
+	int duration = 0;
+	struct test_stack_var_off *skel;
+
+	skel = test_stack_var_off__open_and_load();
+	if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+		return;
+
+	/* Give pid to bpf prog so it doesn't trigger for anyone else. */
+	skel->bss->test_pid = getpid();
+	/* Initialize the probe's input. */
+	skel->bss->input[0] = 2;
+	skel->bss->input[1] = 42;  /* This will be returned in probe_res. */
+
+	if (!ASSERT_OK(test_stack_var_off__attach(skel), "skel_attach"))
+		goto cleanup;
+
+	/* Trigger probe. */
+	usleep(1);
+
+	if (CHECK(skel->bss->probe_res != 42, "check_probe_res",
+		  "wrong probe res: %d\n", skel->bss->probe_res))
+		goto cleanup;
+
+cleanup:
+	test_stack_var_off__destroy(skel);
+}

tools/testing/selftests/bpf/progs/test_stack_var_off.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+int probe_res;
+
+char input[4] = {};
+int test_pid;
+
+SEC("tracepoint/syscalls/sys_enter_nanosleep")
+int probe(void *ctx)
+{
+	/* This BPF program performs variable-offset reads and writes on a
+	 * stack-allocated buffer.
+	 */
+	char stack_buf[16];
+	unsigned long len;
+	unsigned long last;
+
+	if ((bpf_get_current_pid_tgid() >> 32) != test_pid)
+		return 0;
+
+	/* Copy the input to the stack. */
+	__builtin_memcpy(stack_buf, input, 4);
+
+	/* The first byte in the buffer indicates the length. */
+	len = stack_buf[0] & 0xf;
+	last = (len - 1) & 0xf;
+
+	/* Append something to the buffer. The offset where we write is not
+	 * statically known; this is a variable-offset stack write.
+	 */
+	stack_buf[len] = 42;
+
+	/* Index into the buffer at an unknown offset. This is a
+	 * variable-offset stack read.
+	 *
+	 * Note that if it wasn't for the preceding variable-offset write, this
+	 * read would be rejected because the stack slot cannot be verified as
+	 * being initialized. With the preceding variable-offset write, the
+	 * stack slot still cannot be verified, but the write inhibits the
+	 * respective check on the reasoning that, if there was a
+	 * variable-offset to a higher-or-equal spot, we're probably reading
+	 * what we just wrote.
+	 */
+	probe_res = stack_buf[last];
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";