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

// 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);

}

// 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";