1// SPDX-License-Identifier: GPL-2.0
2
3#include <linux/bpf.h>
4#include <bpf/bpf_helpers.h>
5
6int probe_res;
7
8char input[4] = {};
9int test_pid;
10
11SEC("tracepoint/syscalls/sys_enter_nanosleep")
12int probe(void *ctx)
13{
14	/* This BPF program performs variable-offset reads and writes on a
15	 * stack-allocated buffer.
16	 */
17	char stack_buf[16];
18	unsigned long len;
19	unsigned long last;
20
21	if ((bpf_get_current_pid_tgid() >> 32) != test_pid)
22		return 0;
23
24	/* Copy the input to the stack. */
25	__builtin_memcpy(stack_buf, input, 4);
26
27	/* The first byte in the buffer indicates the length. */
28	len = stack_buf[0] & 0xf;
29	last = (len - 1) & 0xf;
30
31	/* Append something to the buffer. The offset where we write is not
32	 * statically known; this is a variable-offset stack write.
33	 */
34	stack_buf[len] = 42;
35
36	/* Index into the buffer at an unknown offset. This is a
37	 * variable-offset stack read.
38	 *
39	 * Note that if it wasn't for the preceding variable-offset write, this
40	 * read would be rejected because the stack slot cannot be verified as
41	 * being initialized. With the preceding variable-offset write, the
42	 * stack slot still cannot be verified, but the write inhibits the
43	 * respective check on the reasoning that, if there was a
44	 * variable-offset to a higher-or-equal spot, we're probably reading
45	 * what we just wrote.
46	 */
47	probe_res = stack_buf[last];
48	return 0;
49}
50
51char _license[] SEC("license") = "GPL";
52