1 // SPDX-License-Identifier: GPL-2.0
2 
3 #include <linux/bpf.h>
4 #include <bpf/bpf_helpers.h>
5 
6 int probe_res;
7 
8 char input[4] = {};
9 int test_pid;
10 
11 SEC("tracepoint/syscalls/sys_enter_nanosleep")
12 int 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 
51 char _license[] SEC("license") = "GPL";
52