// 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";