// 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";