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1// SPDX-License-Identifier: GPL-2.02 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 a15 * 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 not32 * 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 a37 * variable-offset stack read.38 *39 * Note that if it wasn't for the preceding variable-offset write, this40 * read would be rejected because the stack slot cannot be verified as41 * being initialized. With the preceding variable-offset write, the42 * stack slot still cannot be verified, but the write inhibits the43 * respective check on the reasoning that, if there was a44 * variable-offset to a higher-or-equal spot, we're probably reading45 * what we just wrote.46 */47 probe_res = stack_buf[last];48 return 0;49}50 51char _license[] SEC("license") = "GPL";52