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