verifier_mul.c

 1// SPDX-License-Identifier: GPL-2.0
 2/* Copyright (c) 2025 Nandakumar Edamana */
 3#include <linux/bpf.h>
 4#include <bpf/bpf_helpers.h>
 5#include <bpf/bpf_tracing.h>
 6#include "bpf_misc.h"
 7
 8/* Intended to test the abstract multiplication technique(s) used by
 9 * the verifier. Using assembly to avoid compiler optimizations.
10 */
11SEC("fentry/bpf_fentry_test1")
12void BPF_PROG(mul_precise, int x)
13{
14	/* First, force the verifier to be uncertain about the value:
15	 *     unsigned int a = (bpf_get_prandom_u32() & 0x2) | 0x1;
16	 *
17	 * Assuming the verifier is using tnum, a must be tnum{.v=0x1, .m=0x2}.
18	 * Then a * 0x3 would be m0m1 (m for uncertain). Added imprecision
19	 * would cause the following to fail, because the required return value
20	 * is 0:
21	 *     return (a * 0x3) & 0x4);
22	 */
23	asm volatile ("\
24	call %[bpf_get_prandom_u32];\
25	r0 &= 0x2;\
26	r0 |= 0x1;\
27	r0 *= 0x3;\
28	r0 &= 0x4;\
29	if r0 != 0 goto l0_%=;\
30	r0 = 0;\
31	goto l1_%=;\
32l0_%=:\
33	r0 = 1;\
34l1_%=:\
35"	:
36	: __imm(bpf_get_prandom_u32)
37	: __clobber_all);
38}