tools/testing/selftests/bpf/progs/verifier_mul.c

// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2025 Nandakumar Edamana */
#include <linux/bpf.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include "bpf_misc.h"

/* Intended to test the abstract multiplication technique(s) used by
 * the verifier. Using assembly to avoid compiler optimizations.
 */
SEC("fentry/bpf_fentry_test1")
void BPF_PROG(mul_precise, int x)
{
	/* First, force the verifier to be uncertain about the value:
	 *     unsigned int a = (bpf_get_prandom_u32() & 0x2) | 0x1;
	 *
	 * Assuming the verifier is using tnum, a must be tnum{.v=0x1, .m=0x2}.
	 * Then a * 0x3 would be m0m1 (m for uncertain). Added imprecision
	 * would cause the following to fail, because the required return value
	 * is 0:
	 *     return (a * 0x3) & 0x4);
	 */
	asm volatile ("\
	call %[bpf_get_prandom_u32];\
	r0 &= 0x2;\
	r0 |= 0x1;\
	r0 *= 0x3;\
	r0 &= 0x4;\
	if r0 != 0 goto l0_%=;\
	r0 = 0;\
	goto l1_%=;\
l0_%=:\
	r0 = 1;\
l1_%=:\
"	:
	: __imm(bpf_get_prandom_u32)
	: __clobber_all);
}