Commit 2660b9d4 authored by Nandakumar Edamana's avatar Nandakumar Edamana Committed by Andrii Nakryiko
Browse files

bpf: Add selftest to check the verifier's abstract multiplication 



Add new selftest to test the abstract multiplication technique(s) used
by the verifier, following the recent improvement in tnum
multiplication (tnum_mul). One of the newly added programs,
verifier_mul/mul_precise, results in a false positive with the old
tnum_mul, while the program passes with the latest one.

Signed-off-by: default avatarNandakumar Edamana <nandakumar@nandakumar.co.in>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Reviewed-by: default avatarHarishankar Vishwanathan <harishankar.vishwanathan@gmail.com>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20250826034524.2159515-2-nandakumar@nandakumar.co.in
parent 1df7dad4

tools/testing/selftests/bpf/prog_tests/verifier.c

+2 −0
Original line number Diff line number Diff line
@@ -59,6 +59,7 @@ 
#include "verifier_meta_access.skel.h"

#include "verifier_movsx.skel.h"

#include "verifier_mtu.skel.h"

#include "verifier_mul.skel.h"

#include "verifier_netfilter_ctx.skel.h"

#include "verifier_netfilter_retcode.skel.h"

#include "verifier_bpf_fastcall.skel.h"
@@ -194,6 +195,7 @@ void test_verifier_may_goto_1(void) { RUN(verifier_may_goto_1); } 
void test_verifier_may_goto_2(void)           { RUN(verifier_may_goto_2); }

void test_verifier_meta_access(void)          { RUN(verifier_meta_access); }

void test_verifier_movsx(void)                 { RUN(verifier_movsx); }

void test_verifier_mul(void)                  { RUN(verifier_mul); }

void test_verifier_netfilter_ctx(void)        { RUN(verifier_netfilter_ctx); }

void test_verifier_netfilter_retcode(void)    { RUN(verifier_netfilter_retcode); }

void test_verifier_bpf_fastcall(void)         { RUN(verifier_bpf_fastcall); }

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

0 → 100644
+38 −0
Original line number Diff line number Diff line 
// 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);

}