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

This commit adds 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: Nandakumar Edamana <[email protected]>

Author: nandedamana

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

@@ -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

@@ -0,0 +1,38 @@
+// 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);
+}