selftests/bpf: Improve test coverage for kfunc call

On powerpc, immediate load instructions are sign extended. In case
of unsigned types, arguments should be explicitly zero-extended by
the caller. For kfunc call, this needs to be handled in the JIT code.
In bpf_kfunc_call_test4(), that tests for sign-extension of signed
argument types in kfunc calls, add some additional failure checks.
And add bpf_kfunc_call_test5() to test zero-extension of unsigned
argument types in kfunc calls.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260312080113.843408-1-hbathini@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -74,6 +74,8 @@ static struct kfunc_test_params kfunc_tests[] = {
 	TC_TEST(kfunc_call_test1, 12),
 	TC_TEST(kfunc_call_test2, 3),
 	TC_TEST(kfunc_call_test4, -1234),
+	TC_TEST(kfunc_call_test5, 0),
+	TC_TEST(kfunc_call_test5_asm, 0),
 	TC_TEST(kfunc_call_test_ref_btf_id, 0),
 	TC_TEST(kfunc_call_test_get_mem, 42),
 	SYSCALL_TEST(kfunc_syscall_test, 0),

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

@@ -2,8 +2,106 @@
 /* Copyright (c) 2021 Facebook */
 #include <vmlinux.h>
 #include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
 #include "../test_kmods/bpf_testmod_kfunc.h"
 
+SEC("tc")
+int kfunc_call_test5(struct __sk_buff *skb)
+{
+	struct bpf_sock *sk = skb->sk;
+	int ret;
+	u32 val32;
+	u16 val16;
+	u8 val8;
+
+	if (!sk)
+		return -1;
+
+	sk = bpf_sk_fullsock(sk);
+	if (!sk)
+		return -1;
+
+	/*
+	 * Test with constant values to verify zero-extension.
+	 * ISA-dependent BPF asm:
+	 *   With ALU32:    w1 = 0xFF; w2 = 0xFFFF; w3 = 0xFFFFffff
+	 *   Without ALU32: r1 = 0xFF; r2 = 0xFFFF; r3 = 0xFFFFffff
+	 * Both zero-extend to 64-bit before the kfunc call.
+	 */
+	ret = bpf_kfunc_call_test5(0xFF, 0xFFFF, 0xFFFFffffULL);
+	if (ret)
+		return ret;
+
+	val32 = bpf_get_prandom_u32();
+	val16 = val32 & 0xFFFF;
+	val8 = val32 & 0xFF;
+	ret = bpf_kfunc_call_test5(val8, val16, val32);
+	if (ret)
+		return ret;
+
+	/*
+	 * Test multiplication with different operand sizes:
+	 *
+	 * val8 * 0xFF:
+	 *   - Both operands promote to int (32-bit signed)
+	 *   - Result: 32-bit multiplication, truncated to u8, then zero-extended
+	 *
+	 * val16 * 0xFFFF:
+	 *   - Both operands promote to int (32-bit signed)
+	 *   - Result: 32-bit multiplication, truncated to u16, then zero-extended
+	 *
+	 * val32 * 0xFFFFffffULL:
+	 *   - val32 (u32) promotes to unsigned long long (due to ULL suffix)
+	 *   - Result: 64-bit unsigned multiplication, truncated to u32, then zero-extended
+	 */
+	ret = bpf_kfunc_call_test5(val8 * 0xFF, val16 * 0xFFFF, val32 * 0xFFFFffffULL);
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
+/*
+ * Assembly version testing the multiplication edge case explicitly.
+ * This ensures consistent testing across different ISA versions.
+ */
+SEC("tc")
+__naked int kfunc_call_test5_asm(void)
+{
+	asm volatile (
+		/* Get a random u32 value */
+		"call %[bpf_get_prandom_u32];"
+		"r6 = r0;"              /* Save val32 in r6 */
+
+		/* Prepare first argument: val8 * 0xFF */
+		"r1 = r6;"
+		"r1 &= 0xFF;"           /* val8 = val32 & 0xFF */
+		"r7 = 0xFF;"
+		"r1 *= r7;"             /* 64-bit mult: r1 = r1 * r7 */
+
+		/* Prepare second argument: val16 * 0xFFFF */
+		"r2 = r6;"
+		"r2 &= 0xFFFF;"         /* val16 = val32 & 0xFFFF */
+		"r7 = 0xFFFF;"
+		"r2 *= r7;"             /* 64-bit mult: r2 = r2 * r7 */
+
+		/* Prepare third argument: val32 * 0xFFFFffff */
+		"r3 = r6;"              /* val32 */
+		"r7 = 0xFFFFffff;"
+		"r3 *= r7;"             /* 64-bit mult: r3 = r3 * r7 */
+
+		/* Call kfunc with multiplication results */
+		"call bpf_kfunc_call_test5;"
+
+		/* Check return value */
+		"if r0 != 0 goto exit_%=;"
+		"r0 = 0;"
+		"exit_%=: exit;"
+		:
+		: __imm(bpf_get_prandom_u32)
+		: __clobber_all);
+}
+
 SEC("tc")
 int kfunc_call_test4(struct __sk_buff *skb)
 {

tools/testing/selftests/bpf/test_kmods/bpf_testmod.c

@@ -760,12 +760,63 @@ __bpf_kfunc struct sock *bpf_kfunc_call_test3(struct sock *sk)
 
 __bpf_kfunc long noinline bpf_kfunc_call_test4(signed char a, short b, int c, long d)
 {
-	/* Provoke the compiler to assume that the caller has sign-extended a,
+	/*
+	 * Make val as volatile to avoid compiler optimizations.
+	 * Verify that negative signed values remain negative after
+	 * sign-extension (JIT must sign-extend, not zero-extend).
+	 */
+	volatile long val;
+
+	/* val will be positive, if JIT does zero-extension instead of sign-extension */
+	val = a;
+	if (val >= 0)
+		return 1;
+
+	val = b;
+	if (val >= 0)
+		return 2;
+
+	val = c;
+	if (val >= 0)
+		return 3;
+
+	/*
+	 * Provoke the compiler to assume that the caller has sign-extended a,
 	 * b and c on platforms where this is required (e.g. s390x).
 	 */
 	return (long)a + (long)b + (long)c + d;
 }
 
+__bpf_kfunc int bpf_kfunc_call_test5(u8 a, u16 b, u32 c)
+{
+	/*
+	 * Make val as volatile to avoid compiler optimizations on the below checks
+	 * In C, assigning u8/u16/u32 to long performs zero-extension.
+	 */
+	volatile long val = a;
+
+	/* Check zero-extension */
+	if (val != (unsigned long)a)
+		return 1;
+	/* Check no sign-extension */
+	if (val < 0)
+		return 2;
+
+	val = b;
+	if (val != (unsigned long)b)
+		return 3;
+	if (val < 0)
+		return 4;
+
+	val = c;
+	if (val != (unsigned long)c)
+		return 5;
+	if (val < 0)
+		return 6;
+
+	return 0;
+}
+
 static struct prog_test_ref_kfunc prog_test_struct = {
 	.a = 42,
 	.b = 108,
@@ -1228,6 +1279,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test2)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test3)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test4)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test5)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_pass1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail1)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_mem_len_fail2)

tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h

@@ -110,6 +110,7 @@ __u64 bpf_kfunc_call_test1(struct sock *sk, __u32 a, __u64 b,
 int bpf_kfunc_call_test2(struct sock *sk, __u32 a, __u32 b) __ksym;
 struct sock *bpf_kfunc_call_test3(struct sock *sk) __ksym;
 long bpf_kfunc_call_test4(signed char a, short b, int c, long d) __ksym;
+int bpf_kfunc_call_test5(__u8 a, __u16 b, __u32 c) __ksym;
 
 void bpf_kfunc_call_test_pass_ctx(struct __sk_buff *skb) __ksym;
 void bpf_kfunc_call_test_pass1(struct prog_test_pass1 *p) __ksym;