selftests/bpf: Add three test cases for bits_iter

Add more test cases for bits iterator:

(1) huge word test
Verify the multiplication overflow of nr_bits in bits_iter. Without
the overflow check, when nr_words is 67108865, nr_bits becomes 64,
causing bpf_probe_read_kernel_common() to corrupt the stack.
(2) max word test
Verify correct handling of maximum nr_words value (511).
(3) bad word test
Verify early termination of bits iteration when bits iterator
initialization fails.

Also rename bits_nomem to bits_too_big to better reflect its purpose.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20241030100516.3633640-6-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -15,6 +15,8 @@ int bpf_iter_bits_new(struct bpf_iter_bits *it, const u64 *unsafe_ptr__ign,
 int *bpf_iter_bits_next(struct bpf_iter_bits *it) __ksym __weak;
 void bpf_iter_bits_destroy(struct bpf_iter_bits *it) __ksym __weak;
 
+u64 bits_array[511] = {};
+
 SEC("iter.s/cgroup")
 __description("bits iter without destroy")
 __failure __msg("Unreleased reference")
@@ -110,16 +112,16 @@ int bit_index(void)
 }
 
 SEC("syscall")
-__description("bits nomem")
+__description("bits too big")
 __success __retval(0)
-int bits_nomem(void)
+int bits_too_big(void)
 {
 	u64 data[4];
 	int nr = 0;
 	int *bit;
 
 	__builtin_memset(&data, 0xff, sizeof(data));
-	bpf_for_each(bits, bit, &data[0], 513) /* Be greater than 512 */
+	bpf_for_each(bits, bit, &data[0], 512) /* Be greater than 511 */
 		nr++;
 	return nr;
 }
@@ -151,3 +153,56 @@ int zero_words(void)
 		nr++;
 	return nr;
 }
+
+SEC("syscall")
+__description("huge words")
+__success __retval(0)
+int huge_words(void)
+{
+	u64 data[8] = {0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1, 0x1};
+	int nr = 0;
+	int *bit;
+
+	bpf_for_each(bits, bit, &data[0], 67108865)
+		nr++;
+	return nr;
+}
+
+SEC("syscall")
+__description("max words")
+__success __retval(4)
+int max_words(void)
+{
+	volatile int nr = 0;
+	int *bit;
+
+	bits_array[0] = (1ULL << 63) | 1U;
+	bits_array[510] = (1ULL << 33) | (1ULL << 32);
+
+	bpf_for_each(bits, bit, bits_array, 511) {
+		if (nr == 0 && *bit != 0)
+			break;
+		if (nr == 2 && *bit != 32672)
+			break;
+		nr++;
+	}
+	return nr;
+}
+
+SEC("syscall")
+__description("bad words")
+__success __retval(0)
+int bad_words(void)
+{
+	void *bad_addr = (void *)(3UL << 30);
+	int nr = 0;
+	int *bit;
+
+	bpf_for_each(bits, bit, bad_addr, 1)
+		nr++;
+
+	bpf_for_each(bits, bit, bad_addr, 4)
+		nr++;
+
+	return nr;
+}