selftests/bpf: Use -4095 as the bad address for bits iterator

As reported by Byeonguk, the bad_words test in verifier_bits_iter.c
occasionally fails on s390 host. Quoting Ilya's explanation:

  s390 kernel runs in a completely separate address space, there is no
  user/kernel split at TASK_SIZE. The same address may be valid in both
  the kernel and the user address spaces, there is no way to tell by
  looking at it. The config option related to this property is
  ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.

  Also, unfortunately, 0 is a valid address in the s390 kernel address
  space.

Fix the issue by using -4095 as the bad address for bits iterator, as
suggested by Ilya. Verify that bpf_iter_bits_new() returns -EINVAL for
NULL address and -EFAULT for bad address.

Fixes: ebafc1e ("selftests/bpf: Add three test cases for bits_iter")
Reported-by: Byeonguk Jeong <[email protected]>
Closes: https://lore.kernel.org/bpf/ZycSXwjH4UTvx-Cn@ub22/
Signed-off-by: Hou Tao <[email protected]>
Acked-by: Ilya Leoshkevich <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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

@@ -57,9 +57,15 @@ __description("null pointer")
 __success __retval(0)
 int null_pointer(void)
 {
-	int nr = 0;
+	struct bpf_iter_bits iter;
+	int err, nr = 0;
 	int *bit;
 
+	err = bpf_iter_bits_new(&iter, NULL, 1);
+	bpf_iter_bits_destroy(&iter);
+	if (err != -EINVAL)
+		return 1;
+
 	bpf_for_each(bits, bit, NULL, 1)
 		nr++;
 	return nr;
@@ -194,15 +200,33 @@ __description("bad words")
 __success __retval(0)
 int bad_words(void)
 {
-	void *bad_addr = (void *)(3UL << 30);
-	int nr = 0;
+	void *bad_addr = (void *)-4095;
+	struct bpf_iter_bits iter;
+	volatile int nr;
 	int *bit;
+	int err;
+
+	err = bpf_iter_bits_new(&iter, bad_addr, 1);
+	bpf_iter_bits_destroy(&iter);
+	if (err != -EFAULT)
+		return 1;
 
+	nr = 0;
 	bpf_for_each(bits, bit, bad_addr, 1)
 		nr++;
+	if (nr != 0)
+		return 2;
 
+	err = bpf_iter_bits_new(&iter, bad_addr, 4);
+	bpf_iter_bits_destroy(&iter);
+	if (err != -EFAULT)
+		return 3;
+
+	nr = 0;
 	bpf_for_each(bits, bit, bad_addr, 4)
 		nr++;
+	if (nr != 0)
+		return 4;
 
-	return nr;
+	return 0;
 }