[PATCH v3 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[Ilya Leoshkevich <iii@linux.ibm.com>]

test_snprintf_btf fails on s390, because NULL points to a readable
struct lowcore there. Fix by using the last page instead.

Error message example:

    printing 0000000000000000 should generate error, got (361)

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---

v1: https://lore.kernel.org/bpf/20210226135923.114211-1-iii@linux.ibm.com/
v1 -> v2: Yonghong suggested to add the pointer value to the error
          message.
          I've noticed that I've been passing BADPTR as flags, therefore
          the fix worked only by accident. Put it into p.ptr where it
          belongs.

v2: https://lore.kernel.org/bpf/20210226182014.115347-1-iii@linux.ibm.com/
v2 -> v3: Heiko mentioned that using _REGION1_SIZE is not future-proof.
          We had a private discussion and came to the conclusion that
          the the last page is good enough.

 .../testing/selftests/bpf/progs/netif_receive_skb.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/netif_receive_skb.c b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
index 6b670039ea67..c3669967067e 100644
--- a/tools/testing/selftests/bpf/progs/netif_receive_skb.c
+++ b/tools/testing/selftests/bpf/progs/netif_receive_skb.c
@@ -16,6 +16,13 @@ bool skip = false;
 #define STRSIZE			2048
 #define EXPECTED_STRSIZE	256
 
+#if defined(bpf_target_s390)
+/* NULL points to a readable struct lowcore on s390, so take the last page */
+#define BADPTR			((void *)0xFFFFFFFFFFFFF000ULL)
+#else
+#define BADPTR			0
+#endif
+
 #ifndef ARRAY_SIZE
 #define ARRAY_SIZE(x)	(sizeof(x) / sizeof((x)[0]))
 #endif
@@ -113,11 +120,11 @@ int BPF_PROG(trace_netif_receive_skb, struct sk_buff *skb)
 	}
 
 	/* Check invalid ptr value */
-	p.ptr = 0;
+	p.ptr = BADPTR;
 	__ret = bpf_snprintf_btf(str, STRSIZE, &p, sizeof(p), 0);
 	if (__ret >= 0) {
-		bpf_printk("printing NULL should generate error, got (%d)",
-			   __ret);
+		bpf_printk("printing %p should generate error, got (%d)",
+			   BADPTR, __ret);
 		ret = -ERANGE;
 	}
 
-- 
2.29.2