[PATCH v4 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[PATCH v4 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[Ilya Leoshkevich]

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 fffffffffffff000 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.

v3: https://lore.kernel.org/bpf/20210226190908.115706-1-iii@linux.ibm.com/
v3 -> v4: Yonghong suggested to print the non-hashed pointer value.

 .../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..1d8918dfbd3f 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 %llx should generate error, got (%d)",
+			   (unsigned long long)BADPTR, __ret);
 		ret = -ERANGE;
 	}
 
-- 
2.29.2

Re: [PATCH v4 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[Heiko Carstens]

On Sat, Feb 27, 2021 at 06:17:26AM +0100, Ilya Leoshkevich wrote:
> 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 fffffffffffff000 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.
> 
> v3: https://lore.kernel.org/bpf/20210226190908.115706-1-iii@linux.ibm.com/
> v3 -> v4: Yonghong suggested to print the non-hashed pointer value.
> 
>  .../testing/selftests/bpf/progs/netif_receive_skb.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)

Just in case, also for v4:

Acked-by: Heiko Carstens <hca@linux.ibm.com>

Re: [PATCH v4 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[Yonghong Song]

On 2/26/21 9:17 PM, Ilya Leoshkevich wrote:
> 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 fffffffffffff000 should generate error, got (361)
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Acked-by: Yonghong Song <yhs@fb.com>

Re: [PATCH v4 bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf.git (refs/heads/master):

On Sat, 27 Feb 2021 06:17:26 +0100 you wrote:
> 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 fffffffffffff000 should generate error, got (361)
> 
> [...]

Here is the summary with links:
  - [v4,bpf-next] selftests/bpf: Use the last page in test_snprintf_btf on s390
    https://git.kernel.org/bpf/bpf/c/42a382a466a9

You are awesome, thank you!
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