[PATCH] kernel: bpf: remove dead code

[PATCH] kernel: bpf: remove dead code

[Gustavo A. R. Silva]

Execution cannot reach NET_IP_ALIGN inside the following statement:
ip_align = strict ? 2 : NET_IP_ALIGN

Addresses-Coverity-ID: 1409762
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
---
NOTE: variable ip_align could also be removed and use value 2 directly.

 kernel/bpf/verifier.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1eddb71..94f6e46 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -812,7 +812,7 @@ static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg,
 	 * we force this to 2 which is universally what architectures use
 	 * when they don't set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS.
 	 */
-	ip_align = strict ? 2 : NET_IP_ALIGN;
+	ip_align = 2;
 	if ((ip_align + reg_off + off) % size != 0) {
 		verbose("misaligned packet access off %d+%d+%d size %d\n",
 			ip_align, reg_off, off, size);
-- 
2.5.0

Re: [PATCH] kernel: bpf: remove dead code

[David Miller]

From: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Date: Mon, 22 May 2017 09:07:46 -0500

> Execution cannot reach NET_IP_ALIGN inside the following statement:
> ip_align = strict ? 2 : NET_IP_ALIGN
> 
> Addresses-Coverity-ID: 1409762
> Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
> ---
> NOTE: variable ip_align could also be removed and use value 2 directly.

Incorrect.

Some platforms define NET_IP_ALIGN to zero, so the code must remain
as is.

Re: [PATCH] kernel: bpf: remove dead code

[Gustavo A. R. Silva]

Hi David,

Quoting David Miller <davem@davemloft.net>:

> From: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
> Date: Mon, 22 May 2017 09:07:46 -0500
>
>> Execution cannot reach NET_IP_ALIGN inside the following statement:
>> ip_align = strict ? 2 : NET_IP_ALIGN
>>
>> Addresses-Coverity-ID: 1409762
>> Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
>> ---
>> NOTE: variable ip_align could also be removed and use value 2 directly.
>
> Incorrect.
>
> Some platforms define NET_IP_ALIGN to zero, so the code must remain
> as is.

The following piece of code at kernel/bpf/verifier.c:798 is preventing  
value NET_IP_ALIGN to be stored in variable ip_align when _strict_ is  
false:

798        if (!strict || size == 1)
799                return 0;


Thanks

Re: [PATCH] kernel: bpf: remove dead code

[Daniel Borkmann]

On 05/22/2017 04:38 PM, David Miller wrote:
> From: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
> Date: Mon, 22 May 2017 09:07:46 -0500
>
>> Execution cannot reach NET_IP_ALIGN inside the following statement:
>> ip_align = strict ? 2 : NET_IP_ALIGN
>>
>> Addresses-Coverity-ID: 1409762
>> Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
>> ---
>> NOTE: variable ip_align could also be removed and use value 2 directly.
>
> Incorrect.
>
> Some platforms define NET_IP_ALIGN to zero, so the code must remain
> as is.

In the check_pkt_ptr_alignment(), when !strict you would already
return earlier from that function.

So, above test in ip_align will always give 2, meaning technically
the patch is correct, although hard-coded value less clean.

Perhaps something like the below to keep intentions more clear (and
it will get resolved during compile time anyway ...):

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a098d95..3cf1d60 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2297,8 +2297,10 @@ static inline int pskb_network_may_pull(struct sk_buff *skb, unsigned int len)
   * Since this trade off varies between architectures, we allow NET_IP_ALIGN
   * to be overridden.
   */
+#define NET_IP_ALIGN_DEFAULT	2
+
  #ifndef NET_IP_ALIGN
-#define NET_IP_ALIGN	2
+#define NET_IP_ALIGN	NET_IP_ALIGN_DEFAULT
  #endif

  /*
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1eddb71..61f6aaa 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -812,7 +812,7 @@ static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg,
  	 * we force this to 2 which is universally what architectures use
  	 * when they don't set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS.
  	 */
-	ip_align = strict ? 2 : NET_IP_ALIGN;
+	ip_align = NET_IP_ALIGN ? : NET_IP_ALIGN_DEFAULT;
  	if ((ip_align + reg_off + off) % size != 0) {
  		verbose("misaligned packet access off %d+%d+%d size %d\n",
  			ip_align, reg_off, off, size);

Re: [PATCH] kernel: bpf: remove dead code

[David Miller]

From: Daniel Borkmann <daniel@iogearbox.net>
Date: Mon, 22 May 2017 16:52:24 +0200

> On 05/22/2017 04:38 PM, David Miller wrote:
>> From: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
>> Date: Mon, 22 May 2017 09:07:46 -0500
>>
>>> Execution cannot reach NET_IP_ALIGN inside the following statement:
>>> ip_align = strict ? 2 : NET_IP_ALIGN
>>>
>>> Addresses-Coverity-ID: 1409762
>>> Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
>>> ---
>>> NOTE: variable ip_align could also be removed and use value 2
>>> directly.
>>
>> Incorrect.
>>
>> Some platforms define NET_IP_ALIGN to zero, so the code must remain
>> as is.
> 
> In the check_pkt_ptr_alignment(), when !strict you would already
> return earlier from that function.
> 
> So, above test in ip_align will always give 2, meaning technically
> the patch is correct, although hard-coded value less clean.
> 
> Perhaps something like the below to keep intentions more clear (and
> it will get resolved during compile time anyway ...):

Ok I understand the issue now.  Thanks for explaining.

I guess a hard-coded value of 2 and an adjusted comment above the
assignment of ip_align is the way to go.

I'll push the following, thanks everyone:

====================
net: Make IP alignment calulations clearer.

The assignmnet:

	ip_align = strict ? 2 : NET_IP_ALIGN;

in compare_pkt_ptr_alignment() trips up Coverity because we can only
get to this code when strict is true, therefore ip_align will always
be 2 regardless of NET_IP_ALIGN's value.

So just assign directly to '2' and explain the situation in the
comment above.

Reported-by: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 kernel/bpf/verifier.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1eddb71..c72cd41 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -808,11 +808,15 @@ static int check_pkt_ptr_alignment(const struct bpf_reg_state *reg,
 		reg_off += reg->aux_off;
 	}
 
-	/* skb->data is NET_IP_ALIGN-ed, but for strict alignment checking
-	 * we force this to 2 which is universally what architectures use
-	 * when they don't set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS.
+	/* For platforms that do not have a Kconfig enabling
+	 * CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS the value of
+	 * NET_IP_ALIGN is universally set to '2'.  And on platforms
+	 * that do set CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, we get
+	 * to this code only in strict mode where we want to emulate
+	 * the NET_IP_ALIGN==2 checking.  Therefore use an
+	 * unconditional IP align value of '2'.
 	 */
-	ip_align = strict ? 2 : NET_IP_ALIGN;
+	ip_align = 2;
 	if ((ip_align + reg_off + off) % size != 0) {
 		verbose("misaligned packet access off %d+%d+%d size %d\n",
 			ip_align, reg_off, off, size);
-- 
2.4.11

Re: [PATCH] kernel: bpf: remove dead code

[Daniel Borkmann]

On 05/22/2017 06:27 PM, David Miller wrote:
[...]
> Ok I understand the issue now.  Thanks for explaining.
>
> I guess a hard-coded value of 2 and an adjusted comment above the
> assignment of ip_align is the way to go.
>
> I'll push the following, thanks everyone:
>
> ====================
> net: Make IP alignment calulations clearer.
>
> The assignmnet:
>
> 	ip_align = strict ? 2 : NET_IP_ALIGN;
>
> in compare_pkt_ptr_alignment() trips up Coverity because we can only
> get to this code when strict is true, therefore ip_align will always
> be 2 regardless of NET_IP_ALIGN's value.
>
> So just assign directly to '2' and explain the situation in the
> comment above.
>
> Reported-by: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
> Signed-off-by: David S. Miller <davem@davemloft.net>

Yeah, that's fine, thanks!

Acked-by: Daniel Borkmann <daniel@iogearbox.net>