[PATCH net-next] flow_dissector: Fix function argument ordering dependency

Commit c6cc1ca7f4d70c ("flowi: Abstract out functions to get flow hash
based on flowi") introduced a bug in __skb_set_sw_hash where we
require a dependency on evaluating arguments in a function in order.
There is no such ordering enforced in C, so this incorrect. This
patch fixes that by splitting out the arguments. This bug was
found via a compiler warning that keys may be uninitialized.

Signed-off-by: Tom Herbert <redacted>
---
 include/linux/skbuff.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 9e62687..eabfb81 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h

@@ -1035,9 +1035,9 @@ static inline __u32 skb_get_hash_flowi6(struct sk_buff *skb, struct flowi6 *fl6)
 {
 	if (!skb->l4_hash && !skb->sw_hash) {
 		struct flow_keys keys;
+		__u32 hash = __get_hash_from_flowi6(fl6, &keys);
 
-		__skb_set_sw_hash(skb, __get_hash_from_flowi6(fl6, &keys),
-				  flow_keys_have_l4(&keys));
+		__skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
 	}
 
 	return skb->hash;

@@ -1049,9 +1049,9 @@ static inline __u32 skb_get_hash_flowi4(struct sk_buff *skb, struct flowi4 *fl4)
 {
 	if (!skb->l4_hash && !skb->sw_hash) {
 		struct flow_keys keys;
+		__u32 hash = __get_hash_from_flowi4(fl4, &keys);
 
-		__skb_set_sw_hash(skb, __get_hash_from_flowi4(fl4, &keys),
-				  flow_keys_have_l4(&keys));
+		__skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
 	}
 
 	return skb->hash;

-- 
1.8.1

On Tue, 2015-09-01 at 18:11 -0700, Tom Herbert wrote:

Commit c6cc1ca7f4d70c ("flowi: Abstract out functions to get flow hash
based on flowi") introduced a bug in __skb_set_sw_hash where we
require a dependency on evaluating arguments in a function in order.
There is no such ordering enforced in C, so this incorrect. This
patch fixes that by splitting out the arguments. This bug was
found via a compiler warning that keys may be uninitialized.

To help the reader know a bit more about how these
functions operate, perhaps it'd also be useful/better
to mark arguments as const where appropriate.

I didn't look to see if any other functions could
have the flow related args as const.

The __get_hash_from_flowi6 and flow_keys_have_l4
functions could be:
---
 include/net/flow.h           | 2 +-
 include/net/flow_dissector.h | 2 +-
 net/core/flow_dissector.c    | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/net/flow.h b/include/net/flow.h
index dafe97c..96fef26 100644
--- a/include/net/flow.h
+++ b/include/net/flow.h

@@ -244,7 +244,7 @@ void flow_cache_flush(struct net *net);
 void flow_cache_flush_deferred(struct net *net);
 extern atomic_t flow_cache_genid;
 
-__u32 __get_hash_from_flowi6(struct flowi6 *fl6, struct flow_keys *keys);
+__u32 __get_hash_from_flowi6(const struct flowi6 *fl6, struct flow_keys *keys);
 
 static inline __u32 get_hash_from_flowi6(struct flowi6 *fl6)
 {

diff --git a/include/net/flow_dissector.h b/include/net/flow_dissector.h
index 8c8548c..9fdcc3f 100644
--- a/include/net/flow_dissector.h
+++ b/include/net/flow_dissector.h

@@ -177,7 +177,7 @@ struct flow_keys_digest {
 void make_flow_keys_digest(struct flow_keys_digest *digest,
 			   const struct flow_keys *flow);
 
-static inline bool flow_keys_have_l4(struct flow_keys *keys)
+static inline bool flow_keys_have_l4(const struct flow_keys *keys)
 {
 	return (keys->ports.ports || keys->tags.flow_label);
 }

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 345a040..ccece96 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c

@@ -787,7 +787,7 @@ u32 skb_get_poff(const struct sk_buff *skb)
 	return __skb_get_poff(skb, skb->data, &keys, skb_headlen(skb));
 }
 
-__u32 __get_hash_from_flowi6(struct flowi6 *fl6, struct flow_keys *keys)
+__u32 __get_hash_from_flowi6(const struct flowi6 *fl6, struct flow_keys *keys)
 {
 	memset(keys, 0, sizeof(*keys));
 

From: Tom Herbert <redacted>
Date: Tue, 1 Sep 2015 18:11:04 -0700

Commit c6cc1ca7f4d70c ("flowi: Abstract out functions to get flow hash
based on flowi") introduced a bug in __skb_set_sw_hash where we
require a dependency on evaluating arguments in a function in order.
There is no such ordering enforced in C, so this incorrect. This
patch fixes that by splitting out the arguments. This bug was
found via a compiler warning that keys may be uninitialized.

Signed-off-by: Tom Herbert <redacted>

Applied.