[PATCH net-next v5] net: skb: prevent the split of kfree_skb_reason() by gcc

[menglong8.dong@gmail.com]

From: Menglong Dong <imagedong@tencent.com>

Sometimes, gcc will optimize the function by spliting it to two or
more functions. In this case, kfree_skb_reason() is splited to
kfree_skb_reason and kfree_skb_reason.part.0. However, the
function/tracepoint trace_kfree_skb() in it needs the return address
of kfree_skb_reason().

This split makes the call chains becomes:
  kfree_skb_reason() -> kfree_skb_reason.part.0 -> trace_kfree_skb()

which makes the return address that passed to trace_kfree_skb() be
kfree_skb().

Therefore, introduce '__fix_address', which is the combination of
'__noclone' and 'noinline', and apply it to kfree_skb_reason() to
prevent to from being splited or made inline.

(Is it better to simply apply '__noclone oninline' to kfree_skb_reason?
I'm thinking maybe other functions have the same problems)

Meanwhile, wrap 'skb_unref()' with 'unlikely()', as the compiler thinks
it is likely return true and splits kfree_skb_reason().

Signed-off-by: Menglong Dong <imagedong@tencent.com>
---
v5:
- replace optimize("O1") with '__noclone noinline', thanks for
  Segher Boessenkool and Nick Desaulniers's advice.
- wrap 'skb_unref()' with 'unlikely()', as Jakub Kicinski advise

v4:
- move the definition of __nofnsplit to compiler_attributes.h

v3:
- define __nofnsplit only for GCC
- add some document

v2:
- replace 'optimize' with '__optimize__' in __nofnsplit, as Miguel Ojeda
  advised.
---
 include/linux/compiler_attributes.h | 7 +++++++
 include/linux/skbuff.h              | 3 ++-
 net/core/skbuff.c                   | 5 +++--
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 445e80517cab..fc93c9488c76 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -371,4 +371,11 @@
  */
 #define __weak                          __attribute__((__weak__))
 
+/*
+ * Used by functions that use '__builtin_return_address'. These function
+ * don't want to be splited or made inline, which can make
+ * the '__builtin_return_address' get unexpected address.
+ */
+#define __fix_address noinline __noclone
+
 #endif /* __LINUX_COMPILER_ATTRIBUTES_H */
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5d7ff8850eb2..4d0d3b4f0867 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1195,7 +1195,8 @@ static inline bool skb_unref(struct sk_buff *skb)
 	return true;
 }
 
-void kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason);
+void __fix_address
+kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason);
 
 /**
  *	kfree_skb - free an sk_buff with 'NOT_SPECIFIED' reason
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 974bbbbe7138..35d9d5958dc6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -777,9 +777,10 @@ EXPORT_SYMBOL(__kfree_skb);
  *	hit zero. Meanwhile, pass the drop reason to 'kfree_skb'
  *	tracepoint.
  */
-void kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
+void __fix_address
+kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
 {
-	if (!skb_unref(skb))
+	if (unlikely(!skb_unref(skb)))
 		return;
 
 	DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
-- 
2.37.2