[PATCH net-next v2] 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, prevent this kind of optimization to kfree_skb_reason() by
making the optimize level to "O1". I think these should be better
method instead of this "O1", but I can't figure it out......

This optimization CAN happen, which depend on the behavior of gcc.
I'm not able to reproduce it in the latest kernel code, but it happens
in my kernel of version 5.4.119. Maybe the latest code already do someting
that prevent this happen?

Signed-off-by: Menglong Dong <imagedong@tencent.com>
---
v2:
- replace 'optimize' with '__optimize__' in __nofnsplit, as Miguel Ojeda
  advised.
---
 include/linux/compiler_attributes.h | 2 ++
 net/core/skbuff.c                   | 3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 445e80517cab..b910b5775fc7 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -270,6 +270,8 @@
  */
 #define __noreturn                      __attribute__((__noreturn__))
 
+#define __nofnsplit                     __attribute__((__optimize__("O1")))
+
 /*
  * Optional: not supported by gcc.
  * Optional: not supported by icc.
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 974bbbbe7138..ff9ccbc032b9 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -777,7 +777,8 @@ 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 __nofnsplit
+kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
 {
 	if (!skb_unref(skb))
 		return;
-- 
2.36.1