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

From: Menglong Dong <redacted>

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 <redacted>
---
 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..51f7c13bca98 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h

@@ -371,4 +371,6 @@
  */
 #define __weak                          __attribute__((__weak__))
 
+#define __nofnsplit                     __attribute__((optimize("O1")))
+
 #endif /* __LINUX_COMPILER_ATTRIBUTES_H */

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

On Thu, Aug 11, 2022 at 2:07 PM [off-list ref] wrote:

Two notes on this: please use the double underscore form:
`__optimize__` and keep the file sorted (it should go after
`__overloadable__`, since we sort by the actual attribute name).

Thanks!

Cheers,
Miguel

On Thu, Aug 11, 2022 at 4:34 PM Miguel Ojeda
[off-list ref] wrote:

Two notes on this: please use the double underscore form:
`__optimize__` and keep the file sorted (it should go after
`__overloadable__`, since we sort by the actual attribute name).

s/after/before

Cheers,
Miguel

Hi,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on net-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/menglong8-dong-gmail-com/net-skb-prevent-the-split-of-kfree_skb_reason-by-gcc/20220811-200919
base:   https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git f86d1fbbe7858884d6754534a0afbb74fc30bc26
config: riscv-randconfig-r042-20220811 (https://download.01.org/0day-ci/archive/20220812/202208120238.cz3RvV74-lkp@intel.com/config)
compiler: clang version 16.0.0 (https://github.com/llvm/llvm-project 5f1c7e2cc5a3c07cbc2412e851a7283c1841f520)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install riscv cross compiling tool for clang build
        # apt-get install binutils-riscv64-linux-gnu
        # https://github.com/intel-lab-lkp/linux/commit/8e014b654d9e51abed132155237d855b25da06a0
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review menglong8-dong-gmail-com/net-skb-prevent-the-split-of-kfree_skb_reason-by-gcc/20220811-200919
        git checkout 8e014b654d9e51abed132155237d855b25da06a0
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=riscv SHELL=/bin/bash net/core/

If you fix the issue, kindly add following tag where applicable
Reported-by: kernel test robot <redacted>

All warnings (new ones prefixed by >>):

quoted

net/core/skbuff.c:780:6: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]

   void __nofnsplit
        ^~~~~~~~~~~
   include/linux/compiler_attributes.h:374:56: note: expanded from macro '__nofnsplit'
   #define __nofnsplit                     __attribute__((optimize("O1")))
                                                          ^~~~~~~~~~~~~~
   1 warning generated.


vim +/optimize +780 net/core/skbuff.c

   770	
   771	/**
   772	 *	kfree_skb_reason - free an sk_buff with special reason
   773	 *	@skb: buffer to free
   774	 *	@reason: reason why this skb is dropped
   775	 *
   776	 *	Drop a reference to the buffer and free it if the usage count has
   777	 *	hit zero. Meanwhile, pass the drop reason to 'kfree_skb'
   778	 *	tracepoint.
   779	 */
 > 780	void __nofnsplit
   781	kfree_skb_reason(struct sk_buff *skb, enum skb_drop_reason reason)
   782	{
   783		if (!skb_unref(skb))
   784			return;
   785	
   786		DEBUG_NET_WARN_ON_ONCE(reason <= 0 || reason >= SKB_DROP_REASON_MAX);
   787	
   788		trace_kfree_skb(skb, __builtin_return_address(0), reason);
   789		__kfree_skb(skb);
   790	}
   791	EXPORT_SYMBOL(kfree_skb_reason);
   792	

-- 
0-DAY CI Kernel Test Service
https://01.org/lkp

On Thu, Aug 11, 2022 at 10:35 PM Miguel Ojeda
[off-list ref] wrote:

On Thu, Aug 11, 2022 at 4:34 PM Miguel Ojeda
[off-list ref] wrote:

quoted

Two notes on this: please use the double underscore form:
`__optimize__` and keep the file sorted (it should go after
`__overloadable__`, since we sort by the actual attribute name).

s/after/before

Okay......Thanks for your reminds :/

Menglong Dong

Cheers,
Miguel