Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb_reason() by gcc

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Aug 15, 2022 at 8:29 PM <menglong8.dong@gmail.com> wrote:
>
> 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().

Does the existing __noclone function attribute help at all here?

If not, surely there's an attribute that's more precise than "disable
most optimization outright."

https://unix.stackexchange.com/questions/223013/function-symbol-gets-part-suffix-after-compilation
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-noclone-function-attribute

Perhaps noipa might also work here?

>
> 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>
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
> ---
> 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 | 19 +++++++++++++++++++
>  net/core/skbuff.c                   |  3 ++-
>  2 files changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 445e80517cab..968cbafa2421 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -270,6 +270,25 @@
>   */
>  #define __noreturn                      __attribute__((__noreturn__))
>
> +/*
> + * Optional: not supported by clang.
> + * Optional: not supported by icc.
> + *
> + * Prevent function from being splited to multiple part. As what the
> + * document says in gcc/ipa-split.cc, single function will be splited
> + * when necessary:
> + *
> + *   https://github.com/gcc-mirror/gcc/blob/master/gcc/ipa-split.cc
> + *
> + * This optimization seems only take effect on O2 and O3 optimize level.
> + * Therefore, make the optimize level to O1 to prevent this optimization.
> + */
> +#if __has_attribute(__optimize__)
> +# define __nofnsplit                   __attribute__((__optimize__("O1")))
> +#else
> +# define __nofnsplit
> +#endif
> +
>  /*
>   * 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
>


-- 
Thanks,
~Nick Desaulniers