netdev - Re: [PATCH net-next v4] net: skb: prevent the split of kfree_skb

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Message-ID: <87edwo65lw.fsf@oldenburg.str.redhat.com>
Date:   Tue, 06 Sep 2022 14:37:47 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>, kuba@...nel.org,
        miguel.ojeda.sandonis@...il.com, ojeda@...nel.org,
        davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com,
        asml.silence@...il.com, imagedong@...cent.com,
        luiz.von.dentz@...el.com, vasily.averin@...ux.dev,
        jk@...econstruct.com.au, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, kernel test robot <lkp@...el.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH net-next v4] net: skb: prevent the split of
 kfree_skb_reason() by gcc

* Menglong Dong:

> Hello,
>
> On Mon, Aug 22, 2022 at 4:01 PM Florian Weimer <fweimer@...hat.com> wrote:
>>
>> * Menglong Dong:
>>
>> > /*
>> >  * 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' got unexpected address.
>> >  */
>> > #define __fix_address noinline __noclone
>>
>> You need something on the function *declaration* as well, to inhibit
>> sibcalls.
>>
>
> I did some research on the 'sibcalls' you mentioned above. Feel like
> It's a little similar to 'inline', and makes the callee use the same stack
> frame with the caller, which obviously will influence the result of
> '__builtin_return_address'.
>
> Hmm......but I'm not able to find any attribute to disable this optimization.
> Do you have any ideas?

Unless something changed quite recently, GCC does not allow disabling
the optimization with a simple attribute (which would have to apply to
function pointers as well, not functions).  asm ("") barriers that move
out a call out of the tail position are supposed to prevent the
optimization.

Thanks,
Florian