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From:   Florian Weimer <fweimer@redhat.com>
To:     Menglong Dong <menglong8.dong@gmail.com>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        Nick Desaulniers <ndesaulniers@google.com>, kuba@kernel.org,
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        netdev@vger.kernel.org, kernel test robot <lkp@intel.com>,
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Subject: Re: [PATCH net-next v4] net: skb: prevent the split of
 kfree_skb_reason() by gcc
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Date:   Tue, 06 Sep 2022 14:37:47 +0200
In-Reply-To: <CADxym3Z7WpPbX7VSZqVd+nVnbaO6HvxV7ak58TXBCqBqodU+Jg@mail.gmail.com>
        (Menglong Dong's message of "Wed, 24 Aug 2022 00:23:02 +0800")
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* Menglong Dong:

> Hello,
>
> On Mon, Aug 22, 2022 at 4:01 PM Florian Weimer <fweimer@redhat.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