linux-kernel - Re: [RFC 5/8] x86: refcount: prevent gcc distortions

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Date:   Wed, 16 May 2018 16:37:06 +0000
From:   Nadav Amit <namit@...are.com>
To:     Kees Cook <keescook@...omium.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Jan Beulich <JBeulich@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC 5/8] x86: refcount: prevent gcc distortions

Kees Cook <keescook@...omium.org> wrote:

> On Tue, May 15, 2018 at 7:11 AM, Nadav Amit <namit@...are.com> wrote:
>> GCC considers the number of statements in inlined assembly blocks,
>> according to new-lines and semicolons, as an indication to the cost of
>> the block in time and space. This data is distorted by the kernel code,
>> which puts information in alternative sections. As a result, the
>> compiler may perform incorrect inlining and branch optimizations.
>> 
>> The solution is to set an assembly macro and call it from the inlined
>> assembly block. As a result GCC considers the inline assembly block as
>> a single instruction.
>> 
>> This patch allows to inline functions such as __get_seccomp_filter().
>> The effect of the patch is as follows on the kernel size:
>> 
>>   text    data     bss     dec     hex filename
>> 18146418 10064100 2936832 31147350 1db4556 ./vmlinux before
>> 18148228 10063968 2936832 31149028 1db4be4 ./vmlinux after (+1678)
>> 
>> Static text symbols:
>> Before: 39673
>> After:  39649   (-24)
>> 
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: "H. Peter Anvin" <hpa@...or.com>
>> Cc: x86@...nel.org
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Jan Beulich <JBeulich@...e.com>
>> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
>> 
>> Signed-off-by: Nadav Amit <namit@...are.com>
>> ---
>> arch/x86/include/asm/refcount.h | 55 ++++++++++++++++++++-------------
>> 1 file changed, 33 insertions(+), 22 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
>> index 4cf11d88d3b3..a668c534206d 100644
>> --- a/arch/x86/include/asm/refcount.h
>> +++ b/arch/x86/include/asm/refcount.h
>> @@ -14,34 +14,43 @@
>>  * central refcount exception. The fixup address for the exception points
>>  * back to the regular execution flow in .text.
>>  */
>> -#define _REFCOUNT_EXCEPTION                            \
>> -       ".pushsection .text..refcount\n"                \
>> -       "111:\tlea %[counter], %%" _ASM_CX "\n"         \
>> -       "112:\t" ASM_UD2 "\n"                           \
>> -       ASM_UNREACHABLE                                 \
>> -       ".popsection\n"                                 \
>> -       "113:\n"                                        \
>> +
>> +asm ("\n"
>> +       ".macro __REFCOUNT_EXCEPTION counter:vararg\n\t"
> 
> Why are these vararg?

I don’t think it is needed here. I will fix it.

> 
> Also, I think for the whole series, these #define-a-macro cases need a
> comment in the code. It's not obvious from looking at the code why
> they've defined a macro instead of just leaving the asm as it was.

Right. I will add them.

> Beyond that, as long as there is no behavioral changes, I'm fine with
> the changes.

Thanks!

Nadav