linux-kernel - Re: Avoiding unnecessary jump relocations in gas?

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Message-ID: <CALCETrVSN7Mt_tRQ1_x=mvYD-ygoCHM+juvi80bhddaUEDDxdA@mail.gmail.com>
Date:	Mon, 18 May 2015 12:36:44 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H.J. Lu" <hjl.tools@...il.com>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Binutils <binutils@...rceware.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Avoiding unnecessary jump relocations in gas?

On Fri, May 8, 2015 at 1:16 PM, H.J. Lu <hjl.tools@...il.com> wrote:
> On Fri, May 8, 2015 at 5:09 AM, H.J. Lu <hjl.tools@...il.com> wrote:
>> On Thu, May 7, 2015 at 8:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Thu, May 7, 2015 at 9:21 AM, H.J. Lu <hjl.tools@...il.com> wrote:
>>>> On Thu, May 7, 2015 at 4:52 AM, Jan Beulich <JBeulich@...e.com> wrote:
>>>>>>>> On 07.05.15 at 08:02, <luto@...capital.net> wrote:
>>>>>> AFAICT gas will produce relocations for jumps to global labels in the
>>>>>> same file.  This doesn't seem directly harmful to me, except that, on
>>>>>> x86, it forces five-byte jumps instead of two-byte jumps.
>>>>>>
>>>>>> This seems especially unfortunate, since even hidden and protected
>>>>>> symbols have this problem.
>>>>>>
>>>>>> Given that many users don't want interposition support (especially the
>>>>>> kernel and anyone using .hidden or .protected), it would be nice to
>>>>>> have a command-line option to turn this off and probably also to turn
>>>>>> it off by default for hidden and protected symbols.  Can gas do this?
>>>>>
>>>>> I've been running with the below changes (taken off of a bigger set
>>>>> of changes, so the line numbers may look a little odd) for the last
>>>>> couple of years. I never tried to submit this change because so far
>>>>> I couldn't find the time to check whether this would have any
>>>>> unwanted side effects on cases I don't normally use.
>>>>>
>>>>
>>>> This is the patch I checked in.
>>>>
>>>> Thanks.
>>>>
>>>> --
>>>> H.J.
>>>> ---
>>>> Branches to global non-weak symbols defined in the same segment with
>>>> non-default visibility can be optimized the same way as branches to
>>>> local symbols.
>>>
>>> Would it make sense to also add a command line option along the lines
>>> of gcc's -fno-semantic-interposition or some way to override the
>>> default visibility?  AFAICS this patch helps but only if asm code gets
>>> liberally sprinkled with .hidden or .protected directives.
>>>
>>
>> This is what I checked in.  With
>>
>> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>> index 2fda005..186e6f7 100644
>> --- a/arch/x86/Makefile
>> +++ b/arch/x86/Makefile
>> @@ -107,6 +107,10 @@ else
>>          KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args)
>>  endif
>>
>> +NO_SHARED_CFLAGS = $(call as-option,-Wa$(comma)-mno-shared)
>> +KBUILD_CFLAGS += $(NO_SHARED_CFLAGS)
>> +KBUILD_AFLAGS += $(NO_SHARED_CFLAGS)
>> +
>>  # Make sure compiler does not have buggy stack-protector support.
>>  ifdef CONFIG_CC_STACKPROTECTOR
>>    cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh
>>
>> On kernel master branch, I got
>>
>>    text   data    bss    dec    hex filename
>> 10934167 2275232 1609728 14819127 e21f37 vmlinux.old
>> 10934119 2275232 1609728 14819079 e21f07 vmlinux
>>
>> It saves 48 bytes.
>
> This is before I fixed:
>
> /* This is global to keep gas from relaxing the jumps */
> ENTRY(early_idt_handler)
>         cld
>
> in arch/x86/kernel/head_64.S.  With -mno-shared, we must
> make early_idt_handler weak to keep gas from relaxing the jumps.
>

I wonder if it would make sense to have explicit mnemonics for the
one-byte offset and four-byte offset jump variants.  Sometimes users
want a jump with a 32-bit offset for reasons that have nothing to do
with link-time or load-time relocations.

--Andy
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