linux-kernel - Re: RFC: Add -mshared option to x86 ELF assembler

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Message-ID: <CAMe9rOqPabGQTSsK_zaH6gyF-rNKK6ue+HOpMXD03zpO8z7_gA@mail.gmail.com>
Date:	Wed, 13 May 2015 04:50:33 -0700
From:	"H.J. Lu" <hjl.tools@...il.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"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: RFC: Add -mshared option to x86 ELF assembler

On Tue, May 12, 2015 at 5:14 PM, H.J. Lu <hjl.tools@...il.com> wrote:
> 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.
>>
>
> Here is a patch to change the assembler default to optimize out
> relocations to defined non-weak global branch targets with default
> visibility.  It will generate slightly smaller object files.  But Linux
> kernel will be broken unless early_idt_handler is marked weak.
> I am little uncomfortable with -mshare and I don't like -mno-shared
> very much either.  I may just simply remove -mno-shared.
>

I reverted the -mno-shared change.

-- 
H.J.
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