linux-kernel - Re: [committed, PATCH] Add -mshared option to x86 ELF assembler

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Date:	Tue, 2 Jun 2015 09:41:12 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H.J. Lu" <hjl.tools@...il.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Binutils <binutils@...rceware.org>,
	Andy Lutomirski <luto@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [committed, PATCH] Add -mshared option to x86 ELF assembler


* Andy Lutomirski <luto@...capital.net> wrote:

> On Mon, Jun 1, 2015 at 4:05 AM, H.J. Lu <hjl.tools@...il.com> wrote:
> > On Mon, Jun 1, 2015 at 1:32 AM, Markus Trippelsdorf
> > <markus@...ppelsdorf.de> wrote:
> >> On 2015.05.15 at 03:34 -0700, H.J. Lu wrote:
> >>> This patch adds -mshared option to x86 ELF assembler.  By default,
> >>> assembler will optimize out non-PLT relocations against defined non-weak
> >>> global branch targets with default visibility.  The -mshared option tells
> >>> the assembler to generate code which may go into a shared library
> >>> where all non-weak global branch targets with default visibility can
> >>> be preempted.  The resulting code is slightly bigger.  This option
> >>> only affects the handling of branch instructions.
> >>>
> >>> This Linux kernel patch is needed to create a working x86 Linux kernel if
> >>> it hasn't been applied:
> >>>
> >>> diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
> >>> index ae6588b..b91a00c 100644
> >>> --- a/arch/x86/kernel/head_64.S
> >>> +++ b/arch/x86/kernel/head_64.S
> >>> @@ -339,8 +339,8 @@ early_idt_handlers:
> >>>       i = i + 1
> >>>       .endr
> >>>
> >>> -/* This is global to keep gas from relaxing the jumps */
> >>> -ENTRY(early_idt_handler)
> >>> +/* This is weak to keep gas from relaxing the jumps */
> >>> +WEAK(early_idt_handler)
> >>>       cld
> >>
> >> Have you posted the patch to LKML already?
> >> (I hit this issue today and it took me a while to figure out the cause.)
> >
> > A kernel patch will be checked in soon.
> >
> 
> It already is:
> 
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git/commit/?h=x86/asm&id=cdeb6048940fa4bfb429e2f1cba0d28a11e20cd5
> 
> It's currently queued for 4.2.
> 
> Ingo, would it make sense to promote this to 4.1?  It's definitely a violation 
> of the "no development after the merge window" rule, but it's sort of a build 
> fix. [...]

Yeah, agreed - the commit was problem-free in testing, so I've cherry-picked it 
over into tip:x86/urgent and will push it to Linus for v4.1.

Thanks,

	Ingo
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