linux-kernel - Re: [PATCH] objtool, compiler.h: Fix __unreachable section relocation size

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Date:   Wed, 1 Mar 2017 06:56:22 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     hpa@...or.com, linux-tip-commits@...r.kernel.org,
        tip-bot for Josh Poimboeuf <tipbot@...or.com>,
        a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
        tglx@...utronix.de, linux-kernel@...r.kernel.org,
        peterz@...radead.org
Subject: Re: [PATCH] objtool, compiler.h: Fix __unreachable section
 relocation size

On Wed, Mar 01, 2017 at 01:44:01PM +0100, Ingo Molnar wrote:
> 
> * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > On Wed, Mar 01, 2017 at 12:15:38AM -0800, hpa@...or.com wrote:
> > > On March 1, 2017 12:10:59 AM PST, tip-bot for Josh Poimboeuf <tipbot@...or.com> wrote:
> > > >Commit-ID:  90a7e63a31b8f7d630d12ef0d8d37d3ab87f76e5
> > > >Gitweb:    
> > > >http://git.kernel.org/tip/90a7e63a31b8f7d630d12ef0d8d37d3ab87f76e5
> > > >Author:     Josh Poimboeuf <jpoimboe@...hat.com>
> > > >AuthorDate: Wed, 1 Mar 2017 00:05:04 -0600
> > > >Committer:  Ingo Molnar <mingo@...nel.org>
> > > >CommitDate: Wed, 1 Mar 2017 07:38:25 +0100
> > > >
> > > >objtool: Fix __unreachable section relocation size
> > > >
> > > >Linus reported the following commit broke module loading on his laptop:
> > > >
> > > >d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead
> > > >ends")
> > > >
> > > >It showed errors like the following:
> > > >
> > > >  module: overflow in relocation type 10 val ffffffffc02afc81
> > > >  module: 'nvme' likely not compiled with -mcmodel=kernel
> > > >
> > > >The problem is that the __unreachable section addresses are stored
> > > >using
> > > >the '.long' asm directive, which isn't big enough for .text section
> > > >relative kernel addresses.  Use '.quad' instead.
> > > >
> > > >Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > > >Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> > > >Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > >Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > > >Cc: Peter Zijlstra <peterz@...radead.org>
> > > >Cc: Thomas Gleixner <tglx@...utronix.de>
> > > >Fixes: d1091c7fa3d5 ("objtool: Improve detection of BUG() and other
> > > >dead ends")
> > > >Link: http://lkml.kernel.org/r/20170301060504.oltm3iws6fmubnom@treble
> > > >Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > > >---
> > > > include/linux/compiler-gcc.h | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > >diff --git a/include/linux/compiler-gcc.h
> > > >b/include/linux/compiler-gcc.h
> > > >index 76e28c2..91a77a5 100644
> > > >--- a/include/linux/compiler-gcc.h
> > > >+++ b/include/linux/compiler-gcc.h
> > > >@@ -201,7 +201,7 @@
> > > > #define annotate_unreachable() ({					\
> > > > 	asm("%c0:\t\n"							\
> > > > 	    ".pushsection __unreachable, \"a\"\t\n"			\
> > > >-	    ".long %c0b\t\n"						\
> > > >+	    ".quad %c0b\t\n"						\
> > > > 	    ".popsection\t\n" : : "i" (__LINE__));			\
> > > > })
> > > > #else
> > > 
> > > Or perhaps better use relative addresses, so:
> > > 
> > > .long foo - (.+4)
> > 
> > Hm, yeah, something like that would indeed be better as it only needs 4
> > bytes instead of 8.  For this use case, the following seems to work:
> > 
> >   ".long %c0b - .\t\n"
> > 
> > It produces the same relocation section+addend as before but with a
> > X86_64_PC32 relocation.
> > 
> > It looks like the above patch was already merged so I'll add that change
> > to the TODO list unless Ingo can squash it with the above patch.
> 
> I've squashed it - attached the latest version. Does this look good to everyone?
> 
> Thanks,
> 
> 	Ingo
> 
> ================>
> From a6ae17f553bf9b4a0c7f8534e5f0f983bc7955c7 Mon Sep 17 00:00:00 2001
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> Date: Wed, 1 Mar 2017 00:05:04 -0600
> Subject: [PATCH] objtool, compiler.h: Fix __unreachable section relocation size
> 
> Linus reported the following commit broke module loading on his laptop:
> 
>   d1091c7fa3d5 ("objtool: Improve detection of BUG() and other dead ends")
> 
> It showed errors like the following:
> 
>   module: overflow in relocation type 10 val ffffffffc02afc81
>   module: 'nvme' likely not compiled with -mcmodel=kernel
> 
> The problem is that the __unreachable section addresses are stored using
> the '.long' asm directive, which isn't big enough for .text section
> relative kernel addresses.  Use relative addresses instead:

Maybe remove the first "relative" in the last line.  Otherwise it looks
good to me.  Thanks Ingo!

-- 
Josh