linux-kernel - Re: [tip:core/urgent] objtool: Fix __unreachable section relocation size

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Date:   Wed, 1 Mar 2017 06:41:21 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     hpa@...or.com
Cc:     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, mingo@...nel.org
Subject: Re: [tip:core/urgent] objtool: Fix __unreachable section relocation
 size

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.

-- 
Josh