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

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Date:   Wed, 1 Mar 2017 13:44:01 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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: [PATCH] objtool, compiler.h: Fix __unreachable section relocation
 size


* 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:

  ".long %c0b - .\t\n"

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 76e28c229805..b6bb9019d87f 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"						\
+	    ".long %c0b - .\t\n"					\
 	    ".popsection\t\n" : : "i" (__LINE__));			\
 })
 #else