linux-kernel - Re: [tip:x86/urgent] x86: Add back _end symbol that is missing from Symbol.map

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Date:	Sun, 8 Nov 2009 13:38:25 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	hare@...e.de, akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] x86: Add back _end symbol that is missing
	from Symbol.map

On Sun, Nov 08, 2009 at 12:48:29PM +0100, Ingo Molnar wrote:
> 
> * Sam Ravnborg <sam@...nborg.org> wrote:
> 
> > On Sun, Nov 08, 2009 at 11:07:12AM +0000, tip-bot for Hannes Reinecke wrote:
> > > Commit-ID:  d1e4d37b79e944f6351ae38eb1ca7c9f7b506e76
> > > Gitweb:     http://git.kernel.org/tip/d1e4d37b79e944f6351ae38eb1ca7c9f7b506e76
> > > Author:     Hannes Reinecke <hare@...e.de>
> > > AuthorDate: Wed, 4 Nov 2009 07:19:32 -0800
> > > Committer:  Ingo Molnar <mingo@...e.hu>
> > > CommitDate: Sun, 8 Nov 2009 12:05:03 +0100
> > > 
> > > x86: Add back _end symbol that is missing from Symbol.map
> > > 
> > > With 2.6.31 'crash' on x86_64 falls flat on its face as the
> > > '_end' symbol is missing from the System.map file.
> > > 
> > > The culprit is commit 091e52c3551d3031343df24b573b770b4c6c72b6,
> > > which moved the '_end' symbol into it's own section.  Apparently
> > > this causes kallsyms to not reference it properly.
> > > 
> > > So do not include _end in it's own section.
> > > 
> > > Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> > > Cc: Sam Ravnborg <sam@...nborg.org>
> > > Cc: <stable@...nel.org> # .31.x
> > > LKML-Reference: <200911041519.nA4FJWEW030164@...p1.linux-foundation.org>
> > > Signed-off-by: Ingo Molnar <mingo@...e.hu>
> > 
> > I'm afraid this will break a RELOCTABLE i386 kernel.
> > 
> > We fixed this up some time ago where we had to
> > move the symbol definition inside a section
> > to prevent the symbol being absolute.
> 
> ah, indeed. So ... the question is then, why did _end disappear from 
> System.map?
I think ld discards zero size sections. No need to keep them around
if they are empty.

Following patch should fix it up in a better way.
I reused most of Hannes changelog.

	Sam

[PATCH] x86: Add back _end symbol that is missing from Symbol.map

With 2.6.31 'crash' on x86_64 falls flat on its face as the
'_end' symbol is missing from the System.map file.

The culprit is commit 091e52c3551d3031343df24b573b770b4c6c72b6,
which moved the '_end' symbol into it's own section.

The problem is likely that ld discards empty sections.
So move the definition of _end into the last section defined.

Cc: Hannes Reinecke <hare@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: <stable@...nel.org> # .31.x
Signed-off-by: Sam Ravnborg <sam@...nborg.org>


diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 92929fb..c58a6e3 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -289,9 +289,12 @@ SECTIONS
 		. += 64 * 1024;		/* 64k alignment slop space */
 		*(.brk_reservation)	/* areas brk users have reserved */
 		__brk_limit = .;
-	}
 
-	.end : AT(ADDR(.end) - LOAD_OFFSET) {
+		/*
+		 * To support RELOCTABLE kernel _end must be inside a section.
+		 * And ld is know to discard empty section so we locate _end in
+		 * last section we define.
+		 */
 		_end = .;
 	}
 
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