linux-kernel - Re: missing .data.shared_align placement in vmlinux

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Date:	Wed, 4 Aug 2010 22:28:06 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Alan Jenkins <alan-jenkins@...fmail.co.uk>
Subject: Re: missing .data.shared_align placement in vmlinux

On Wed, Aug 04, 2010 at 02:12:03PM -0400, Mike Frysinger wrote:
> On Wed, Aug 4, 2010 at 07:56, Sam Ravnborg wrote:
> > On Fri, May 21, 2010 at 04:58:53PM -0400, Mike Frysinger wrote:
> >> am i missing something or does the .data.shared_align section lack
> >> definition in vmlinux.lds.h and all arch vmlinux.lds.S files ?
> >>
> >> with the recent change "net: remove time limit in process_backlog()",
> >> the softnet_data variable changed from "DEFINE_PER_CPU()" to
> >> "DEFINE_PER_CPU_ALIGNED()" which moved it from the .data section to
> >> the .data.shared_align section.  i'm not saying this patch is wrong,
> >> just that is what caused me to notice this larger problem.  no one
> >> else in the kernel is using this aligned macro variant, so i imagine
> >> that's why no one has noticed yet.
> >>
> >> since .data.shared_align isnt declared in any vmlinux files that i can
> >> see, the linker just places it last.  this "just works" for most
> >> people, but when building a ROM kernel on Blackfin systems, it causes
> >> section overlap errors:
> >> bfin-uclinux-ld.real: section .init.data [00000000202e06b8 ->
> >> 00000000202e48b7] overlaps section .data.shared_aligned
> >> [00000000202e06b8 -> 00000000202e0723]
> >> i imagine other arches which support the ROM config option and thus do
> >> funky placement would see similar issues ...
> >>
> >> on x86, it is stuck in a dedicated section:
> >>   [ 8] .data             PROGBITS        ffffffff810ec000 2ec000
> >> 0303a8 00  WA  0   0 4096
> >>   [ 9] .data.shared_alig PROGBITS        ffffffff8111c3c0 31c3c0
> >> 0000c8 00  WA  0   0 64
> >>
> >> the ifdef forest in asm-generic/percpu.h is beyond a quick glance &
> >> fix, so i leave it up to someone else ;)
> >
> > as there any resolution on this?
> > I briefly looked at it some time ago.
> > And it looks like a plain oversight.
> 
> no, it's still broken in 2.6.35 :(
> $ make ARCH=blackfin BF537-STAMP_defconfig
> $ make ARCH=blackfin -s -j4
> $ readelf -WS vmlinux | grep '\<data\>'
>   [10] .data             PROGBITS        0018369c 17269c 012964 00  WA  0   0  4
>   [11] .data..shared_aligned PROGBITS        00196000 185000 00006c 00
>  WA  0   0  4

Following simple patch should deal with it.
Jeremy - you introduced this in ("x86/i386: Put aligned
stack-canary in percpu shared_aligned section")
53f824520b6d84ca5b4a8fd71addc91dbf64357e

Does the following simple fix look correct to you?

	Sam

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 030a954..e87260f 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -150,6 +150,7 @@
 #define DATA_DATA							\
 	*(.data)							\
 	*(.ref.data)							\
+	*(.data..shared_aligned) /* percpu related */			\
 	DEV_KEEP(init.data)						\
 	DEV_KEEP(exit.data)						\
 	CPU_KEEP(init.data)						\
--
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