linux-kernel - Re: [PATCH 1/5] vmlinux.lds.h: Include *(.text.*) in TEXT

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Date:	Thu, 17 Jun 2010 22:19:07 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	James Bottomley <James.Bottomley@...senpartnership.com>
Cc:	Tim Abbott <tabbott@...lice.com>,
	Matt Fleming <matt@...sole-pimps.org>,
	linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-kernel@...r.kernel.org, Sam Ravnborg <sam@...nborg.org>,
	Michal Marek <mmarek@...e.cz>,
	Parisc List <linux-parisc@...r.kernel.org>
Subject: Re: [PATCH 1/5] vmlinux.lds.h: Include *(.text.*) in TEXT_TEXT

>> This is doable in asm, yes. For .bss, we need to not forget about
>> @nobits too: section .bss.foo,"aw",@nobits
>
> That's only for bss ... we have about a handful of such statements and
> they always use the assembler .bss directive (which doesn't need flags).
>
>> > Actually, as I said, that would be .data-

You are right, in assembly we can specify needed attributes.

I am more concerned about C:

arch/x86/include/asm/cache.h:
#define __read_mostly __attribute__((__section__(".data..read_mostly")))

If we change it to

#define __read_mostly __attribute__((__section__(".data-read_mostly")))

What makes this section have correct attributes?

With current gcc, __attribute__((__section__(".bss-page_aligned")))
does get wrong attributes. That's why we settled on .bss..foo
scheme.

> I thought I just refuted that in the above: we don't care what the
> assembler sections are flagged as because the linker script gets to pick
> the flags anyway ... so most bugs arrived at this way have no visible
> side effects ... and section merging problems have to be accounted for
> anyway in the final linker scripts.

When I was working on a older iteration of this patch,
I renamed .bss.page_aligned to .page_aligned.bss
and was bitten by linker bug: linker tried to merge
the sections and corrupted them.

Aha, here is it:
http://sourceware.org/bugzilla/show_bug.cgi?id=5006

It was since fixed, and if I read the ld patch correctly,
now ld emits a warning and switches entire target section
to PROGBITS - not what we want to happen to bss.

-- 
vda
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