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Message-ID: <4B072E31.4000906@tuffmail.co.uk>
Date: Sat, 21 Nov 2009 00:02:57 +0000
From: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
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To: Tony Luck <tony.luck@gmail.com>
CC: Rusty Russell <rusty@rustcorp.com.au>, Sam Ravnborg <sam@ravnborg.org>,
       Mike Frysinger <vapier.adi@gmail.com>, greg@kroah.com,
       linux-kbuild@vger.kernel.org, carmelo73@gmail.com,
       linux-kernel@vger.kernel.org, achiang@hp.com, kyle@mcmartin.ca,
       deller@gmx.de, jejb@parisc-linux.org,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>, paulus@samba.org
Subject: Re: [PATCH 05/10] kbuild: sort the list of symbols exported by the
 kernel (__ksymtab)
References: <9b2b86520911020852q49c55695rb05d87090fa9ad33@mail.gmail.com>	 <20091104171916.GA6323@merkur.ravnborg.org>	 <4AF2E00A.3010107@tuffmail.co.uk>	 <200911091347.57286.rusty@rustcorp.com.au> <12c511ca0911201420i5f2d4751m25ae5288b69dbabb@mail.gmail.com>
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Tony Luck wrote:
> The sorted ksymtab breaks ia64 (and possibly ppc64 and
> parisc too).
>
> Alex Chiang did the bisect to find this change as
> the cause of the breakage.  The problem is that ia64
> expects that the first item in each ksymtab entry to be
> a function pointer.  The code in modpost that creates
> .tmp_exports-asm.S doesn't know about types of exported
> objects, so it uses __EXPORT_SYMBOL from linux/mod_export.h
> for everything. This results in
>
>         PTR SYM(sym);
>         PTR SYM(__kstrtab_##sym);
>
> which the preprocessor expands to entries like:
>
>         .long ____pagevec_lru_add
>         .long __kstrtab____pagevec_lru_add
>
> which puts the address of the first instruction of the
> function into the table, rather than the address of a
> function pointer (which on ia64 is a two element data
> object containing the code address and the global data
> pointer).
>
> The syntax you need for this* is:
>
>         .long @fptr(____pagevec_lru_add)
>         .long __kstrtab____pagevec_lru_add
>
> Note that you must only use the @fptr(name) syntax for
> function exports. Exported data items just need an address.
>
> -Tony
>
> * On ia64 ... powerpc and parisc might need something else.
>   

Thanks!  It doesn't sound too hard to retro-fit your suggestion.

Still, I can't help wondering if I've done this all wrong :-/.  Perhaps 
I should avoid the assembler.  Instead, I could write a tool to sort the 
ksymtab elf sections in-place (and mangle their relocations 
accordingly).  That should preserve any special handling for function 
symbols without arch-specific special cases.  It would also concentrate 
all the magic in one tool - rather than it being scattered between the 
modpost tool, mod_export.h, tmp_exports.S, and vmlinux.lds.h.

Alan
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