Date:   Wed, 23 Nov 2016 09:33:32 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:     Nicholas Piggin <npiggin@...il.com>, Arnd Bergmann <arnd@...db.de>,
        Uwe Kleine-König <uwe@...ine-koenig.org>,
        linux-arm-kernel@...ts.infradead.org, viro@...iv.linux.org.uk,
        linux-kbuild@...r.kernel.org, linux-arch@...r.kernel.org,
        regressions@...mhuis.info, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] kbuild: provide include/asm/asm-prototypes.h for ARM

On Tue, Nov 22, 2016 at 08:35:48PM -0500, Nicolas Pitre wrote:
> On Wed, 23 Nov 2016, Nicholas Piggin wrote:
> 
> > On Tue, 22 Nov 2016 11:34:48 -0500 (EST)
> > Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> > 
> > > On Tue, 22 Nov 2016, Arnd Bergmann wrote:
> > > 
> > > > This adds an asm/asm-prototypes.h header for ARM to fix the broken symbol
> > > > versioning for symbols exported from assembler files.
> > > > 
> > > > I couldn't find the correct prototypes for the compiler builtins,
> > > > so I went with the fake 'void f(void)' prototypes that we had
> > > > before, restoring the state before they were moved.
> > > > 
> > > > Originally I assumed that the problem was just a harmless warning
> > > > in unusual configurations, but as Uwe found, we actually need this
> > > > to load most modules when symbol versioning is enabled, as it is
> > > > in many distro kernels.
> > > > 
> > > > Cc: Uwe Kleine-König <uwe@...ine-koenig.org>
> > > > Fixes: 4dd1837d7589 ("arm: move exports to definitions")
> > > > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> > > > ---
> > > > Compared to the earlier version, I dropped the changes to the
> > > > csumpartial files, which now get handled correctly by Kbuild
> > > > even when the export comes from a macro, and I also dropped the
> > > > changes to the bitops files, which were already fixed in a
> > > > patch from Nico.
> > > > 
> > > > The patch applies cleanly on top of the rmk/fixes tree but has
> > > > no effect there, as it also needs 4efca4ed05cb ("kbuild: modversions
> > > > for EXPORT_SYMBOL() for asm") and cc6acc11cad1 ("kbuild: be more
> > > > careful about matching preprocessed asm ___EXPORT_SYMBOL").
> > > > 
> > > > With the combination of rmk/fixes, torvalds/master and these two
> > > > patches, symbol versioning works again on ARM. As it is still
> > > > broken on almost all other architectures (powerpc is fixed,
> > > > x86 has a patch), I wonder if we should make CONFIG_MODVERSIONS
> > > > as broken for everything else.  
> > > 
> > > I'm not sure I like this at all.
> > > 
> > > The goal for moving EXPORT_SYMBOL() to assembly code where symbols were 
> > > defined is to make things close together and avoid those centralized 
> > > list of symbols that you can easily miss when modifying the actual code.
> > 
> > Right.
> > 
> > > 
> > > This series is therefore bringing back a centralized list of symbols in 
> > > a slightly different form, nullifying the advantages from having moved 
> > > EXPORT_SYMBOL() to asm code.  To me this looks like a big step backward.
> > 
> > Exported symbols have C declarations in headers already. For the most
> > part, anyway -- these ones Arnd adds are for compiler runtime which is
> > why some architectures haven't had the prototypes.
> 
> Hmmm. That's right.  That makes it much more justifiable.
> My main objection is withdrawn.

I don't see it makes any difference - the armksyms.c originally had
the same:

-#include <linux/export.h>
-#include <linux/sched.h>
-#include <linux/string.h>
-#include <linux/cryptohash.h>
-#include <linux/delay.h>
-#include <linux/in6.h>
-#include <linux/syscalls.h>
-#include <linux/uaccess.h>
-#include <linux/io.h>
-#include <linux/arm-smccc.h>
-
-#include <asm/checksum.h>
-#include <asm/ftrace.h>

followed by prototypes for the GCC internal functions, and:

-extern void fpundefinstr(void);
-
-void mmioset(void *, unsigned int, size_t);
-void mmiocpy(void *, const void *, size_t);

So, the asm-prototypes.h approach is just the same, only that we now
have a bunch of prototypes in a header file, and the EXPORT_SYMBOL()s
in the assembly files.

As the C prototypes are remote from the definitions, it means that
the C prototypes are going to get forgotten about in exactly the same
way that armksyms.c would've been forgotten about too.

It _is_ worse than that though - with the armksyms.c approach, if the
assembly code for it is removed, you get a build error reminding you
to remove the export (and prototype).  With this approach, you get no
reminder to touch asm-prototypes.h.

It's also error prone for another reason - adding a new assembly level
export, if you forget to add it to asm-prototypes.h, we're back into
the problem we have right now with MODVERSIONS breaking.

So, I still think the whole approach is wrong - it's added extra
fragility that wasn't there with the armksyms.c approach.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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