Re: [PATCH 2/2] ARM: copypage: do not use naked functions - Russell King

From: Russell King - ARM Linux <linux@armlinux.org.uk>
To: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Stefan Agner <stefan@agner.ch>,
	arnd@arndb.de, linus.walleij@linaro.org,
	linux-kernel@vger.kernel.org, joel@jms.id.au,
	ulli.kroll@googlemail.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 2/2] ARM: copypage: do not use naked functions
Date: Tue, 16 Oct 2018 09:33:41 +0100	[thread overview]
Message-ID: <20181016083341.GE30658@n2100.armlinux.org.uk> (raw)
In-Reply-To: <nycvar.YSQ.7.76.1810151908450.1498@knanqh.ubzr>

On Mon, Oct 15, 2018 at 07:27:43PM -0400, Nicolas Pitre wrote:
> It's hard to see what that commit was actually fixing, but the operands 
> usage is wrong as explained already. Maybe the generated code has been 
> OK for all those years but that is due to luck rather than correctness.
...
> No idea. Maybe Russell remembers?
> Maybe digging into the mailing list archive might tell.

I found this as a reply to the patch by Mikael Pettersson:

I've tested and verified that this bit enables a gcc-4.5 compiled kernel
to boot on TS-119 (Kirkwood) when combined with my fix for __naked.
With neither or only one of the patches applied, the kernel oopses hard
in copy_user_page() as it tries to start /sbin/init.
...
- the asm() bodies of these __naked functions have inadequate input
  parameter constraints, in particular they fail to declare any
  dependencies on the functions' formal parameters; gcc-4.5 sees this
  and skips the parameter setup before calling these functions, causing
  runtime crashes; Khem's patch (this one) fixes that
  (copypage-xscale.c already had correct asm() constraints so it works
  with only the __naked fix, these other copypage-*.c files need both
  patches to work)

So, while wrong to the GCC manual, it's fixing a bug that is present
with gcc-4.5 and who-knows what other GCC versions.  Reverting the
commit has the chance to cause regressions with GCC.

It looks like any change here needs to be validated on a range of
GCC versions, because there are versions of GCC known not to follow
it's manual!

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