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From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Linus Walleij <linus.walleij@linaro.org>,
	"# 3.4.x" <stable@vger.kernel.org>
Subject: Re: [PATCH] ARM: Thumb2: align ALT_UP() sections sufficiently
Date: Tue, 18 Jan 2022 11:35:22 +0000	[thread overview]
Message-ID: <Yeal+oNFCvj911nt@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAMj1kXHTXeLPWbnQpkEen2uy6ameVL27QfeN2MZpdBB21Wj14w@mail.gmail.com>

On Tue, Jan 18, 2022 at 12:32:55PM +0100, Ard Biesheuvel wrote:
> On Tue, 18 Jan 2022 at 12:21, Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Tue, Jan 18, 2022 at 11:27:56AM +0100, Ard Biesheuvel wrote:
> > > When building for Thumb2, the .alt.smp.init sections that are emitted by
> > > the ALT_UP() patching code may not be 32-bit aligned, even though the
> > > fixup_smp_on_up() routine expects that. This results in alignment faults
> > > at module load time, which need to be fixed up by the fault handler.
> > >
> > > So let's align those sections explicitly, and avoid this from occurring.
> >
> > Are you seeing a problem that this patch fixes?
> >
> > This really should not matter. .alt.smp.init contents are always a whole
> > number of 32-bit words. These are gathered by the linker into the
> > .init.smpalt section, so the contents should always be a whole number
> > of 32-bit words.
> >
> > This follows the .init.tagtable section, which is also a 32-bit word
> > aligned structure built by the linker... which follows the
> > .init.arch.info section and .init.proc.info sections which all have
> > 32-bit alignment requirements.
> >
> 
> This only affects modules, not the core kernel. The .alt.smp.init
> section in a module is visible to the module loader, which means the
> module loader will make no attempt to position it at a 32-bit aligned
> address if the ELF alignment is only 16 bits, which appears to be the
> default in my Thumb2 build [gcc version 10.3.1 20211117 (Debian
> 10.3.0-13)]
> 
> I only spotted this because do_fixup_smp_on_up() was shown as the most
> recent in-kernel fixup location in /proc/cpu/alignment.

Ok, thanks for the explanation.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

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WARNING: multiple messages have this Message-ID (diff)
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Linus Walleij <linus.walleij@linaro.org>,
	"# 3.4.x" <stable@vger.kernel.org>
Subject: Re: [PATCH] ARM: Thumb2: align ALT_UP() sections sufficiently
Date: Tue, 18 Jan 2022 11:35:22 +0000	[thread overview]
Message-ID: <Yeal+oNFCvj911nt@shell.armlinux.org.uk> (raw)
In-Reply-To: <CAMj1kXHTXeLPWbnQpkEen2uy6ameVL27QfeN2MZpdBB21Wj14w@mail.gmail.com>

On Tue, Jan 18, 2022 at 12:32:55PM +0100, Ard Biesheuvel wrote:
> On Tue, 18 Jan 2022 at 12:21, Russell King (Oracle)
> <linux@armlinux.org.uk> wrote:
> >
> > On Tue, Jan 18, 2022 at 11:27:56AM +0100, Ard Biesheuvel wrote:
> > > When building for Thumb2, the .alt.smp.init sections that are emitted by
> > > the ALT_UP() patching code may not be 32-bit aligned, even though the
> > > fixup_smp_on_up() routine expects that. This results in alignment faults
> > > at module load time, which need to be fixed up by the fault handler.
> > >
> > > So let's align those sections explicitly, and avoid this from occurring.
> >
> > Are you seeing a problem that this patch fixes?
> >
> > This really should not matter. .alt.smp.init contents are always a whole
> > number of 32-bit words. These are gathered by the linker into the
> > .init.smpalt section, so the contents should always be a whole number
> > of 32-bit words.
> >
> > This follows the .init.tagtable section, which is also a 32-bit word
> > aligned structure built by the linker... which follows the
> > .init.arch.info section and .init.proc.info sections which all have
> > 32-bit alignment requirements.
> >
> 
> This only affects modules, not the core kernel. The .alt.smp.init
> section in a module is visible to the module loader, which means the
> module loader will make no attempt to position it at a 32-bit aligned
> address if the ELF alignment is only 16 bits, which appears to be the
> default in my Thumb2 build [gcc version 10.3.1 20211117 (Debian
> 10.3.0-13)]
> 
> I only spotted this because do_fixup_smp_on_up() was shown as the most
> recent in-kernel fixup location in /proc/cpu/alignment.

Ok, thanks for the explanation.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

  reply	other threads:[~2022-01-18 11:36 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-18 10:27 [PATCH] ARM: Thumb2: align ALT_UP() sections sufficiently Ard Biesheuvel
2022-01-18 10:27 ` Ard Biesheuvel
2022-01-18 11:21 ` Russell King (Oracle)
2022-01-18 11:21   ` Russell King (Oracle)
2022-01-18 11:32   ` Ard Biesheuvel
2022-01-18 11:32     ` Ard Biesheuvel
2022-01-18 11:35     ` Russell King (Oracle) [this message]
2022-01-18 11:35       ` Russell King (Oracle)

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