linux-kernel - Re: [PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after

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Message-ID: <aBnrCXJWKh3O0Bai@e129823.arm.com>
Date: Tue, 6 May 2025 11:57:13 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ryan Roberts <ryan.roberts@....com>, Ard Biesheuvel <ardb@...nel.org>,
	will@...nel.org, nathan@...nel.org, nick.desaulniers+lkml@...il.com,
	morbo@...gle.com, justinstitt@...gle.com, broonie@...nel.org,
	maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
	shameerali.kolothum.thodi@...wei.com, james.morse@....com,
	hardevsinh.palaniya@...iconsignals.io,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with
 ro_after_init to prevent wrong idmap generation

> On Tue, May 06, 2025 at 11:47:44AM +0100, Yeoreum Yun wrote:
> > > On 06/05/2025 10:41, Ard Biesheuvel wrote:
> > > > On Tue, 6 May 2025 at 10:16, Ryan Roberts <ryan.roberts@....com> wrote:
> > > >>
> > > >> On 06/05/2025 09:09, Yeoreum Yun wrote:
> > > >>> Hi Catalin,
> > > >>>
> > > >>>> On Sat, May 03, 2025 at 09:23:27PM +0100, Yeoreum Yun wrote:
> > > >>>>> Hi Catalin,
> > > >>>>>
> > > >>>>>> On Sat, May 03, 2025 at 11:16:12AM +0100, Catalin Marinas wrote:
> > > >>>>>>> On Fri, 02 May 2025 19:04:12 +0100, Yeoreum Yun wrote:
> > > >>>>>>>> create_init_idmap() could be called before .bss section initialization
> > > >>>>>>>> which is done in early_map_kernel().
> > > >>>>>>>> Therefore, data/test_prot could be set incorrectly by PTE_MAYBE_NG macro.
> > > >>>>>>>>
> > > >>>>>>>> PTE_MAYBE_NG macro set NG bit according to value of "arm64_use_ng_mappings".
> > > >>>>>>>> and this variable places in .bss section.
> > > >>>>>>>>
> > > >>>>>>>> [...]
> > > >>>>>>>
> > > >>>>>>> Applied to arm64 (for-next/fixes), with some slight tweaking of the
> > > >>>>>>> comment, thanks!
> > > >>>>>>>
> > > >>>>>>> [1/1] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after_init to prevent wrong idmap generation
> > > >>>>>>>       https://git.kernel.org/arm64/c/12657bcd1835
> > > >>>>>>
> > > >>>>>> I'm going to drop this for now. The kernel compiled with a clang 19.1.5
> > > >>>>>> version I have around (Debian sid) fails to boot, gets stuck early on:
> > > >>>>>>
> > > >>>>>> $ clang --version
> > > >>>>>> Debian clang version 19.1.5 (1)
> > > >>>>>> Target: aarch64-unknown-linux-gnu
> > > >>>>>> Thread model: posix
> > > >>>>>> InstalledDir: /usr/lib/llvm-19/bin
> > > >>>>>>
> > > >>>>>> I didn't have time to investigate, disassemble etc. I'll have a look
> > > >>>>>> next week.
> > > >>>>>
> > > >>>>> Just for your information.
> > > >>>>> When I see the debian package, clang 19.1.5-1 doesn't supply anymore:
> > > >>>>>  - https://ftp.debian.org/debian/pool/main/l/llvm-toolchain-19/
> > > >>>>>
> > > >>>>> and the default version for sid is below:
> > > >>>>>
> > > >>>>> $ clang-19 --version
> > > >>>>> Debian clang version 19.1.7 (3)
> > > >>>>> Target: aarch64-unknown-linux-gnu
> > > >>>>> Thread model: posix
> > > >>>>> InstalledDir: /usr/lib/llvm-19/bin
> > > >>>>>
> > > >>>>> When I tested with above version with arm64-linux's for-next/fixes
> > > >>>>> including this patch. it works well.
> > > >>>>
> > > >>>> It doesn't seem to be toolchain related. It fails with gcc as well from
> > > >>>> Debian stable but you'd need some older CPU (even if emulated, e.g.
> > > >>>> qemu). It fails with Cortex-A72 (guest on Raspberry Pi 4) but not
> > > >>>> Neoverse-N2. Also changing the annotation from __ro_after_init to
> > > >>>> __read_mostly also works.
> > > >>
> > > >> I think this is likely because __ro_after_init is also "ro before init" - i.e.
> > > >> if you try to write to it in the PI code an exception is generated due to it
> > > >> being mapped RO. Looks like early_map_kernel() is writiing to it.
> > > >>
> > > >
> > > > Indeed.
> > > >
> > > >> I've noticed a similar problem in the past and it would be nice to fix it so
> > > >> that PI code maps __ro_after_init RW.
> > > >>
> > > >
> > > > The issue is that the store occurs via the ID map, which only consists
> > > > of one R-X and one RW- section. I'm not convinced that it's worth the
> > > > hassle to relax this.
> > > >
> > > > If moving the variable to .data works, then let's just do that.
> > >
> > > Yeah, fair enough.
> >
> > Thanks.
> > I've success to reproduce. and after check, Sending with this fix.
>
> No need to resend, I modified it locally. Thanks.
>

Thanks. and Sorry to all for making this because of my lack :\

--
Sincerely,
Yeoreum Yun