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

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Message-ID: <aBnDqvY5c6a3qQ4H@e129823.arm.com>
Date: Tue, 6 May 2025 09:09:14 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: 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, ardb@...nel.org,
	ryan.roberts@....com, 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

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.

Thanks to let me know. But still I've failed to reproduce this
on Cortex-a72 and any older cpu on qeum.
If you don't mind, would you share your Kconfig?

> I haven't debugged it yet but I wonder whether something wants to write
> this variable after it was made read-only (well, I couldn't find any by
> grep'ing the code, so it needs some step-by-step debugging).
>
[...]

Thanks!

--
Sincerely,
Yeoreum Yun