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

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Message-ID: <fbfded61-cfe2-4416-9098-ef39ef3e2b62@arm.com>
Date: Tue, 6 May 2025 09:15:48 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Yeoreum Yun <yeoreum.yun@....com>,
 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,
 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 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.

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.

Thanks,
Ryan

> 
> 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