Re: [PATCH v2] arm64/cpufeature: annotate arm64_use_ng_mappings with ro_after_init to prevent wrong idmap generation

[Yeoreum Yun <yeoreum.yun@arm.com>]

> On 06/05/2025 10:41, Ard Biesheuvel wrote:
> > On Tue, 6 May 2025 at 10:16, Ryan Roberts <ryan.roberts@arm.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.

--
Sincerely,
Yeoreum Yun