linux-kernel - Re: [PATCH v7 1/1] riscv: Allow to downgrade paging mode from the command line

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Date:   Mon, 27 Feb 2023 13:56:55 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Conor Dooley <conor.dooley@...rochip.com>
Cc:     Alexandre Ghiti <alexghiti@...osinc.com>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Conor Dooley <conor@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Björn Töpel <bjorn@...osinc.com>,
        nathan@...nel.org, llvm@...ts.linux.dev, ndesaulniers@...gle.com
Subject: Re: [PATCH v7 1/1] riscv: Allow to downgrade paging mode from the
 command line

On Mon, 27 Feb 2023 at 13:32, Conor Dooley <conor.dooley@...rochip.com> wrote:
>
> Hey Alex, clang/llvm folk,
>
> On Fri, Feb 24, 2023 at 11:02:18AM +0100, Alexandre Ghiti wrote:
> > Add 2 early command line parameters that allow to downgrade satp mode
> > (using the same naming as x86):
> > - "no5lvl": use a 4-level page table (down from sv57 to sv48)
> > - "no4lvl": use a 3-level page table (down from sv57/sv48 to sv39)
> >
> > Note that going through the device tree to get the kernel command line
> > works with ACPI too since the efi stub creates a device tree anyway with
> > the command line.
> >
> > In KASAN kernels, we can't use the libfdt that early in the boot process
> > since we are not ready to execute instrumented functions. So instead of
> > using the "generic" libfdt, we compile our own versions of those functions
> > that are not instrumented and that are prefixed so that they do not
> > conflict with the generic ones. We also need the non-instrumented versions
> > of the string functions and the prefixed versions of memcpy/memmove.
> >
> > This is largely inspired by commit aacd149b6238 ("arm64: head: avoid
> > relocating the kernel twice for KASLR") from which I removed compilation
> > flags that were not relevant to RISC-V at the moment (LTO, SCS, pie).
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
> > Tested-by: Björn Töpel <bjorn@...osinc.com>
> > Reviewed-by: Björn Töpel <bjorn@...osinc.com>
> > ---
> >  .../admin-guide/kernel-parameters.txt         |  5 +-
> >  arch/riscv/kernel/Makefile                    |  2 +
> >  arch/riscv/kernel/pi/Makefile                 | 37 +++++++++++
> >  arch/riscv/kernel/pi/cmdline_early.c          | 62 +++++++++++++++++++
>
> +      4 ld.lld: warning: vmlinux.a(arch/riscv/kernel/pi/string.pi.o):(.init.sdata) is being placed in '.init.sdata'
>
> I'm getting 4 new linker warnings with LLVM=1 clang-15 allmodconfig
> builds - but I don't really understand this issue.
> Is this spurious, and a just consequence of...
>
> > diff --git a/arch/riscv/kernel/pi/Makefile b/arch/riscv/kernel/pi/Makefile
> > new file mode 100644
> > index 000000000000..4002ed94b6d3
> > --- /dev/null
> > +++ b/arch/riscv/kernel/pi/Makefile
> > @@ -0,0 +1,37 @@
> > +# SPDX-License-Identifier: GPL-2.0
> > +# This file was copied from arm64/kernel/pi/Makefile.
> > +
> > +KBUILD_CFLAGS        := $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
> > +                -Os -DDISABLE_BRANCH_PROFILING $(DISABLE_STACKLEAK_PLUGIN) \
> > +                $(call cc-option,-mbranch-protection=none) \
> > +                -I$(srctree)/scripts/dtc/libfdt -fno-stack-protector \
> > +                -D__DISABLE_EXPORTS -ffreestanding \
> > +                -fno-asynchronous-unwind-tables -fno-unwind-tables \
> > +                $(call cc-option,-fno-addrsig)
> > +
> > +CFLAGS_cmdline_early.o += -D__NO_FORTIFY
> > +CFLAGS_lib-fdt_ro.o += -D__NO_FORTIFY
> > +
> > +GCOV_PROFILE := n
> > +KASAN_SANITIZE       := n
> > +KCSAN_SANITIZE       := n
> > +UBSAN_SANITIZE       := n
> > +KCOV_INSTRUMENT      := n
> > +
> > +$(obj)/%.pi.o: OBJCOPYFLAGS := --prefix-symbols=__pi_ \
> > +                            --remove-section=.note.gnu.property \
> > +                            --prefix-alloc-sections=.init
>
> ...this?
>

Yes. if the input object has a .sdata section, the output .pi.o object
will have a .init.sdata section, and if the common linker script does
not place it explicitly, you will get a warning. Note that we switched
to --orphan-handling=error on other arches, as the linker sometimes
does unexpected things with sections it doesn't know about.

I'd imagine this needs to be combined with .init.data