Re: [PATCH v7 1/1] riscv: Allow to downgrade paging mode from the command line
From: Alexandre Ghiti
Date: Mon Feb 27 2023 - 08:27:22 EST
On 2/27/23 13:56, Ard Biesheuvel wrote:
On Mon, 27 Feb 2023 at 13:32, Conor Dooley <conor.dooley@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
Tested-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
Reviewed-by: Björn Töpel <bjorn@xxxxxxxxxxxx>
---
.../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
It seems orphan-handling is set to "error" only when WERROR is set (see
CONFIG_LD_ORPHAN_WARN_LEVEL).
does unexpected things with sections it doesn't know about.
I'd imagine this needs to be combined with .init.data
Yes, gcc does not produce this section but others that I placed in a new
section called .init.pidata: I did not want to expand INIT_DATA_SECTION
macro because we won't need that anymore when we implement all the
string functions.
So I'll add this new .init.sdata section to .init.pidata in the v8!
Thanks,
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