Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: Ard Biesheuvel
Date: Fri Mar 08 2024 - 19:01:35 EST

On Fri, 8 Mar 2024 at 16:37, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 8 Mar 2024 at 15:27, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> >
> > On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > >
> > > On Thu, Mar 7, 2024, at 16:12, Yuntao Liu wrote:
> > > > The current arm32 architecture does not yet support the
> > > > HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. arm32 is widely used in
> > > > embedded scenarios, and enabling this feature would be beneficial for
> > > > reducing the size of the kernel image.
> > > >
> > > > In order to make this work, we keep the necessary tables by annotating
> > > > them with KEEP, also it requires further changes to linker script to KEEP
> > > > some tables and wildcard compiler generated sections into the right place.
> > > >
> > > > It boots normally with defconfig, vexpress_defconfig and tinyconfig.
> > > >
> > > > The size comparison of zImage is as follows:
> > > > defconfig vexpress_defconfig tinyconfig
> > > > 5137712 5138024 424192 no dce
> > > > 5032560 4997824 298384 dce
> > > > 2.0% 2.7% 29.7% shrink
> > > >
> > > > When using smaller config file, there is a significant reduction in the
> > > > size of the zImage.
> > > >
> > > > We also tested this patch on a commercially available single-board
> > > > computer, and the comparison is as follows:
> > > > a15eb_config
> > > > 2161384 no dce
> > > > 2092240 dce
> > > > 3.2% shrink
> > > >
> > > > The zImage size has been reduced by approximately 3.2%, which is 70KB on
> > > > 2.1M.
> > > >
> > > > Signed-off-by: Yuntao Liu <liuyuntao12@xxxxxxxxxx>
> > >
> > > I've retested with both gcc-13 and clang-18, and so no
> > > more build issues. Your previous version already worked
> > > fine for me.
> > >
> > > I did some tests combining this with CONFIG_TRIM_UNUSED_KSYMS,
> > > which showed a significant improvement as expected. I also
> > > tried combining it with an experimental CONFIG_LTO_CLANG
> > > patch, but that did not show any further improvements.
> > >
> > > Tested-by: Arnd Bergmann <arnd@xxxxxxxx>
> > > Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
> > >
> > > Adding Ard Biesheuvel and Fangrui Song to Cc, so they can comment
> > > on the ARM_VECTORS_TEXT workaround. I don't understand enough of
> > > the details of what is going on here.
> > >
> >
> > Thanks for the cc
> >
> > > Full quote of the patch below so they can see the whole thing.
> > >
> > > If they are also happy with the patch, I think you can send it
> > > into Russell's patch tracker at
> > > https://www.armlinux.org.uk/developer/patches/info.php
> > >
> >
> > No, not happy at all :-)
> >
> > The resulting kernel does not boot (built with GCC or Clang). And the
> > patch is buggy (see below)
> >
> > > > ---
> > > > v2:
> > > > - Support config XIP_KERNEL.
> > > > - Support LLVM compilation.
> > > >
> > > > v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@xxxxxxxxxx/
> > > > ---
> > > > arch/arm/Kconfig | 1 +
> > > > arch/arm/boot/compressed/vmlinux.lds.S | 4 ++--
> > > > arch/arm/include/asm/vmlinux.lds.h | 18 +++++++++++++++++-
> > > > arch/arm/kernel/vmlinux-xip.lds.S | 8 ++++++--
> > > > arch/arm/kernel/vmlinux.lds.S | 10 +++++++---
> > > > 5 files changed, 33 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > > > index 0af6709570d1..de78ceb821df 100644
> > > > --- a/arch/arm/Kconfig
> > > > +++ b/arch/arm/Kconfig
> > > > @@ -113,6 +113,7 @@ config ARM
> > > > select HAVE_KERNEL_XZ
> > > > select HAVE_KPROBES if !XIP_KERNEL && !CPU_ENDIAN_BE32 && !CPU_V7M
> > > > select HAVE_KRETPROBES if HAVE_KPROBES
> > > > + select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> > > > select HAVE_MOD_ARCH_SPECIFIC
> > > > select HAVE_NMI
> > > > select HAVE_OPTPROBES if !THUMB2_KERNEL
> > > > diff --git a/arch/arm/boot/compressed/vmlinux.lds.S
> > > > b/arch/arm/boot/compressed/vmlinux.lds.S
> > > > index 3fcb3e62dc56..da21244aa892 100644
> > > > --- a/arch/arm/boot/compressed/vmlinux.lds.S
> > > > +++ b/arch/arm/boot/compressed/vmlinux.lds.S
> > > > @@ -89,7 +89,7 @@ SECTIONS
> > > > * The EFI stub always executes from RAM, and runs strictly before
> > > > the
> > > > * decompressor, so we can make an exception for its r/w data, and
> > > > keep it
> > > > */
> > > > - *(.data.efistub .bss.efistub)
> > > > + *(.data.* .bss.*)
> >
> > Why is this necessary? There is a reason we don't allow .data in the
> > decompressor.
> >
> > > > __pecoff_data_end = .;
> > > >
> > > > /*
> > > > @@ -125,7 +125,7 @@ SECTIONS
> > > >
> > > > . = BSS_START;
> > > > __bss_start = .;
> > > > - .bss : { *(.bss) }
> > > > + .bss : { *(.bss .bss.*) }
> > > > _end = .;
> > > >
> > > > . = ALIGN(8); /* the stack must be 64-bit aligned */
> > > > diff --git a/arch/arm/include/asm/vmlinux.lds.h
> > > > b/arch/arm/include/asm/vmlinux.lds.h
> > > > index 4c8632d5c432..dfe2b6ad6b51 100644
> > > > --- a/arch/arm/include/asm/vmlinux.lds.h
> > > > +++ b/arch/arm/include/asm/vmlinux.lds.h
> > > > @@ -42,7 +42,7 @@
> > > > #define PROC_INFO \
> > > > . = ALIGN(4); \
> > > > __proc_info_begin = .; \
> > > > - *(.proc.info.init) \
> > > > + KEEP(*(.proc.info.init)) \
> > > > __proc_info_end = .;
> > > >
> > > > #define IDMAP_TEXT \
> > > > @@ -87,6 +87,22 @@
> > > > *(.vfp11_veneer) \
> > > > *(.v4_bx)
> > > >
> > > > +/*
> > > > +When CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled, it is important
> > > > to
> > > > +annotate .vectors sections with KEEP. While linking with ld, it is
> > > > +acceptable to directly use KEEP with .vectors sections in ARM_VECTORS.
> > > > +However, when using ld.lld for linking, KEEP is not recognized within
> > > > the
> > > > +OVERLAY command; it is treated as a regular string. Hence, it is
> > > > advisable
> > > > +to define a distinct section here that explicitly retains the .vectors
> > > > +sections when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is turned on.
> > > > +*/
> > > > +#define ARM_VECTORS_TEXT \
> > > > + .vectors.text : { \
> > > > + KEEP(*(.vectors)) \
> > > > + KEEP(*(.vectors.bhb.loop8)) \
> > > > + KEEP(*(.vectors.bhb.bpiall)) \
> > > > + }
> > > > +
> >
> > This looks fishy to me. How is this supposed to work? You cannot emit
> > these sections into some random other place in the binary.
> >
> > And also, ARM_VECTORS_TEXT is never used (by accident, see below)
> >
>
> The below appears to work for me:
>
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -1076,7 +1076,12 @@
> W(b) vector_irq
> W(b) vector_fiq
>
> + .text
> + .reloc ., R_ARM_NONE, .vectors
> #ifdef CONFIG_HARDEN_BRANCH_HISTORY
> + .reloc ., R_ARM_NONE, .vectors.bhb.loop8
> + .reloc ., R_ARM_NONE, .vectors.bhb.bpiall
> +
> .section .vectors.bhb.loop8, "ax", %progbits
> W(b) vector_rst
> W(b) vector_bhb_loop8_und

.. or even better:

--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -1066,4 +1066,5 @@

.section .vectors, "ax", %progbits
+ .reloc .text, R_ARM_NONE, .
W(b) vector_rst
W(b) vector_und
@@ -1079,4 +1080,5 @@
#ifdef CONFIG_HARDEN_BRANCH_HISTORY
.section .vectors.bhb.loop8, "ax", %progbits
+ .reloc .text, R_ARM_NONE, .
W(b) vector_rst
W(b) vector_bhb_loop8_und
@@ -1091,4 +1093,5 @@

.section .vectors.bhb.bpiall, "ax", %progbits
+ .reloc .text, R_ARM_NONE, .
W(b) vector_rst
W(b) vector_bhb_bpiall_und