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

From: Ard Biesheuvel
Date: Fri Mar 08 2024 - 09:28:21 EST

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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)

> > #define ARM_TEXT \
> > IDMAP_TEXT \
> > __entry_text_start = .; \
> > diff --git a/arch/arm/kernel/vmlinux-xip.lds.S
> > b/arch/arm/kernel/vmlinux-xip.lds.S
> > index c16d196b5aad..035fa18060b3 100644
> > --- a/arch/arm/kernel/vmlinux-xip.lds.S
> > +++ b/arch/arm/kernel/vmlinux-xip.lds.S
> > @@ -63,7 +63,7 @@ SECTIONS
> > . = ALIGN(4);
> > __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
> > __start___ex_table = .;
> > - ARM_MMU_KEEP(*(__ex_table))
> > + ARM_MMU_KEEP(KEEP(*(__ex_table)))
> > __stop___ex_table = .;
> > }
> >
> > @@ -83,7 +83,7 @@ SECTIONS
> > }
> > .init.arch.info : {
> > __arch_info_begin = .;
> > - *(.arch.info.init)
> > + KEEP(*(.arch.info.init))
> > __arch_info_end = .;
> > }
> > .init.tagtable : {
> > @@ -135,6 +135,10 @@ SECTIONS
> > ARM_TCM
> > #endif
> >
> > +#ifdef LD_DEAD_CODE_DATA_ELIMINATION

This should be CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

> > + ARM_VECTORS_TEXT
> > +#endif
> > +
> > /*
> > * End of copied data. We need a dummy section to get its LMA.
> > * Also located before final ALIGN() as trailing padding is not stored
> > diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> > index bd9127c4b451..2cfb890c93fb 100644
> > --- a/arch/arm/kernel/vmlinux.lds.S
> > +++ b/arch/arm/kernel/vmlinux.lds.S
> > @@ -74,7 +74,7 @@ SECTIONS
> > . = ALIGN(4);
> > __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
> > __start___ex_table = .;
> > - ARM_MMU_KEEP(*(__ex_table))
> > + ARM_MMU_KEEP(KEEP(*(__ex_table)))
> > __stop___ex_table = .;
> > }
> >
> > @@ -99,7 +99,7 @@ SECTIONS
> > }
> > .init.arch.info : {
> > __arch_info_begin = .;
> > - *(.arch.info.init)
> > + KEEP(*(.arch.info.init))
> > __arch_info_end = .;
> > }
> > .init.tagtable : {
> > @@ -116,7 +116,7 @@ SECTIONS
> > #endif
> > .init.pv_table : {
> > __pv_table_begin = .;
> > - *(.pv_table)
> > + KEEP(*(.pv_table))
> > __pv_table_end = .;
> > }
>
> I previously asked about this bit, since it appeared that this
> might prevent a lot of code from being discarded when
> CONFIG_ARM_PATCH_PHYS_VIRT is set. I tested this again now,
> and found this makes very little practical difference, so
> it's all good.
>
> > @@ -134,6 +134,10 @@ SECTIONS
> > ARM_TCM
> > #endif
> >
> > +#ifdef LD_DEAD_CODE_DATA_ELIMINATION

Same here

> > + ARM_VECTORS_TEXT
> > +#endif
> > +
> > #ifdef CONFIG_STRICT_KERNEL_RWX
> > . = ALIGN(1<<SECTION_SHIFT);
> > #else

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In reply to: Arnd Bergmann: "Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION"
Next in thread: Ard Biesheuvel: "Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION"
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