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

[Ard Biesheuvel]

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