Re: [PATCH] ARM: change link order of $(mmy-y) to avoid veneers

[Masahiro Yamada]

On Mon, Jun 19, 2023 at 11:37 PM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> The kernel compiled with multi_v7_defconfig + CONFIG_KASAN=y +
> CONFIG_KASAN_INLINE=y does not boot.
>
> I do not think KASAN is the direct reason of the boot failure.
> CONFIG_KASAN_INLINE is just one example configuration that grows the
> image size significantly and makes a big distance between function
> callers and callees.
>
> I see some veneers for __get_user_* in the bad kernel image. I am
> not perfectly clear, but __get_user_* may not work with veneers for
> some reasons.
>
> If I move the link order of arch/arm/lib/getuser.S, the veneers are
> gone, and the kernel gets working again.
>
> I do not see a good reason that $(mmu-y) must be added to lib-y because
> all the code in $(mmu-y) is mandatory. Add it to obj-y to move the code
> to lower address.
>
> [1] multi_v7_defconfig (works)
>
>  $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
>  c072a450 T __get_user_1
>  c17ea033 r __kstrtab___get_user_1
>  c18119fe r __kstrtabns___get_user_1
>  c17c4878 r __ksymtab___get_user_1
>
> [2] multi_v7_defconfig + CONFIG_KASAN_INLINE (does not work)
>
>  $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
>  c341ec2c T __get_user_1
>  c06e3580 t ____get_user_1_veneer
>  c0adc6c0 t ____get_user_1_veneer
>  c12cf054 t ____get_user_1_veneer
>  c43f42cc r __kstrtab___get_user_1
>  c441c128 r __kstrtabns___get_user_1
>  c43cead8 r __ksymtab___get_user_1
>
> [3] multi_v7_defconfig + CONFIG_KASAN_INLINE + this patch (works)
>
>  $ arm-linux-gnueabihf-nm vmlinux | grep __get_user_1
>  c10975b0 T __get_user_1
>  c43f42cc r __kstrtab___get_user_1
>  c441c128 r __kstrtabns___get_user_1
>  c43cead8 r __ksymtab___get_user_1
>
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> ---
>
>  arch/arm/lib/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
> index 650404be6768..4d092ef87a1d 100644
> --- a/arch/arm/lib/Makefile
> +++ b/arch/arm/lib/Makefile
> @@ -28,7 +28,7 @@ endif
>  # using lib_ here won't override already available weak symbols
>  obj-$(CONFIG_UACCESS_WITH_MEMCPY) += uaccess_with_memcpy.o
>
> -lib-$(CONFIG_MMU) += $(mmu-y)
> +obj-$(CONFIG_MMU) += $(mmu-y)
>
>  ifeq ($(CONFIG_CPU_32v3),y)
>    lib-y        += io-readsw-armv3.o io-writesw-armv3.o
> --
> 2.39.2
>



I also wonder why __get_user* is defined in
arch/arm/lib/ instead of arch/arm/kernel/,
but I might be a convention.

x86 puts it in the lib directory,
  arch/x86/lib/getuser.S
then other architectures copy the code structure.






-- 
Best Regards
Masahiro Yamada