Re: [RFC PATCH v2 13/13] objtool: arm64: Enable stack validation for arm64

[Ard Biesheuvel]

On Wed, 3 Mar 2021 at 18:10, Julien Thierry <jthierry@xxxxxxxxxx> wrote:
>
> From: Raphael Gault <raphael.gault@xxxxxxx>
>
> Add build option to run stack validation at compile time.
>
> When requiring stack validation, jump tables are disabled as it
> simplifies objtool analysis (without having to introduce unreliable
> artifacs). In local testing, this does not appear to significaly
> affect final binary size nor system performance.
>
> Signed-off-by: Raphael Gault <raphael.gault@xxxxxxx>
> Signed-off-by: Julien Thierry <jthierry@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig  | 1 +
>  arch/arm64/Makefile | 4 ++++
>  2 files changed, 5 insertions(+)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 1f212b47a48a..928323c03318 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -187,6 +187,7 @@ config ARM64
>         select MMU_GATHER_RCU_TABLE_FREE
>         select HAVE_RSEQ
>         select HAVE_STACKPROTECTOR
> +       select HAVE_STACK_VALIDATION
>         select HAVE_SYSCALL_TRACEPOINTS
>         select HAVE_KPROBES
>         select HAVE_KRETPROBES
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 5b84aec31ed3..b819fb2e8eda 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -136,6 +136,10 @@ ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_REGS),y)
>    CC_FLAGS_FTRACE := -fpatchable-function-entry=2
>  endif
>
> +ifeq ($(CONFIG_STACK_VALIDATION),y)
> +KBUILD_CFLAGS  += -fno-jump-tables
> +endif
> +

This is a bit misleading: the Kconfig option in question is selected
automatically in all cases, so jump tables are always disabled.

However, I think disabling jump tables make sense anyway, at least
when building the relocatable kernel for KASLR: we currently don't use
-fpic/fpie in that case when building the vmlinux objects (because we
don't want/need GOT tables), and so jump tables are emitted using
absolute addresses, which induce some space overhead in the image. (24
bytes of RELA data per absolute address)

... unless I am missing something, and jump tables can/will be emitted
as relative, even when not compiling in PIC mode?





>  # Default value
>  head-y         := arch/arm64/kernel/head.o
>
> --
> 2.25.4
>