Re: [PATCH v2] rust: add flags for shadow call stack sanitizer

[Valentin Obst]

> Add flags to support the shadow call stack sanitizer, both in the
> dynamic and non-dynamic modes.
>
> Right now, the compiler will emit the warning "unknown feature specified
> for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
> passes it to the codegen backend, so the flag will work just fine. Once
> rustc starts recognizing the flag (or provides another way to enable the
> feature), it will stop emitting this warning. See [1] for the relevant
> issue.
>
> Currently, the compiler thinks that the aarch64-unknown-none target
> doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
> you enable shadow call stack in non-dynamic mode. However, I still think
> it is reasonable to add the flag now, as it will at least fail the build
> when using an invalid configuration, until the Rust compiler is fixed to
> list -Zsanitizer=shadow-call-stack as supported for the target. See [2]
> for the feature request to add this.
>
> I have tested this change with Rust Binder on an Android device using
> CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=+reserve-x18 flag, the
> phone crashes immediately on boot, and with the flag, the phone appears
> to work normally.
>
> This contains a TODO to add the -Zuse-sync-unwind=n flag. The flag
> defaults to n, so it isn't a problem today, but the flag is unstable, so
> the default could change in a future compiler release.
>
> Link: https://github.com/rust-lang/rust/issues/121970 [1]
> Link: https://github.com/rust-lang/rust/issues/121972 [2]
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
> This patch raises the question of whether we should change the Rust
> aarch64 support to use a custom target.json specification. If we do
> that, then we can fix both the warning for dynamic SCS and the
> build-failure for non-dynamic SCS without waiting for a new version of
> rustc with the mentioned issues fixed.
> ---
> Changes in v2:
> - Add -Cforce-unwind-tables flag.
> - Link to v1: https://lore.kernel.org/r/20240304-shadow-call-stack-v1-1-f055eaf40a2c@xxxxxxxxxx
> ---
>
>  Makefile            | 1 +
>  arch/arm64/Makefile | 4 ++++
>  2 files changed, 5 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index 0e36eff14608..345066643a76 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -936,6 +936,7 @@ ifdef CONFIG_SHADOW_CALL_STACK
>  ifndef CONFIG_DYNAMIC_SCS
>  CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
>  KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
> +KBUILD_RUSTFLAGS += -Zsanitizer=shadow-call-stack
>  endif
>  export CC_FLAGS_SCS
>  endif
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index a88cdf910687..9bd5522c18e9 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -48,9 +48,12 @@ KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
>  ifneq ($(CONFIG_UNWIND_TABLES),y)
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
>  KBUILD_AFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
> +KBUILD_RUSTFLAGS += -Cforce-unwind-tables=n
>  else
>  KBUILD_CFLAGS	+= -fasynchronous-unwind-tables
>  KBUILD_AFLAGS	+= -fasynchronous-unwind-tables
> +# TODO: Pass -Zuse-sync-unwind=n once we upgrade to Rust 1.77.0
> +KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y
>  endif
>

That's the setup I used for my previous testing at [1], offering:

  Tested-by: Valentin Obst <kernel@xxxxxxxxxxxxxxx>
  Reviewed-by: Valentin Obst <kernel@xxxxxxxxxxxxxxx>

    - Best Valentin

Link: https://lore.kernel.org/all/20240305112017.125061-1-kernel@xxxxxxxxxxxxxxx/ [1]

>  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> @@ -103,6 +106,7 @@ endif
>
>  ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
>  KBUILD_CFLAGS	+= -ffixed-x18
> +KBUILD_RUSTFLAGS += -Ctarget-feature=+reserve-x18
>  endif
>
>  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)