Re: [PATCH v4 2/2] rust: support for shadow call stack sanitizer

[Will Deacon <will@kernel.org>]

Hi Alice,

Just some minor comments on this:

On Mon, Jul 29, 2024 at 02:22:50PM +0000, Alice Ryhl wrote:
> To use the shadow call stack sanitizer, you must pass special flags:
> 
> * On arm64, you must pass -ffixed-x18 to your compiler.
> * On riscv, you must pass --no-relax-gp to your linker.

Since this patch doesn't touch riscv, I think you can just talk about
arm64 in the commit message.

> These requirements also apply to Rust code. When using Rust on arm64,
> you must pass the -Zfixed-x18 flag to rustc, which has the same effect
> as the -ffixed-x18 flag does for C code. The -Zfixed-x18 flag requires
> rustc version 1.80.0 or greater.
> 
> There is no need to pass any flags to rustc on riscv as only the linker
> requires additional flags on this platform.
> 
> On older versions of Rust, it is still possible to use shadow call stack
> by passing -Ctarget-feature=+reserve-x18 instead of -Zfixed-x18.
> However, this flag emits a warning during the build, so this patch does
> not add support for using it.
> 
> Currently, the compiler thinks that the aarch64-unknown-none target

"Currently" will probably age badly -- can you talk about a compiler
version instead (e.g. "prior to version nnn, the compiler thinks...").

> doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
> you enable shadow call stack in non-dynamic mode. See [1] for the
> relevant feature request. To avoid this compilation failure, Kconfig is
> set up to reject such configurations.
> 
> The `depends on` clause is placed on `config RUST` to avoid a situation
> where enabling Rust silently turns off the sanitizer. Instead, turning
> on the sanitizer results in Rust being disabled. We generally do not
> want changes to CONFIG_RUST to result in any mitigations being changed
> or turned off.
> 
> Link: https://github.com/rust-lang/rust/issues/121972 [1]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  Makefile            | 1 +
>  arch/arm64/Makefile | 3 +++
>  init/Kconfig        | 2 +-
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index 2b5f9f098b6f..66daca7a9b57 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -928,6 +928,7 @@ 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 f6bc3da1ef11..b058c4803efb 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -57,9 +57,11 @@ 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
> +KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y -Zuse-sync-unwind=n

These unwind entries aren't mentioned at all in the commit message.
Please can you explain what you're doing here? I guess it's something to
do with the PAC patching? Maybe this hunk would be better as a separate
patch?

Will