Re: [PATCH v3 2/2] rust: add flags for shadow call stack sanitizer

[Nathan Chancellor <nathan@kernel.org>]

Hi Alice,

On Thu, Jul 04, 2024 at 03:07:58PM +0000, Alice Ryhl wrote:
> As of rustc 1.80.0, the Rust compiler supports the -Zfixed-x18 flag, so
> we can now use Rust with the shadow call stack sanitizer.
> 
> On older versions of Rust, it is possible to use shadow call stack by
> passing -Ctarget-feature=+reserve-x18 instead of -Zfixed-x18. However,
> this flag emits a warning, so this patch does not add support for that.
> 
> 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. See [2] for the

                                                        ^ this should be [1]?

> feature request to add this. Kconfig is not configured to reject this
> configuration because that leads to cyclic Kconfig rules.

While it probably does not matter much given Rust for Linux is still "in
the works", I think it would be good to avoid these build failures.
Perhaps something like this could work (which basically just forces on
UNWIND_PATCH_PAC_INTO_SCS when Rust is enabled).

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5d91259ee7b5..a9f08a2bd1c6 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -89,7 +89,7 @@ config ARM64
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	select ARCH_SUPPORTS_HUGETLBFS
 	select ARCH_SUPPORTS_MEMORY_FAILURE
-	select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
+	select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK && (!RUST || CAN_UNWIND_PATCH_PAC_INTO_SCS)
 	select ARCH_SUPPORTS_LTO_CLANG if CPU_LITTLE_ENDIAN
 	select ARCH_SUPPORTS_LTO_CLANG_THIN
 	select ARCH_SUPPORTS_CFI_CLANG
@@ -2262,12 +2262,16 @@ config STACKPROTECTOR_PER_TASK
 	def_bool y
 	depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_SYSREG
 
-config UNWIND_PATCH_PAC_INTO_SCS
-	bool "Enable shadow call stack dynamically using code patching"
+config CAN_UNWIND_PATCH_PAC_INTO_SCS
+	def_bool y
 	# needs Clang with https://github.com/llvm/llvm-project/commit/de07cde67b5d205d58690be012106022aea6d2b3 incorporated
 	depends on CC_IS_CLANG && CLANG_VERSION >= 150000
 	depends on ARM64_PTR_AUTH_KERNEL && CC_HAS_BRANCH_PROT_PAC_RET
-	depends on SHADOW_CALL_STACK
+
+config UNWIND_PATCH_PAC_INTO_SCS
+	bool "Enable shadow call stack dynamically using code patching" if !RUST
+	depends on SHADOW_CALL_STACK && CAN_UNWIND_PATCH_PAC_INTO_SCS
+	default y if RUST
 	select UNWIND_TABLES
 	select DYNAMIC_SCS
 

Otherwise, it might be good to wait to enable this until [1] is
addressed, but I don't really feel that strongly about it.

From a Kconfig/Kbuild perspective, the rest of the patch seems fine.

> Link: https://github.com/rust-lang/rust/issues/121972 [1]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
>  Makefile            | 1 +
>  arch/Kconfig        | 2 +-
>  arch/arm64/Makefile | 3 +++
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index c11a10c8e710..4ae741601a1c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -945,6 +945,7 @@ 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/Kconfig b/arch/Kconfig
> index 238448a9cb71..5a6e296df5e6 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -690,7 +690,7 @@ config SHADOW_CALL_STACK
>  	bool "Shadow Call Stack"
>  	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
>  	depends on DYNAMIC_FTRACE_WITH_ARGS || DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
> -	depends on !RUST
> +	depends on !RUST || RUSTC_VERSION >= 108000
>  	depends on MMU
>  	help
>  	  This option enables the compiler's Shadow Call Stack, which
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 3f0f35fd5bb7..bbf313ddd700 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
>  endif
>  
>  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> @@ -114,6 +116,7 @@ endif
>  
>  ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
>  KBUILD_CFLAGS	+= -ffixed-x18
> +KBUILD_RUSTFLAGS += -Zfixed-x18
>  endif
>  
>  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
> 
> -- 
> 2.45.2.803.g4e1b14247a-goog
>