Re: [PATCH 1/2] kbuild: rust: enable KCSAN support

[Boqun Feng <boqun@kernel.org>]

On Mon, Oct 12, 2026 at 01:39:14PM +0000, Marco Elver wrote:
> Rust supports ThreadSanitizer instrumentation via LLVM, which is what
> KCSAN relies on; however, Rust code is not currently instrumented.
> 
> Pass -Zsanitizer=thread and the same TSan backend options used for C
> code to rustc. Wire up KCSAN_SANITIZE in scripts/Makefile.lib for parity
> with C (though rustc currently rejects linking crates with mismatched
> instrumentation).
> 
> rustc only accepts -Zsanitizer=thread for targets that declare support
> for it. Of the targets used by the kernel, this is only possible for the
> generated x86_64 target specification, so restrict instrumentation of
> Rust code to x86_64 for now via HAVE_KCSAN_RUSTC. On other architectures
> Rust code remains uninstrumented as before.
> 
> All TSan options used are supported by the LLVM versions of the minimum
> supported rustc, so no rustc-option checks are needed (they would also
> fail, because x86_64-unknown-none used for option checks does not
> support -Zsanitizer=thread).
> 
> With this enabled we can detect data races in kernel Rust code and
> between Rust and C code.
> 

Help has finally arrived! :)

Thank you for looking into this, for both patches:

Acked-by: Boqun Feng <boqun@kernel.org>

Regards,
Boqun

> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  Documentation/dev-tools/kcsan.rst |  7 +++++++
>  lib/Kconfig.kcsan                 |  7 +++++++
>  scripts/Makefile.kcsan            | 13 +++++++++++++
>  scripts/Makefile.lib              |  3 +++
>  scripts/generate_rust_target.rs   |  2 +-
>  5 files changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/dev-tools/kcsan.rst b/Documentation/dev-tools/kcsan.rst
> index 8575178aa87f..3bb8754f844b 100644
> --- a/Documentation/dev-tools/kcsan.rst
> +++ b/Documentation/dev-tools/kcsan.rst
> @@ -14,6 +14,9 @@ Usage
>  KCSAN is supported by both GCC and Clang. With GCC we require version 11 or
>  later, and with Clang also require version 11 or later.
>  
> +Rust code is instrumented as well on architectures whose Rust target supports
> +it (currently only x86_64; see ``CONFIG_HAVE_KCSAN_RUSTC``).
> +
>  To enable KCSAN configure the kernel with::
>  
>      CONFIG_KCSAN = y
> @@ -121,6 +124,10 @@ the below options are available:
>  
>      KCSAN_SANITIZE := n
>  
> +  Note that ``rustc`` rejects linking crates built with and without KCSAN
> +  instrumentation, so ``KCSAN_SANITIZE`` and ``KCSAN_SANITIZE_<target>.o``
> +  cannot currently be used for Rust code.
> +
>  .. _"Marking Shared-Memory Accesses" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
>  
>  Furthermore, it is possible to tell KCSAN to show or hide entire classes of
> diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan
> index 4ce4b0c0109c..f0096e1c8b61 100644
> --- a/lib/Kconfig.kcsan
> +++ b/lib/Kconfig.kcsan
> @@ -10,6 +10,13 @@ config HAVE_KCSAN_COMPILER
>  	  For the list of compilers that support KCSAN, please see
>  	  <file:Documentation/dev-tools/kcsan.rst>.
>  
> +config HAVE_KCSAN_RUSTC
> +	def_bool RUST && X86_64
> +	help
> +	  rustc only accepts -Zsanitizer=thread for targets that declare
> +	  support for it; of the kernel's Rust targets, only x86_64 does.
> +	  Elsewhere, Rust code is not instrumented.
> +
>  menuconfig KCSAN
>  	bool "KCSAN: dynamic data race detector"
>  	depends on HAVE_ARCH_KCSAN && HAVE_KCSAN_COMPILER
> diff --git a/scripts/Makefile.kcsan b/scripts/Makefile.kcsan
> index 19f693b68a96..01af6fd71006 100644
> --- a/scripts/Makefile.kcsan
> +++ b/scripts/Makefile.kcsan
> @@ -24,3 +24,16 @@ kcsan-cflags += $(call cc-option,$(call cc-param,tsan-instrument-func-entry-exit
>  endif
>  
>  export CFLAGS_KCSAN := $(kcsan-cflags)
> +
> +ifdef CONFIG_HAVE_KCSAN_RUSTC
> +# The minimum supported rustc's LLVM supports all of these options.
> +rustc-param = -Cllvm-args=-$(1)
> +kcsan-rustflags := -Zsanitizer=thread \
> +	$(call rustc-param,tsan-compound-read-before-write=1) \
> +	$(call rustc-param,tsan-distinguish-volatile=1)
> +ifndef CONFIG_KCSAN_WEAK_MEMORY
> +kcsan-rustflags += $(call rustc-param,tsan-instrument-func-entry-exit=0)
> +endif
> +endif
> +
> +export RUSTFLAGS_KCSAN := $(kcsan-rustflags)
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 0a4fdd8bd975..78f1c23e54be 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -98,6 +98,9 @@ ifeq ($(CONFIG_KCSAN),y)
>  _c_flags += $(if $(patsubst n%,, \
>  	$(KCSAN_SANITIZE_$(target-stem).o)$(KCSAN_SANITIZE)$(is-kernel-object)), \
>  	$(CFLAGS_KCSAN))
> +_rust_flags += $(if $(patsubst n%,, \
> +	$(KCSAN_SANITIZE_$(target-stem).o)$(KCSAN_SANITIZE)$(is-kernel-object)), \
> +	$(RUSTFLAGS_KCSAN))
>  # Some uninstrumented files provide implied barriers required to avoid false
>  # positives: set KCSAN_INSTRUMENT_BARRIERS for barrier instrumentation only.
>  _c_flags += $(if $(patsubst n%,, \
> diff --git a/scripts/generate_rust_target.rs b/scripts/generate_rust_target.rs
> index 7687b0dd5474..4b02bffa6b46 100644
> --- a/scripts/generate_rust_target.rs
> +++ b/scripts/generate_rust_target.rs
> @@ -231,7 +231,7 @@ fn main() {
>          }
>          ts.push("features", features);
>          ts.push("llvm-target", "x86_64-linux-gnu");
> -        ts.push("supported-sanitizers", ["kcfi", "kernel-address"]);
> +        ts.push("supported-sanitizers", ["kcfi", "kernel-address", "thread"]);
>          if cfg.rustc_version_atleast(1, 91, 0) {
>              ts.push("target-pointer-width", 64);
>          } else {
> -- 
> 2.56.0.385.gd3acb90ef8-goog
>