rust: kasan: add support for Software Tag-Based KASAN

This adds support for Software Tag-Based KASAN (KASAN_SW_TAGS) when
CONFIG_RUST is enabled. This requires that rustc includes support for
the kernel-hwaddress sanitizer, which is available since 1.96.0 [1].

Unlike with clang, we need to pass -Zsanitizer-recover in addition to
-Zsanitizer because the option is not implied automatically.

The kasan makefile uses different names for the flags depending on
whether CC is clang or gcc, but as we require that CC is clang when
using KASAN, we do not need to try to handle mixed gcc/llvm builds when
Rust is enabled.

Link: https://github.com/rust-lang/rust/pull/153049 [1]
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260408-kasan-rust-sw-tags-v3-2-e07964d14363@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

init/Kconfig

@@ -2199,7 +2199,7 @@ config RUST
 	depends on !CFI || HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC
 	select CFI_ICALL_NORMALIZE_INTEGERS if CFI
 	depends on !KASAN || CC_IS_CLANG
-	depends on !KASAN_SW_TAGS
+	depends on !KASAN_SW_TAGS || RUSTC_VERSION >= 109600
 	help
 	  Enables Rust support in the kernel.
 

scripts/Makefile.kasan

@@ -71,8 +71,6 @@ ifdef CONFIG_KASAN_SW_TAGS
 
 CFLAGS_KASAN := -fsanitize=kernel-hwaddress
 
-# This sets flags that will enable SW_TAGS KASAN once enabled in Rust. These
-# will not work today, and is guarded against in dependencies for CONFIG_RUST.
 RUSTFLAGS_KASAN := -Zsanitizer=kernel-hwaddress \
 		   -Zsanitizer-recover=kernel-hwaddress