[PATCH v2] rust: kasan/kbuild: fix rustc-option when cross-compiling

[Alice Ryhl]

The Makefile version of rustc-option currently checks whether the option
exists for the host target instead of the target actually being compiled
for. It was done this way in commit 46e24a545cdb ("rust: kasan/kbuild:
fix missing flags on first build") to avoid a circular dependency on
target.json. However, because of this, rustc-option currently does not
function when cross-compiling from x86_64 to aarch64 if
CONFIG_SHADOW_CALL_STACK is enabled. This is because KBUILD_RUSTFLAGS
contains -Zfixed-x18 under this configuration. Since that flag does not
exist on the host target, rustc-option runs into a compilation failure
every time, leading to all flags being rejected as unsupported.

To fix this, update rustc-option to pass a --target parameter so that
the host target is not used. For targets using target.json, use a
built-in target that is as close as possible to the target created with
target.json to avoid the circular dependency on target.json.

One scenario where this causes a boot failure:
* Cross-compiled from x86_64 to aarch64.
* With CONFIG_SHADOW_CALL_STACK=y
* With CONFIG_KASAN_SW_TAGS=y
* With CONFIG_KASAN_INLINE=n
Then the resulting kernel image will fail to boot when it first calls
into Rust code with a crash along the lines of "Unable to handle kernel
paging request at virtual address 0ffffffc08541796". This is because the
call threshold is not specified, so rustc will inline kasan operations,
but the kasan shadow offset is not specified, which leads to the inlined
kasan instructions being incorrect.

Note that the -Zsanitizer=kernel-hwaddress parameter itself does not
lead to a rustc-option failure despite being aarch64-specific because
RUSTFLAGS_KASAN has not yet been added to KBUILD_RUSTFLAGS when
rustc-option is evaluated by the kasan Makefile.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: 46e24a545cdb ("rust: kasan/kbuild: fix missing flags on first build")
Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
---
Changes in v2:
- Fix inaccurate statement in commit message.
- Link to v1: https://lore.kernel.org/r/20260504-rustc-option-cross-v1-1-09f2726b680f@xxxxxxxxxx
---
 arch/x86/Makefile         | 4 ++++
 arch/x86/Makefile.um      | 8 ++++++++
 scripts/Makefile.compiler | 2 +-
 3 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 46fec0b08487..1d526a5d2a83 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -77,6 +77,10 @@ KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -mno-avx -mno-sse4a
 KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
 KBUILD_RUSTFLAGS += -Ctarget-feature=-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-avx,-avx2
 
+# The target.json file is not available when invoking rustc-option, so use the
+# built-in target when checking whether flags are supported instead.
+KBUILD_RUSTFLAGS_OPTION_CHKS += --target=x86_64-unknown-none
+
 #
 # CFLAGS for compiling floating point code inside the kernel.
 #
diff --git a/arch/x86/Makefile.um b/arch/x86/Makefile.um
index 19c13afa474e..9adecd65639f 100644
--- a/arch/x86/Makefile.um
+++ b/arch/x86/Makefile.um
@@ -14,6 +14,14 @@ endif
 
 KBUILD_RUSTFLAGS += --target=$(objtree)/scripts/target.json
 
+# The target.json file is not available when invoking rustc-option, so use the
+# built-in target when checking whether flags are supported instead.
+ifeq ($(CONFIG_X86_32),y)
+KBUILD_RUSTFLAGS_OPTION_CHKS += --target=i686-unknown-linux-gnu
+else
+KBUILD_RUSTFLAGS_OPTION_CHKS += --target=x86_64-unknown-linux-gnu
+endif
+
 ifeq ($(CONFIG_X86_32),y)
 START := 0x8048000
 
diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
index ef91910de265..06bbe29c846c 100644
--- a/scripts/Makefile.compiler
+++ b/scripts/Makefile.compiler
@@ -80,7 +80,7 @@ ld-option = $(call try-run, $(LD) $(KBUILD_LDFLAGS) $(1) -v,$(1),$(2),$(3))
 # TODO: remove RUSTC_BOOTSTRAP=1 when we raise the minimum GNU Make version to 4.4
 __rustc-option = $(call try-run,\
 	echo '$(pound)![allow(missing_docs)]$(pound)![feature(no_core)]$(pound)![no_core]' | RUSTC_BOOTSTRAP=1\
-	$(1) --sysroot=/dev/null $(filter-out --sysroot=/dev/null --target=%,$(2)) $(3)\
+	$(1) --sysroot=/dev/null $(KBUILD_RUSTFLAGS_OPTION_CHKS) $(filter-out --sysroot=/dev/null --target=%target.json,$(2)) $(3)\
 	--crate-type=rlib --out-dir=$(TMPOUT) --emit=obj=- - >/dev/null,$(3),$(4))
 
 # rustc-option

---
base-commit: 7fd2df204f342fc17d1a0bfcd474b24232fb0f32
change-id: 20260504-rustc-option-cross-7b524fc354d8

Best regards,
-- 
Alice Ryhl <aliceryhl@xxxxxxxxxx>