rust: kasan/kbuild: fix rustc-option when cross-compiling

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@vger.kernel.org
Fixes: 46e24a545cdb ("rust: kasan/kbuild: fix missing flags on first build")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260507-rustc-option-cross-v2-1-2f650a49c2b5@google.com
[ Edited slightly:
    - Reset variable to avoid using the environment.
    - Use a simply expanded variable flavor for simplicity.
    - Export variable so that behavior in sub-`make`s is consistent.

  This matches other variables. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

diff --git a/Makefile b/Makefile
index b7b80e84e1eb26c31927fca6d28c96185c55e807..1f205a004437285ba789a5099bb3b156ecb10f8c 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -607,6 +607,7 @@ KBUILD_RUSTFLAGS := $(rust_common_flags) \
                    -Crelocation-model=static \
                    -Zfunction-sections=n \
                    -Wclippy::float_arithmetic
+KBUILD_RUSTFLAGS_OPTION_CHKS :=
 
 KBUILD_AFLAGS_KERNEL :=
 KBUILD_CFLAGS_KERNEL :=
@@ -643,7 +644,7 @@ export KBUILD_USERCFLAGS KBUILD_USERLDFLAGS
 
 export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
 export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
-export KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE
+export KBUILD_RUSTFLAGS RUSTFLAGS_KERNEL RUSTFLAGS_MODULE KBUILD_RUSTFLAGS_OPTION_CHKS
 export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
 export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_RUSTFLAGS_MODULE KBUILD_LDFLAGS_MODULE
 export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL KBUILD_RUSTFLAGS_KERNEL

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 46fec0b08487abf931bba0aa5e1d7d8b0a49f011..1d526a5d2a831046a333129639cae32fc35fb4a3 100644 (file)
--- 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 19c13afa474e9da57d38e94ff9c8db7509614af7..9adecd65639f3e5f12042631d9ba286e878edfeb 100644 (file)
--- 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 ef91910de265d266717714a8d6e6d3cb51c37118..06bbe29c846ce1faae980eedd6a59ede62ac66fb 100644 (file)
--- 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