[PATCH v2 23/33] s390/boot: Add the KMSAN runtime stub

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It should be possible to have inline functions in the s390 header
files, which call kmsan_unpoison_memory(). The problem is that these
header files might be included by the decompressor, which does not
contain KMSAN runtime, causing linker errors.

Not compiling these calls if __SANITIZE_MEMORY__ is not defined -
either by changing kmsan-checks.h or at the call sites - may cause
unintended side effects, since calling these functions from an
uninstrumented code that is linked into the kernel is valid use case.

One might want to explicitly distinguish between the kernel and the
decompressor. Checking for a decompressor-specific #define is quite
heavy-handed, and will have to be done at all call sites.

A more generic approach is to provide a dummy kmsan_unpoison_memory()
definition. This produces some runtime overhead, but only when building
with CONFIG_KMSAN. The benefit is that it does not disturb the existing
KMSAN build logic and call sites don't need to be changed.

Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>
 arch/s390/boot/Makefile | 1 +
 arch/s390/boot/kmsan.c  | 6 ++++++
 2 files changed, 7 insertions(+)
 create mode 100644 arch/s390/boot/kmsan.c

diff --git a/arch/s390/boot/Makefile b/arch/s390/boot/Makefile
index fb10fcd21221..096216a72e98 100644
--- a/arch/s390/boot/Makefile
+++ b/arch/s390/boot/Makefile
@@ -44,6 +44,7 @@ obj-$(findstring y, $(CONFIG_PROTECTED_VIRTUALIZATION_GUEST) $(CONFIG_PGSTE))	+=
 obj-$(CONFIG_RANDOMIZE_BASE)	+= kaslr.o
 obj-y	+= $(if $(CONFIG_KERNEL_UNCOMPRESSED),,decompressor.o) info.o
 obj-$(CONFIG_KERNEL_ZSTD) += clz_ctz.o
+obj-$(CONFIG_KMSAN) += kmsan.o
 obj-all := $(obj-y) piggy.o syms.o
 targets	:= bzImage section_cmp.boot.data section_cmp.boot.preserved.data $(obj-y)
diff --git a/arch/s390/boot/kmsan.c b/arch/s390/boot/kmsan.c
new file mode 100644
index 000000000000..e7b3ac48143e
--- /dev/null
+++ b/arch/s390/boot/kmsan.c
@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kmsan-checks.h>
+void kmsan_unpoison_memory(const void *address, size_t size)

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