[PATCH 1/2] kcsan: xtensa: Add atomic builtin stubs for 32-bit systems

[Rohan McLure <rmclure@linux.ibm.com>]

KCSAN instruments calls to atomic builtins, and will in turn call these
builtins itself. As such, architectures supporting KCSAN must have
compiler support for these atomic primitives.

Since 32-bit systems are unlikely to have 64-bit compiler builtins,
provide a stub for each missing builtin, and use BUG() to assert
unreachability.

In commit 725aea873261 ("xtensa: enable KCSAN"), xtensa implements these
locally. Move these definitions to be accessible to all 32-bit
architectures that do not provide the necessary builtins, with opt in
for PowerPC and xtensa.

Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Reviewed-by: Max Filippov <jcmvbkbc@gmail.com>
---
Previously issued as a part of a patch series adding KCSAN support to
64-bit.
Link: https://lore.kernel.org/linuxppc-dev/167646486000.1421441.10070059569986228558.b4-ty@ellerman.id.au/T/#t
v1: Remove __has_builtin check, as gcc is not obligated to inline
builtins detected using this check, but instead is permitted to supply
them in libatomic:
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108734
Instead, opt-in PPC32 and xtensa.
---
 arch/xtensa/lib/Makefile                              | 1 -
 kernel/kcsan/Makefile                                 | 2 ++
 arch/xtensa/lib/kcsan-stubs.c => kernel/kcsan/stubs.c | 0
 3 files changed, 2 insertions(+), 1 deletion(-)
 rename arch/xtensa/lib/kcsan-stubs.c => kernel/kcsan/stubs.c (100%)

diff --git a/arch/xtensa/lib/Makefile b/arch/xtensa/lib/Makefile
index 7ecef0519a27..d69356dc97df 100644
--- a/arch/xtensa/lib/Makefile
+++ b/arch/xtensa/lib/Makefile
@@ -8,5 +8,4 @@ lib-y	+= memcopy.o memset.o checksum.o \
 	   divsi3.o udivsi3.o modsi3.o umodsi3.o mulsi3.o umulsidi3.o \
 	   usercopy.o strncpy_user.o strnlen_user.o
 lib-$(CONFIG_PCI) += pci-auto.o
-lib-$(CONFIG_KCSAN) += kcsan-stubs.o
 KCSAN_SANITIZE_kcsan-stubs.o := n
diff --git a/kernel/kcsan/Makefile b/kernel/kcsan/Makefile
index 8cf70f068d92..86dd713d8855 100644
--- a/kernel/kcsan/Makefile
+++ b/kernel/kcsan/Makefile
@@ -12,6 +12,8 @@ CFLAGS_core.o := $(call cc-option,-fno-conserve-stack) \
 	-fno-stack-protector -DDISABLE_BRANCH_PROFILING
 
 obj-y := core.o debugfs.o report.o
+obj-$(CONFIG_PPC32) += stubs.o
+obj-$(CONFIG_XTENSA) += stubs.o
 
 KCSAN_INSTRUMENT_BARRIERS_selftest.o := y
 obj-$(CONFIG_KCSAN_SELFTEST) += selftest.o
diff --git a/arch/xtensa/lib/kcsan-stubs.c b/kernel/kcsan/stubs.c
similarity index 100%
rename from arch/xtensa/lib/kcsan-stubs.c
rename to kernel/kcsan/stubs.c
-- 
2.37.2