[PATCH] selftests: probe for sanitizer support in fchmodat2 and openat2

[Sasha Levin <sashal@kernel.org>]

The fchmodat2 and openat2 tests are always built with -fsanitize=address
and -fsanitize=undefined. Toolchains without the sanitizer runtimes,
such as clang without the compiler-rt package, cannot link them and the
tests fail to build:

  ld: cannot find /usr/lib/llvm-21/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.asan_static.a: No such file or directory

The same happens with USERLDFLAGS=-static, which AddressSanitizer does
not support. KernelCI's clang builds fail this way on every
architecture.

Link an empty program with the sanitizer flags first, and only add them
when that works. With the runtimes available the tests are built as
before.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Tested on x86_64 with gcc 16 and clang 21, building both suites with
O=: with the runtimes installed the binaries still link ASan as before.
With clang pointed at a resource directory without the runtimes, and
with gcc and USERLDFLAGS=-static, the build failed before and now
builds all five programs without the sanitizers. fchmodat2_test and the
openat2 tests give the same results from both builds.

 tools/testing/selftests/fchmodat2/Makefile     | 18 +++++++++++++-----
 .../selftests/filesystems/openat2/Makefile     | 18 +++++++++++++-----
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/fchmodat2/Makefile b/tools/testing/selftests/fchmodat2/Makefile
index 4373cea79b794..21f6b0b0cdc8f 100644
--- a/tools/testing/selftests/fchmodat2/Makefile
+++ b/tools/testing/selftests/fchmodat2/Makefile
@@ -1,15 +1,23 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
-CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined $(KHDR_INCLUDES)
+CFLAGS += -Wall -O2 -g $(KHDR_INCLUDES)
+
+TEST_GEN_PROGS := fchmodat2_test
+
+include ../lib.mk
 
 # gcc requires -static-libasan in order to ensure that Address Sanitizer's
 # library is the first one loaded. However, clang already statically links the
 # Address Sanitizer if -fsanitize is specified. Therefore, simply omit
 # -static-libasan for clang builds.
+SANITIZER_CFLAGS := -fsanitize=address -fsanitize=undefined
 ifeq ($(LLVM),)
-    CFLAGS += -static-libasan
+    SANITIZER_CFLAGS += -static-libasan
 endif
 
-TEST_GEN_PROGS := fchmodat2_test
-
-include ../lib.mk
+# Build without the sanitizers when the toolchain cannot link them.
+ifeq ($(shell echo 'int main(void) { return 0; }' | $(CC) $(CFLAGS) \
+	$(SANITIZER_CFLAGS) $(LDFLAGS) -x c - -o /dev/null >/dev/null 2>&1 && \
+	echo y),y)
+CFLAGS += $(SANITIZER_CFLAGS)
+endif
diff --git a/tools/testing/selftests/filesystems/openat2/Makefile b/tools/testing/selftests/filesystems/openat2/Makefile
index d848aac96bded..2f712dfcbf75c 100644
--- a/tools/testing/selftests/filesystems/openat2/Makefile
+++ b/tools/testing/selftests/filesystems/openat2/Makefile
@@ -1,17 +1,25 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 CFLAGS += $(KHDR_INCLUDES)
-CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined $(TOOLS_INCLUDES)
+CFLAGS += -Wall -O2 -g $(TOOLS_INCLUDES)
 TEST_GEN_PROGS := openat2_test resolve_test rename_attack_test emptypath_test
 
+LOCAL_HDRS += helpers.h
+
+include ../../lib.mk
+
 # gcc requires -static-libasan in order to ensure that Address Sanitizer's
 # library is the first one loaded. However, clang already statically links the
 # Address Sanitizer if -fsanitize is specified. Therefore, simply omit
 # -static-libasan for clang builds.
+SANITIZER_CFLAGS := -fsanitize=address -fsanitize=undefined
 ifeq ($(LLVM),)
-    CFLAGS += -static-libasan
+    SANITIZER_CFLAGS += -static-libasan
 endif
 
-LOCAL_HDRS += helpers.h
-
-include ../../lib.mk
+# Build without the sanitizers when the toolchain cannot link them.
+ifeq ($(shell echo 'int main(void) { return 0; }' | $(CC) $(CFLAGS) \
+	$(SANITIZER_CFLAGS) $(LDFLAGS) -x c - -o /dev/null >/dev/null 2>&1 && \
+	echo y),y)
+CFLAGS += $(SANITIZER_CFLAGS)
+endif
-- 
2.53.0