[RFC PATCH bpf-next v1 12/16] selftests/bpf: Build BPF objects with compiler-inserted typed arena casts

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Clang inserts typed_arena_cast where a pointer to a typed record is used,
but only with the +typed-arena target feature, and it defines
__BPF_FEATURE_TYPED_ARENA_CAST only then: a struct with kptr fields lives in
map values of programs that run on kernels without the instruction, so the
insertion has to be opted into. Probe the feature the way cpu v4 is probed,
by asking the compiler for the macro, and pass it to every flavor when the
compiler has it. The typed arena tests are gated on the macro and skip with
a compiler that lacks the feature.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 tools/testing/selftests/bpf/Makefile | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index afa589a27b15..bd1f84f55edc 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -15,6 +15,13 @@ ifneq ($(shell $(CLANG) --target=bpf -mcpu=help 2>&1 | grep 'v4'),)
 CLANG_CPUV4 := 1
 endif
 
+# Check whether clang inserts typed arena casts; the feature defines the macro
+# the typed arena tests are gated on, so they skip with a clang without it
+ifneq ($(shell echo | $(CLANG) --target=bpf -Xclang -target-feature -Xclang +typed-arena \
+		-dM -E - 2>/dev/null | grep __BPF_FEATURE_TYPED_ARENA_CAST),)
+CLANG_TYPED_ARENA := -Xclang -target-feature -Xclang +typed-arena
+endif
+
 # Order correspond to 'make run_tests' order
 TEST_GEN_PROGS = test_verifier test_tag test_maps test_lru_map test_progs \
 	test_sockmap \
@@ -510,7 +517,7 @@ BINARY := test_progs
 BPF_CC := $(CLANG)
 BPF_CC_MSG := CLNG-BPF
 BPF_SYS_INCLUDES := $(CLANG_SYS_INCLUDES)
-BPF_CC_FLAGS := -O2 $(BPF_TARGET_ENDIAN) -mcpu=v3
+BPF_CC_FLAGS := -O2 $(BPF_TARGET_ENDIAN) -mcpu=v3 $(CLANG_TYPED_ARENA)
 BPF_DEFINES := -DENABLE_ATOMICS_TESTS
 include Makefile.skel
 
@@ -526,12 +533,12 @@ $(OUTPUT)/test_progs: $(RUNNER_PREREQS) $(BPF_OBJS) $(ALL_SKELS) FORCE
 
 $(OUTPUT)/test_progs-no_alu32: $(RUNNER_PREREQS) FORCE
 	+$(Q)$(RUNNER_MAKE) RUNNER=test_progs FLAVOR=no_alu32 TESTS_DIR=prog_tests \
-		$(CLANG_RUNNER_ARGS) BPF_CC_FLAGS='-O2 $(BPF_TARGET_ENDIAN) -mcpu=v2'
+		$(CLANG_RUNNER_ARGS) BPF_CC_FLAGS='-O2 $(BPF_TARGET_ENDIAN) -mcpu=v2 $(CLANG_TYPED_ARENA)'
 
 ifneq ($(CLANG_CPUV4),)
 $(OUTPUT)/test_progs-cpuv4: $(RUNNER_PREREQS) FORCE
 	+$(Q)$(RUNNER_MAKE) RUNNER=test_progs FLAVOR=cpuv4 TESTS_DIR=prog_tests \
-		$(CLANG_RUNNER_ARGS) BPF_CC_FLAGS='-O2 $(BPF_TARGET_ENDIAN) -mcpu=v4' \
+		$(CLANG_RUNNER_ARGS) BPF_CC_FLAGS='-O2 $(BPF_TARGET_ENDIAN) -mcpu=v4 $(CLANG_TYPED_ARENA)' \
 		BPF_DEFINES=-DENABLE_ATOMICS_TESTS
 endif
 
-- 
2.53.0