[PATCH v2 19/20] gcov: add MC/DC condition coverage support

[Sasha Levin <sashal@kernel.org>]

Support CONFIG_COVERAGE_MCDC with gcov, using GCC's MC/DC condition
coverage instrumentation (-fcondition-coverage), which is available
since GCC 14. The data is shown by "gcov --conditions".

Add -fcondition-coverage to the gcov compiler flags when enabled, and
disable the warnings about decisions it leaves out, as is done for
Clang. Both are gated on the config option to avoid Clang warnings
about unknown options.

Also add -fcondition-coverage to CFLAGS_REMOVE in the x86 UML vDSO
Makefile to prevent instrumentation of userspace vDSO code.

Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/um/vdso/Makefile | 4 ++--
 kernel/coverage/Kconfig   | 7 +++++--
 kernel/coverage/gcov/fs.c | 5 +++--
 scripts/Makefile.gcov     | 5 +++++
 4 files changed, 15 insertions(+), 6 deletions(-)

diff --git a/arch/x86/um/vdso/Makefile b/arch/x86/um/vdso/Makefile
index 7664cbedbe30f..adfc57c1a726d 100644
--- a/arch/x86/um/vdso/Makefile
+++ b/arch/x86/um/vdso/Makefile
@@ -42,8 +42,8 @@ $(vobjs): KBUILD_CFLAGS += $(CFL)
 #
 # vDSO code runs in userspace and -pg doesn't help with profiling anyway.
 #
-CFLAGS_REMOVE_vdso-note.o = -pg -fprofile-arcs -ftest-coverage
-CFLAGS_REMOVE_um_vdso.o = -pg -fprofile-arcs -ftest-coverage
+CFLAGS_REMOVE_vdso-note.o = -pg -fprofile-arcs -ftest-coverage -fcondition-coverage
+CFLAGS_REMOVE_um_vdso.o = -pg -fprofile-arcs -ftest-coverage -fcondition-coverage
 
 #
 # The DSO images are built using a special linker script.
diff --git a/kernel/coverage/Kconfig b/kernel/coverage/Kconfig
index 8d9d5f063aea6..3429bc902c934 100644
--- a/kernel/coverage/Kconfig
+++ b/kernel/coverage/Kconfig
@@ -108,8 +108,8 @@ config COVERAGE_PROFILE_ALL
 
 config COVERAGE_MCDC
 	bool "Measure modified condition/decision coverage (MC/DC)"
-	depends on LLVM_COV_KERNEL
-	depends on CLANG_VERSION >= 220000 || !WERROR
+	depends on LLVM_COV_KERNEL || (GCOV_KERNEL && CC_HAS_CONDITION_COVERAGE)
+	depends on !LLVM_COV_KERNEL || CLANG_VERSION >= 220000 || !WERROR
 	help
 	  MC/DC verifies that each condition of a boolean decision has been
 	  shown to independently affect the outcome of the decision. It is
@@ -119,6 +119,9 @@ config COVERAGE_MCDC
 	  Decisions that the compiler can't instrument are not measured, and
 	  only get branch coverage.
 
+	  With gcov this requires GCC 14 or later, and the data is shown by
+	  "gcov --conditions".
+
 	  If unsure, say N.
 
 config LLVM_COV_MCDC_MAX_CONDITIONS
diff --git a/kernel/coverage/gcov/fs.c b/kernel/coverage/gcov/fs.c
index a761cfb837414..689b8b9a0d16a 100644
--- a/kernel/coverage/gcov/fs.c
+++ b/kernel/coverage/gcov/fs.c
@@ -866,8 +866,9 @@ void gcov_event(enum gcov_action action, struct gcov_info *info)
 }
 
 static const struct coverage_backend gcov_backend = {
-	.name	= "gcov",
-	.reset	= gcov_reset_all,
+	.name		= "gcov",
+	.features	= IS_ENABLED(CONFIG_COVERAGE_MCDC) ? "mcdc" : NULL,
+	.reset		= gcov_reset_all,
 };
 
 /* Create debugfs entries. */
diff --git a/scripts/Makefile.gcov b/scripts/Makefile.gcov
index 1cfe04d89b0a2..977c8ee023f68 100644
--- a/scripts/Makefile.gcov
+++ b/scripts/Makefile.gcov
@@ -18,4 +18,9 @@ CFLAGS_COVERAGE	+= $(call try-run,\
 	cmp -s "$$TMP.ubase" "$$TMP.utest",\
 	-fprofile-update=prefer-atomic)
 endif
+ifdef CONFIG_COVERAGE_MCDC
+CFLAGS_COVERAGE	+= -fcondition-coverage
+# As for Clang: KCFLAGS=-Wcoverage-too-many-conditions lists what's left out.
+KBUILD_CFLAGS	+= -Wno-coverage-too-many-conditions
+endif
 export CFLAGS_COVERAGE
-- 
2.53.0