[RFC PATCH 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 and -Wno-error=coverage-too-many-conditions to
the gcov compiler flags when enabled. Both flags 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     | 3 +++
 4 files changed, 13 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 fab3da51305bb..5990e366db994 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 produce build warnings
 	  and are not measured.
 
+	  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 23d366dcfc2f7..dfaccc01e06f3 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 1620b81398eaa..b00dd40f2167d 100644
--- a/scripts/Makefile.gcov
+++ b/scripts/Makefile.gcov
@@ -19,4 +19,7 @@ CFLAGS_COVERAGE	+= $(call try-run,\
 	cmp -s "$$TMP.ubase" "$$TMP.utest",\
 	-fprofile-update=prefer-atomic)
 endif
+ifdef CONFIG_COVERAGE_MCDC
+CFLAGS_COVERAGE	+= -fcondition-coverage -Wno-error=coverage-too-many-conditions
+endif
 export CFLAGS_COVERAGE
-- 
2.53.0