fuzz: link fuzz programs with make all on Linux

Since 5e47215 (fuzz: add basic fuzz testing target., 2018-10-12), we
have compiled object files for the fuzz tests as part of the default
'make all' target. This helps prevent bit-rot in lesser-used parts of
the codebase, by making sure that incompatible changes are caught at
build time.

However, since we never linked the fuzzer executables, this did not
protect us from link-time errors. As of 8b9a42b (fuzz: fix fuzz test
build rules, 2024-01-19), it's now possible to link the fuzzer
executables without using a fuzzing engine and a variety of
compiler-specific (and compiler-version-specific) flags, at least on
Linux. So let's add a platform-specific option in config.mak.uname to
link the executables as part of the default `make all` target.

Since linking the fuzzer executables without a fuzzing engine does not
require a C++ compiler, we can change the FUZZ_PROGRAMS build rule to
use $(CC) by default. This avoids compiler mis-match issues when
overriding $(CC) but not $(CXX). When we *do* want to actually link with
a fuzzing engine, we can set $(FUZZ_CXX). The build instructions in the
CI fuzz-smoke-test job and in the Makefile comment have been updated
accordingly.

While we're at it, we can consolidate some of the fuzzer build
instructions into one location in the Makefile.

Suggested-by: Junio C Hamano <[email protected]>
Helped-by: Jeff King <[email protected]>
Signed-off-by: Josh Steadmon <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Makefile

@@ -409,6 +409,9 @@ include shared.mak
 # to the "<name>" of the corresponding `compat/fsmonitor/fsm-settings-<name>.c`
 # that implements the `fsm_os_settings__*()` routines.
 #
+# Define LINK_FUZZ_PROGRAMS if you want `make all` to also build the fuzz test
+# programs in oss-fuzz/.
+#
 # === Optional library: libintl ===
 #
 # Define NO_GETTEXT if you don't want Git output to be translated.
@@ -752,23 +755,6 @@ SCRIPTS = $(SCRIPT_SH_GEN) \
 
 ETAGS_TARGET = TAGS
 
-# If you add a new fuzzer, please also make sure to run it in
-# ci/run-build-and-minimal-fuzzers.sh so that we make sure it still links and
-# runs in the future.
-FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
-FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
-FUZZ_OBJS += oss-fuzz/fuzz-config.o
-FUZZ_OBJS += oss-fuzz/fuzz-date.o
-FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o
-FUZZ_OBJS += oss-fuzz/fuzz-pack-idx.o
-.PHONY: fuzz-objs
-fuzz-objs: $(FUZZ_OBJS)
-
-# Always build fuzz objects even if not testing, to prevent bit-rot.
-all:: $(FUZZ_OBJS)
-
-FUZZ_PROGRAMS += $(patsubst %.o,%,$(filter-out %dummy-cmd-main.o,$(FUZZ_OBJS)))
-
 # Empty...
 EXTRA_PROGRAMS =
 
@@ -2372,6 +2358,29 @@ ifndef NO_TCLTK
 endif
 	$(QUIET_SUBDIR0)templates $(QUIET_SUBDIR1) SHELL_PATH='$(SHELL_PATH_SQ)' PERL_PATH='$(PERL_PATH_SQ)'
 
+# If you add a new fuzzer, please also make sure to run it in
+# ci/run-build-and-minimal-fuzzers.sh so that we make sure it still links and
+# runs in the future.
+FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
+FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
+FUZZ_OBJS += oss-fuzz/fuzz-config.o
+FUZZ_OBJS += oss-fuzz/fuzz-date.o
+FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o
+FUZZ_OBJS += oss-fuzz/fuzz-pack-idx.o
+.PHONY: fuzz-objs
+fuzz-objs: $(FUZZ_OBJS)
+
+# Always build fuzz objects even if not testing, to prevent bit-rot.
+all:: $(FUZZ_OBJS)
+
+FUZZ_PROGRAMS += $(patsubst %.o,%,$(filter-out %dummy-cmd-main.o,$(FUZZ_OBJS)))
+
+# Build fuzz programs when possible, even without the necessary fuzzing support,
+# to prevent bit-rot.
+ifdef LINK_FUZZ_PROGRAMS
+all:: $(FUZZ_PROGRAMS)
+endif
+
 please_set_SHELL_PATH_to_a_more_modern_shell:
 	@$$(:)
 
@@ -3857,22 +3866,22 @@ cover_db_html: cover_db
 #
 # An example command to build against libFuzzer from LLVM 11.0.0:
 #
-# make CC=clang CXX=clang++ \
+# make CC=clang FUZZ_CXX=clang++ \
 #      CFLAGS="-fsanitize=fuzzer-no-link,address" \
 #      LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
 #      fuzz-all
 #
+FUZZ_CXX ?= $(CC)
 FUZZ_CXXFLAGS ?= $(ALL_CFLAGS)
 
 .PHONY: fuzz-all
+fuzz-all: $(FUZZ_PROGRAMS)
 
 $(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS
-	$(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) -o $@ $(ALL_LDFLAGS) \
+	$(QUIET_LINK)$(FUZZ_CXX) $(FUZZ_CXXFLAGS) -o $@ $(ALL_LDFLAGS) \
 		-Wl,--allow-multiple-definition \
 		$(filter %.o,$^) $(filter %.a,$^) $(LIBS) $(LIB_FUZZING_ENGINE)
 
-fuzz-all: $(FUZZ_PROGRAMS)
-
 $(UNIT_TEST_PROGS): $(UNIT_TEST_BIN)/%$X: $(UNIT_TEST_DIR)/%.o $(UNIT_TEST_DIR)/test-lib.o $(GITLIBS) GIT-LDFLAGS
 	$(call mkdir_p_parent_template)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \

ci/run-build-and-minimal-fuzzers.sh

@@ -7,7 +7,7 @@
 
 group "Build fuzzers" make \
 	CC=clang \
-	CXX=clang++ \
+	FUZZ_CXX=clang++ \
 	CFLAGS="-fsanitize=fuzzer-no-link,address" \
 	LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
 	fuzz-all

config.mak.uname

@@ -68,6 +68,7 @@ ifeq ($(uname_S),Linux)
 	ifneq ($(findstring .el7.,$(uname_R)),)
 		BASIC_CFLAGS += -std=c99
 	endif
+	LINK_FUZZ_PROGRAMS = YesPlease
 endif
 ifeq ($(uname_S),GNU/kFreeBSD)
 	HAVE_ALLOCA_H = YesPlease