Run limited fuzz tests in GitHub CI

Add a simple smoke test in CI to make sure that the fuzz tests can build and execute properly. While we already compile the fuzz-test objects in the default make target, we don't link the executables due to these requiring clang-specific support. However, this means that the fuzz tests have been vulnerable to unnoticed build breakages as the code that they link against has changed over time.

Adding this CI test should make such build breakages more visible more quickly.

Josh Steadmon (2):
fuzz: fix fuzz test build rules
ci: build and run minimal fuzzers in GitHub CI

 .github/workflows/main.yml          | 11 +++++++++++
 Makefile                            | 17 +++++++++++------
 ci/run-build-and-minimal-fuzzers.sh | 19 +++++++++++++++++++
 oss-fuzz/dummy-cmd-main.c           | 14 ++++++++++++++
 4 files changed, 55 insertions(+), 6 deletions(-)
 create mode 100755 ci/run-build-and-minimal-fuzzers.sh
 create mode 100644 oss-fuzz/dummy-cmd-main.c

base-commit: 186b115d3062e6230ee296d1ddaa0c4b72a464b5

2.43.0.429.g432eaa2c6b-goog

When we originally added the fuzz tests in 5e47215080 (fuzz: add basic fuzz testing target., 2018-10-12), we went to some trouble to create a Makefile rule that allowed linking the fuzz executables without pulling in common-main.o. This was necessary to prevent the fuzzing-engine-provided main() from clashing with Git's main().

However, since 19d75948ef (common-main.c: move non-trace2 exit() behavior out of trace2.c, 2022-06-02), it has been necessary to link common-main.o due to moving the common_exit() function to that file. Ævar suggested a set of compiler flags to allow this in [1], but this was never reflected in the Makefile.

Since we now must include common-main.o, there's no reason to pick and choose a subset of object files to link, so simplify the Makefile rule for the fuzzer executables to just use libgit.a. While we're at it, include the necessary linker flag to allow multiple definitions directly in the Makefile rule, rather than requiring it to be passed on the command-line each time. This means the Makefile rule as written is now more compiler-specific, but this was already the case for the fuzzers themselves anyway.

[1] https://lore.kernel.org/git/[email protected]/

---
 Makefile                  | 14 ++++++++------
 oss-fuzz/dummy-cmd-main.c | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 6 deletions(-)
 create mode 100644 oss-fuzz/dummy-cmd-main.c

index 15990ff312..1e9bd6430f 100644
@@ -752,6 +752,7 @@ SCRIPTS = $(SCRIPT_SH_GEN) \
 
 ETAGS_TARGET = TAGS
 
+FUZZ_OBJS += oss-fuzz/dummy-cmd-main.o
 FUZZ_OBJS += oss-fuzz/fuzz-commit-graph.o
 FUZZ_OBJS += oss-fuzz/fuzz-date.o
 FUZZ_OBJS += oss-fuzz/fuzz-pack-headers.o

@@ -762,7 +763,7 @@ fuzz-objs: $(FUZZ_OBJS)
 # Always build fuzz objects even if not testing, to prevent bit-rot.
 all:: $(FUZZ_OBJS)
 
-FUZZ_PROGRAMS += $(patsubst %.o,%,$(FUZZ_OBJS))
+FUZZ_PROGRAMS += $(patsubst %.o,%,$(filter-out %dummy-cmd-main.o,$(FUZZ_OBJS)))
 
 # Empty...
 EXTRA_PROGRAMS =

@@ -3850,16 +3851,17 @@ cover_db_html: cover_db
 #
 # make CC=clang CXX=clang++ \
 #      CFLAGS="-fsanitize=fuzzer-no-link,address" \
-#      LIB_FUZZING_ENGINE="-fsanitize=fuzzer" \
+#      LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
 #      fuzz-all
 #
-FUZZ_CXXFLAGS ?= $(CFLAGS)
+FUZZ_CXXFLAGS ?= $(ALL_CFLAGS)
 
 .PHONY: fuzz-all
 
-$(FUZZ_PROGRAMS): all
-	$(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) $(LIB_OBJS) $(BUILTIN_OBJS) \
-		$(XDIFF_OBJS) $(EXTLIBS) git.o [email protected] $(LIB_FUZZING_ENGINE) -o $@
+$(FUZZ_PROGRAMS): %: %.o oss-fuzz/dummy-cmd-main.o $(GITLIBS) GIT-LDFLAGS
+	$(QUIET_LINK)$(CXX) $(FUZZ_CXXFLAGS) -o $@ $(ALL_LDFLAGS) \
+		-Wl,--allow-multiple-definition \
+		$(filter %.o,$^) $(filter %.a,$^) $(LIBS) $(LIB_FUZZING_ENGINE)
 
 fuzz-all: $(FUZZ_PROGRAMS)
 

new file mode 100644
index 0000000000..071cb231ba
@@ -0,0 +1,14 @@
+#include "git-compat-util.h"
+
+/*
+ * When linking the fuzzers, we link against common-main.o to pick up some
+ * symbols. However, even though we ignore common-main:main(), we still need to
+ * provide all the symbols it references. In the fuzzers' case, we need to
+ * provide a dummy cmd_main() for the linker to be happy. It will never be
+ * executed.
+ */
+
+int cmd_main(int argc, const char **argv) {
+	BUG("We should not execute cmd_main() from a fuzz target");
+	return 1;
+}

2.43.0.429.g432eaa2c6b-goog

But there is this target. I wonder if it makes it even better to update the "always build fuzz objects" one? Given that some folks may not have the necessary clang toochain for linking, it may probably be a bit too much, perhaps?

It definitely is an improvement to build them in the CI environment, like you have in [2/2].

Thanks. Will queue.

Indeed, this would have caught the previous common-main issue. I'll send a followup to fix this, thanks for the suggestion!

To prevent bitrot, we would like to regularly exercise the fuzz tests in order to make sure they still link & run properly. We already compile the fuzz test objects as part of the default make target, but we do not link the executables due to the fuzz tests needing specific compilers and compiler features. This has lead to frequent build breakages for the fuzz tests.

To remedy this, we can add a CI step to actually link the fuzz executables, and run them (with finite input rather than the default infinite random input mode) to verify that they execute properly.

Since the main use of the fuzz tests is via OSS-Fuzz [1], and OSS-Fuzz only runs tests on Linux [2], we only set up a CI test for the fuzzers on Linux.

[1] https://github.com/google/oss-fuzz
[2] https://google.github.io/oss-fuzz/further-reading/fuzzer-environment/

---
 .github/workflows/main.yml          | 11 +++++++++++
 Makefile                            |  3 +++
 ci/run-build-and-minimal-fuzzers.sh | 19 +++++++++++++++++++
 3 files changed, 33 insertions(+)
 create mode 100755 ci/run-build-and-minimal-fuzzers.sh

index 9fdbd54028..4d97da57ec 100644
@@ -309,6 +309,17 @@ jobs:
       with:
         name: failed-tests-${{matrix.vector.jobname}}
         path: ${{env.FAILED_TEST_ARTIFACTS}}
+  fuzz-smoke-test:
+    name: fuzz smoke test
+    needs: ci-config
+    if: needs.ci-config.outputs.enabled == 'yes'
+    env:
+      CC: clang
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - run: ci/install-dependencies.sh
+    - run: ci/run-build-and-minimal-fuzzers.sh
   dockerized:
     name: ${{matrix.vector.jobname}} (${{matrix.vector.image}})
     needs: ci-config

index 1e9bd6430f..2e94c566e0 100644
@@ -752,6 +752,9 @@ 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-date.o

new file mode 100755
index 0000000000..8ba486f659
@@ -0,0 +1,19 @@
+#!/bin/sh
+#
+# Build and test Git's fuzzers
+#
+
+. ${0%/*}/lib.sh
+
+group "Build fuzzers" make \
+	CC=clang \
+	CXX=clang++ \
+	CFLAGS="-fsanitize=fuzzer-no-link,address" \
+	LIB_FUZZING_ENGINE="-fsanitize=fuzzer,address" \
+	fuzz-all
+
+for fuzzer in commit-graph date pack-headers pack-idx ; do
+	begin_group "fuzz-$fuzzer"
+	./oss-fuzz/fuzz-$fuzzer -verbosity=0 -runs=1 || exit 1
+	end_group "fuzz-$fuzzer"
+done

2.43.0.429.g432eaa2c6b-goog

Nice.