[libgit2] Use upstreamed fuzzers (#1684)

To get the ball rolling more quickly, the download_refs fuzzer for
libgit2 was first implemented directly in oss-fuzz. But as we want to
ensure that we're not breaking fuzzers and thus at least build them as
part of our CI, the goal has been to upstream them into libgit2.

This commit removes the download_refs fuzzer and its corpus in favor of
using the upstreamed fuzzer. Furthermore, the build script is
generalized to automatically pick up new fuzzers named according to a
certain pattern, as we already added a second packfile fuzzer.

projects/libgit2/Dockerfile

@@ -20,5 +20,4 @@ RUN apt-get update && apt-get install -y make autoconf automake libtool cmake
 RUN git clone --depth 1 https://github.com/libgit2/libgit2 libgit2
 WORKDIR libgit2
 
-COPY download_refs_fuzzer.cc build.sh $SRC/
-COPY corpora $SRC/corpora
+COPY build.sh $SRC/

projects/libgit2/build.sh

@@ -28,8 +28,14 @@ cmake .. -DCMAKE_INSTALL_PREFIX="$WORK" \
 make -j$(nproc)
 make install
 
-$CXX $CXXFLAGS -std=c++11 -I"$WORK/include" \
-    /src/download_refs_fuzzer.cc -o $OUT/download_refs_fuzzer \
-    -lFuzzingEngine "$WORK/lib/libgit2.a"
+for fuzzer in ../fuzzers/*_fuzzer.c
+do
+    fuzzer_name=$(basename "${fuzzer%.c}")
 
-zip -j "$OUT/download_refs_fuzzer_seed_corpus.zip" $SRC/corpora/download_refs/*
+    $CC $CFLAGS -c -I"$WORK/include" "$fuzzer" -o "$WORK/$fuzzer_name.o"
+    $CXX $CXXFLAGS -std=c++11 -o "$OUT/$fuzzer_name" \
+        -lFuzzingEngine "$WORK/$fuzzer_name.o" "$WORK/lib/libgit2.a"
+
+    zip -j "$OUT/${fuzzer_name}_seed_corpus.zip" \
+        ../fuzzers/corpora/${fuzzer_name%_fuzzer}/*
+done

projects/libgit2/download_refs_fuzzer.cc

@@ -1,156 +0,0 @@
-#include <git2.h>
-#include <git2/sys/transport.h>
-
-#include <cstring>
-#include <cstdlib>
-#include <sys/stat.h>
-
-#include <string>
-
-struct fuzz_buffer {
-    const uint8_t *data;
-    size_t size;
-};
-
-class fuzzer_stream {
-public:
-    git_smart_subtransport_stream base;
-    fuzzer_stream(fuzz_buffer data) : readp(data.data), endp(data.data + data.size) {
-        base.read = fuzzer_stream::read;
-        base.write = fuzzer_stream::write;
-        base.free = fuzzer_stream::free;
-    }
-
-    int do_read(char *buffer, size_t buf_size, size_t *bytes_read) {
-        size_t avail = endp - readp;
-        *bytes_read = std::min(buf_size, avail);
-        memcpy(buffer, readp, *bytes_read);
-        readp += *bytes_read;
-        return 0;
-    }
-
-    static int read(git_smart_subtransport_stream *stream,
-                    char *buffer,
-                    size_t buf_size,
-                    size_t *bytes_read) {
-        fuzzer_stream *fs = reinterpret_cast<fuzzer_stream*>(stream);
-        return fs->do_read(buffer, buf_size, bytes_read);
-    }
-
-    static int write(git_smart_subtransport_stream *stream,
-              const char *buffer,
-              size_t len) {
-        return 0;
-    }
-
-    static void free(git_smart_subtransport_stream *stream) {
-        fuzzer_stream *fs = reinterpret_cast<fuzzer_stream*>(stream);
-        delete fs;
-    }
-private:
-    const uint8_t *readp;
-    const uint8_t *endp;
-};
-
-class fuzzer_subtransport {
-public:
-    git_smart_subtransport base;
-    fuzzer_subtransport(git_transport *owner, fuzz_buffer data) : owner(owner), data(data) {
-        base.action = fuzzer_subtransport::action;
-        base.close = fuzzer_subtransport::close;
-        base.free = fuzzer_subtransport::free;
-    }
-
-    int do_action(git_smart_subtransport_stream **out,
-                  git_smart_subtransport *transport,
-                  const char *url,
-                  git_smart_service_t action) {
-        fuzzer_stream *stream = new fuzzer_stream(this->data);
-        *out = &stream->base;
-        return 0;
-    }
-
-    static int action(git_smart_subtransport_stream **out,
-                      git_smart_subtransport *transport,
-                      const char *url,
-                      git_smart_service_t action) {
-        fuzzer_subtransport *ft = reinterpret_cast<fuzzer_subtransport*>(transport);
-        return ft->do_action(out, transport, url, action);
-    }
-
-    static int close(git_smart_subtransport *transport) {
-        return 0;
-    }
-
-    static void free(git_smart_subtransport *transport) {
-        fuzzer_subtransport *ft = reinterpret_cast<fuzzer_subtransport*>(transport);
-        delete ft;
-    }
-
-private:
-    git_transport *owner;
-    fuzz_buffer data;
-};
-
-int fuzzer_subtransport_cb(git_smart_subtransport **out,
-                           git_transport* owner,
-                           void* param) {
-    fuzz_buffer *buf = static_cast<fuzz_buffer*>(param);
-    fuzzer_subtransport *sub = new fuzzer_subtransport(owner, *buf);
-
-    *out = &sub->base;
-    return 0;
-}
-
-int create_fuzzer_transport(git_transport **out, git_remote *owner, void *param) {
-    git_smart_subtransport_definition fuzzer_subtransport {fuzzer_subtransport_cb, 1, param};
-    return git_transport_smart(out, owner, &fuzzer_subtransport);
-}
-
-void fuzzer_git_abort(const char *op) {
-    const git_error *err  = giterr_last();
-    fprintf(stderr, "unexpected libgit error: %s: %s\n",
-            op, err ? err->message : "<none>");
-    abort();
-}
-
-
-extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
-  static git_repository *repo = nullptr;
-  if (repo == nullptr) {
-      git_libgit2_init();
-      char tmp[] = "/tmp/git2.XXXXXX";
-      if (mkdtemp(tmp) != tmp) {
-          abort();
-      }
-      int err = git_repository_init(&repo, tmp, true);
-      if (err != 0) {
-          fuzzer_git_abort("git_repository_init");
-      }
-  }
-
-  int err;
-  git_remote *remote;
-  err = git_remote_create_anonymous(&remote, repo, "fuzzer://remote-url");
-  if (err != 0) {
-      fuzzer_git_abort("git_remote_create");
-  }
-
-
-  fuzz_buffer buffer = {data, size};
-  git_remote_callbacks callbacks = GIT_REMOTE_CALLBACKS_INIT;
-  callbacks.transport = create_fuzzer_transport;
-  callbacks.payload = &buffer;
-
-  err = git_remote_connect(remote, GIT_DIRECTION_FETCH, &callbacks, nullptr, nullptr);
-  if (err != 0) {
-      goto out;
-  }
-
-  git_remote_download(remote, nullptr, nullptr);
-
- out:
-  git_remote_free(remote);
-
-  return 0;
-}