fix(fuzz): build a minimal libpcap for oss-fuzz targets instead of linking the system one (#1092)

test/fuzz/oss-fuzz/build.sh

@@ -34,16 +34,51 @@ cd "$SRC/tcpreplay"
 ./configure --disable-local-libopts
 make -j"$(nproc)"
 
+# The Dockerfile installs libpcap-dev so this build (and tcpreplay's own
+# normal build) can find pcap.h/link against something - but the fuzz targets
+# link against a *separately built* libpcap.a below, not that system package.
+#
+# Debian/Ubuntu's libpcap is built with D-Bus, Bluetooth and USB sniffing
+# support enabled, each pulling in its own shared library (libdbus-1,
+# libsystemd, and libsystemd's own dependency on libcap in turn). All of that
+# is dynamically linked into libpcap.so, so the *build* step here links fine
+# - the base-builder image has every one of those .so files. OSS-Fuzz's
+# separate runner image, where the built binaries actually execute, does not:
+# it installs only libcap2 (see infra/base-images/base-runner/install_deps.sh
+# upstream), not libdbus-1 or libsystemd. The result was every target dying
+# at startup with "error while loading shared libraries: libpcap.so.0.8:
+# cannot open shared object file" - a build that reported success but
+# produced nothing that could actually run.
+#
+# Linking the *system* libpcap.a statically doesn't fix this - it just moves
+# the missing .so from libpcap itself to whatever libpcap.a still pulls in
+# (dbus, systemd, and transitively libcap's cap_* symbols, none of which are
+# needed for reading pcap files). None of that is needed to fuzz the pcap
+# read path, so it's built out entirely: a self-contained libpcap.a with the
+# capture backends that need those disabled, leaving nothing for a fuzz
+# target to depend on beyond libc/libstdc++/libm - which the runner image, or
+# any Linux system, always has.
+cd "$SRC/libpcap"
+mkdir -p build
+cd build
+cmake -DBUILD_SHARED_LIBS=OFF -DDISABLE_DBUS=ON -DDISABLE_BLUETOOTH=ON \
+      -DDISABLE_LINUX_USBMON=ON -DDISABLE_RDMA=ON ..
+make -j"$(nproc)" pcap_static
+LIBPCAP_A="$SRC/libpcap/build/libpcap.a"
+cd "$SRC/tcpreplay"
+
 # libdnet is optional, and fragroute is the only target that needs it - Debian/
 # Ubuntu (including this image) package it as libdumbnet, not libdnet, so the
 # link flag has to come from wherever ./configure already worked that out
 # rather than being guessed here. test/fuzz/Makefile has it after configure
-# runs above, whether or not fragroute itself is actually available.
+# runs above, whether or not fragroute itself is actually available. Statically
+# linked for the same reason as libpcap above; libdumbnet has no equivalent
+# transitive dependency problem, so the system copy is fine to use as-is.
 FRAGROUTE_LIB=""
 FRAGROUTE_LIBNET=""
 if [ -f src/fragroute/libfragroute.a ]; then
     FRAGROUTE_LIB="src/fragroute/libfragroute.a"
-    FRAGROUTE_LIBNET=$(sed -n 's/^LDNETLIB = //p' test/fuzz/Makefile)
+    FRAGROUTE_LIBNET="-Wl,-Bstatic $(sed -n 's/^LDNETLIB = //p' test/fuzz/Makefile) -Wl,-Bdynamic"
 fi
 
 for target in fuzz_services fuzz_pcap fuzz_fragroute; do
@@ -52,9 +87,13 @@ for target in fuzz_services fuzz_pcap fuzz_fragroute; do
         continue
     fi
 
-    libs="src/common/libcommon.a -lpcap"
+    # Link order matters for static archives: they only resolve symbols for
+    # things listed *after* them, so each library has to come before whatever
+    # it depends on - libfragroute needs libcommon and libdumbnet, libcommon
+    # needs libpcap.
+    libs="src/common/libcommon.a $LIBPCAP_A"
     if [ "$target" = "fuzz_fragroute" ]; then
-        libs="$FRAGROUTE_LIB $libs $FRAGROUTE_LIBNET"
+        libs="$FRAGROUTE_LIB src/common/libcommon.a $LIBPCAP_A $FRAGROUTE_LIBNET"
     fi
 
     $CC $CFLAGS -DHAVE_CONFIG_H -I. -Isrc -Itest/fuzz -c "test/fuzz/${target}.c" -o "$WORK/${target}.o"