OSS-Fuzz: Add a new fuzzer targets client read handle

fuzzing/Makefile

@@ -5,6 +5,7 @@ all:
 	zip -r corpora/db_dump_seed_corpus.zip ../test/apps/db_dump/data/
 	$(MAKE) -C apps $@
 	$(MAKE) -C broker $@
+	$(MAKE) -C client $@
 	$(MAKE) -C libcommon $@
 	$(MAKE) -C plugins $@
 
@@ -14,5 +15,6 @@ clean:
 	-rm -f corpora/db_dump_seed_corpus.zip
 	$(MAKE) -C apps $@
 	$(MAKE) -C broker $@
+	$(MAKE) -C client $@
 	$(MAKE) -C libcommon $@
 	$(MAKE) -C plugins $@

fuzzing/client/Makefile

@@ -0,0 +1,34 @@
+R=../..
+include ${R}/fuzzing/config.mk
+
+.PHONY: all clean
+
+PACKET_FUZZERS:= \
+	client_fuzz_read_handle
+
+# IMPORTANT: these -D flags must exactly match the ones used to compile
+# lib/libmosquitto.a (see config.mk with WITH_FUZZING=yes). A mismatch changes
+# the layout of `struct mosquitto` between this harness and the library and
+# leads to memory corruption. In particular WITH_WEBSOCKETS=WS_IS_BUILTIN adds
+# fields to the struct, and WITH_BROKER must NOT be defined for the client lib.
+LOCAL_CPPFLAGS+=-I${R}/include/ -I${R}/lib -I${R} -I${R}/common \
+	-I${R}/deps -I${R}/deps/picohttpparser \
+	-DWITH_THREADING -DWITH_TLS -DWITH_TLS_PSK -DWITH_SOCKS \
+	-DWITH_UNIX_SOCKETS -DWITH_WEBSOCKETS=WS_IS_BUILTIN -DWITH_FUZZING
+LOCAL_CXXFLAGS+=-g -Wall -Werror -pthread
+LOCAL_CFLAGS+=-g -Wall -Werror -pthread
+LOCAL_LDFLAGS+=
+LOCAL_LIBADD+=$(LIB_FUZZING_ENGINE) -lssl -lcrypto -lcjson -lm \
+	${R}/libcommon/libmosquitto_common.a \
+	-Wl,-Bdynamic -Wl,-Bstatic -largon2 -Wl,-Bdynamic
+LIBMOSQ_A=${R}/lib/libmosquitto.a
+
+all: $(PACKET_FUZZERS)
+
+${PACKET_FUZZERS} : %: %.c ${R}/lib/libmosquitto.a
+	$(CC) $(LOCAL_CFLAGS) $(LOCAL_CPPFLAGS) $(LOCAL_LDFLAGS) -o $@ $< $(LIBMOSQ_A) $(LOCAL_LIBADD)
+	install $@ ${OUT}/$@
+	cp ${R}/fuzzing/corpora/client_packet_seed_corpus.zip ${OUT}/$@_seed_corpus.zip
+
+clean:
+	rm -f *.o $(PACKET_FUZZERS)

fuzzing/client/client_fuzz_read_handle.c

@@ -0,0 +1,80 @@
+/*
+Copyright (c) 2026 Cedalo GmbH
+
+All rights reserved. This program and the accompanying materials
+are made available under the terms of the Eclipse Public License 2.0
+and Eclipse Distribution License v1.0 which accompany this distribution.
+
+The Eclipse Public License is available at
+   https://www.eclipse.org/legal/epl-2.0/
+and the Eclipse Distribution License is available at
+  http://www.eclipse.org/org/documents/edl-v10.php.
+
+SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
+
+Contributors:
+   AdaLogics - client-side read-handle fuzzer.
+*/
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "mosquitto.h"
+#include "mosquitto/libcommon_memory.h"
+#include "mosquitto_internal.h"
+#include "packet_mosq.h"
+#include "read_handle.h"
+
+#define kMinInputLength 3
+#define kMaxInputLength 268435455U
+
+static int initialised = 0;
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+	struct mosquitto *mosq;
+	uint8_t *payload;
+	size_t payload_len;
+
+	if(size < kMinInputLength || size > kMaxInputLength){
+		return 0;
+	}
+
+	if(!initialised){
+		mosquitto_lib_init();
+		initialised = 1;
+	}
+
+	mosq = mosquitto_new("fuzz-client", true, NULL);
+	if(!mosq){
+		return 0;
+	}
+
+	/* Put the client in a connected state; v5 also exercises the property parser */
+	mosq->protocol = (data[0] & 0x01) ? mosq_p_mqtt311 : mosq_p_mqtt5;
+	mosq->state = mosq_cs_connected;
+	mosq->alias_max_l2r = 10;
+
+	payload_len = size - 1;
+	payload = (uint8_t *)mosquitto_malloc(payload_len);
+	if(!payload){
+		mosquitto_destroy(mosq);
+		return 0;
+	}
+	memcpy(payload, &data[1], payload_len);
+
+	/* Stage the bytes as a received packet with the command byte consumed (pos=1) */
+	mosq->in_packet.command = payload[0];
+	mosq->in_packet.payload = payload;
+	mosq->in_packet.packet_length = (uint32_t)payload_len;
+	mosq->in_packet.remaining_length = (uint32_t)(payload_len - 1);
+	mosq->in_packet.pos = 1;
+
+	handle__packet(mosq);
+
+	packet__cleanup(&mosq->in_packet);
+	mosquitto_destroy(mosq);
+
+	return 0;
+}