[fix] Always return dstSize_tooSmall when it is the case

tests/fuzz/decompress_dstSize_tooSmall.c

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2016-2020, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+/**
+ * This fuzz target attempts to decompress a valid compressed frame into
+ * an output buffer that is too small to ensure we always get
+ * ZSTD_error_dstSize_tooSmall.
+ */
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include "fuzz_helpers.h"
+#include "zstd.h"
+#include "zstd_errors.h"
+#include "zstd_helpers.h"
+#include "fuzz_data_producer.h"
+
+static ZSTD_CCtx *cctx = NULL;
+static ZSTD_DCtx *dctx = NULL;
+
+int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
+{
+    /* Give a random portion of src data to the producer, to use for
+    parameter generation. The rest will be used for (de)compression */
+    FUZZ_dataProducer_t *producer = FUZZ_dataProducer_create(src, size);
+    size_t rBufSize = FUZZ_dataProducer_uint32Range(producer, 0, size);
+    size = FUZZ_dataProducer_remainingBytes(producer);
+    /* Ensure the round-trip buffer is too small. */
+    if (rBufSize >= size) {
+        rBufSize = size > 0 ? size - 1 : 0;
+    }
+    size_t const cBufSize = ZSTD_compressBound(size);
+
+    if (!cctx) {
+        cctx = ZSTD_createCCtx();
+        FUZZ_ASSERT(cctx);
+    }
+    if (!dctx) {
+        dctx = ZSTD_createDCtx();
+        FUZZ_ASSERT(dctx);
+    }
+
+    void *cBuf = FUZZ_malloc(cBufSize);
+    void *rBuf = FUZZ_malloc(rBufSize);
+    size_t const cSize = ZSTD_compressCCtx(cctx, cBuf, cBufSize, src, size, 1);
+    FUZZ_ZASSERT(cSize);
+    size_t const rSize = ZSTD_decompressDCtx(dctx, rBuf, rBufSize, cBuf, cSize);
+    if (size == 0) {
+        FUZZ_ASSERT(rSize == 0);
+    } else {
+        FUZZ_ASSERT(ZSTD_isError(rSize));
+        FUZZ_ASSERT(ZSTD_getErrorCode(rSize) == ZSTD_error_dstSize_tooSmall);
+    }
+    free(cBuf);
+    free(rBuf);
+    FUZZ_dataProducer_free(producer);
+#ifndef STATEFUL_FUZZING
+    ZSTD_freeCCtx(cctx); cctx = NULL;
+    ZSTD_freeDCtx(dctx); dctx = NULL;
+#endif
+    return 0;
+}