[ossfuzz] Improve the fuzzers

* Run more decompression variants
* Round trip the compression fuzzer and do partial decompression as well
* Add a compression fuzzer that compresses into a smaller output buffer
  and test the destSize variant

These fuzzers caught 2 bugs that were fixed in the previous commit.
* Input buffer over-read in partial decompress
* Partial decompress fails if output size is 0

8 files changed, 273 insertions, 44 deletions

diff --git a/ossfuzz/Makefile b/ossfuzz/Makefile
index bd01123..4d24944 100644
--- a/ossfuzz/Makefile
+++ b/ossfuzz/Makefile
@@ -57,3 +57,4 @@ $(LZ4DIR)/liblz4.a:
 
 .PHONY: clean
 clean: compress_fuzzer_clean decompress_fuzzer_clean
+	$(MAKE) -C $(LZ4DIR) clean
diff --git a/ossfuzz/compress_fuzzer.c b/ossfuzz/compress_fuzzer.c
index 3908534..7021624 100644
--- a/ossfuzz/compress_fuzzer.c
+++ b/ossfuzz/compress_fuzzer.c
@@ -1,25 +1,51 @@
+/**
+ * This fuzz target attempts to compress the fuzzed data with the simple
+ * compression function with an output buffer that may be too small to
+ * ensure that the compressor never crashes.
+ */
+
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
-#include "lz4.h"
+#include <string.h>
 
-#define CHECK(COND)   if (!(COND)) { abort(); }
+#include "fuzz_helpers.h"
+#include "lz4.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
-  size_t const compressed_dest_size = LZ4_compressBound(size);
-  char *const dest_buffer = (char *)malloc(compressed_dest_size);
+    uint32_t seed = FUZZ_seed(&data, &size);
+    size_t const dstCapacity = FUZZ_rand32(&seed, 0, LZ4_compressBound(size));
+    char* const dst = (char*)malloc(dstCapacity);
+    char* const rt = (char*)malloc(size);
+
+    FUZZ_ASSERT(dst);
+    FUZZ_ASSERT(rt);
 
-  CHECK(dest_buffer != NULL);
+    /* If compression succeeds it must round trip correctly. */
+    {
+        int const dstSize = LZ4_compress_default((const char*)data, dst,
+                                                 size, dstCapacity);
+        if (dstSize > 0) {
+            int const rtSize = LZ4_decompress_safe(dst, rt, dstSize, size);
+            FUZZ_ASSERT_MSG(rtSize == size, "Incorrect regenerated size");
+            FUZZ_ASSERT_MSG(!memcmp(data, rt, size), "Corruption!");
+        }
+    }
 
-  // Allocation succeeded, try compressing the incoming data.
-  int result = LZ4_compress_default((const char*)data,
-                                    dest_buffer,
-                                    size,
-                                    compressed_dest_size);
-  CHECK(result != 0);
+    if (dstCapacity > 0) {
+        /* Compression succeeds and must round trip correctly. */
+        int compressedSize = size;
+        int const dstSize = LZ4_compress_destSize((const char*)data, dst,
+                                                  &compressedSize, dstCapacity);
+        FUZZ_ASSERT(dstSize > 0);
+        int const rtSize = LZ4_decompress_safe(dst, rt, dstSize, size);
+        FUZZ_ASSERT_MSG(rtSize == compressedSize, "Incorrect regenerated size");
+        FUZZ_ASSERT_MSG(!memcmp(data, rt, compressedSize), "Corruption!");
+    }
 
-  free(dest_buffer);
+    free(dst);
+    free(rt);
 
-  return 0;
+    return 0;
 }
diff --git a/ossfuzz/decompress_fuzzer.c b/ossfuzz/decompress_fuzzer.c
index e6e14c4..0267c93 100644
--- a/ossfuzz/decompress_fuzzer.c
+++ b/ossfuzz/decompress_fuzzer.c
@@ -1,28 +1,58 @@
+/**
+ * This fuzz target attempts to decompress the fuzzed data with the simple
+ * decompression function to ensure the decompressor never crashes.
+ */
+
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
-#include "lz4.h"
+#include <string.h>
 
-#define CHECK(COND)   if (!(COND)) { abort(); }
+#include "fuzz_helpers.h"
+#include "lz4.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
 {
-  // TODO: Size input buffer pseudo-randomly based on seed extracted from input
-  size_t const buffer_size = 10 * 1024 * 1024;
-  char *const dest_buffer = (char *)malloc(buffer_size);
 
-  CHECK(dest_buffer != NULL);
+    uint32_t seed = FUZZ_seed(&data, &size);
+    size_t const dstCapacity = FUZZ_rand32(&seed, 0, 4 * size);
+    size_t const smallDictSize = size + 1;
+    size_t const largeDictSize = 64 * 1024 - 1;
+    size_t const dictSize = MAX(smallDictSize, largeDictSize);
+    char* const dst = (char*)malloc(dstCapacity);
+    char* const dict = (char*)malloc(dictSize + size);
+    char* const largeDict = dict;
+    char* const dataAfterDict = dict + dictSize;
+    char* const smallDict = dataAfterDict - smallDictSize;
 
-  // Allocation succeeded, try decompressing the incoming data.
-  int result = LZ4_decompress_safe((const char*)data,
-                                   dest_buffer,
-                                   size,
-                                   buffer_size);
+    FUZZ_ASSERT(dst);
+    FUZZ_ASSERT(dict);
 
-  // Ignore the result of decompression.
-  (void)result;
+    /* Prepare the dictionary. The data doesn't matter for decompression. */
+    memset(dict, 0, dictSize);
+    memcpy(dataAfterDict, data, size);
 
-  free(dest_buffer);
+    /* Decompress using each possible dictionary configuration. */
+    /* No dictionary. */
+    LZ4_decompress_safe_usingDict((char const*)data, dst, size,
+                                  dstCapacity, NULL, 0);
+    /* Small external dictonary. */
+    LZ4_decompress_safe_usingDict((char const*)data, dst, size,
+                                  dstCapacity, smallDict, smallDictSize);
+    /* Large external dictionary. */
+    LZ4_decompress_safe_usingDict((char const*)data, dst, size,
+                                  dstCapacity, largeDict, largeDictSize);
+    /* Small prefix. */
+    LZ4_decompress_safe_usingDict((char const*)dataAfterDict, dst, size,
+                                  dstCapacity, smallDict, smallDictSize);
+    /* Large prefix. */
+    LZ4_decompress_safe_usingDict((char const*)data, dst, size,
+                                  dstCapacity, largeDict, largeDictSize);
+    /* Partial decompression. */
+    LZ4_decompress_safe_partial((char const*)data, dst, size,
+                                dstCapacity, dstCapacity);
+    free(dst);
+    free(dict);
 
-  return 0;
+    return 0;
 }
diff --git a/ossfuzz/fuzz.h b/ossfuzz/fuzz.h
new file mode 100644
index 0000000..eefac63
--- /dev/null
+++ b/ossfuzz/fuzz.h
@@ -0,0 +1,48 @@
+/**
+ * Fuzz target interface.
+ * Fuzz targets have some common parameters passed as macros during compilation.
+ * Check the documentation for each individual fuzzer for more parameters.
+ *
+ * @param FUZZ_RNG_SEED_SIZE:
+ *        The number of bytes of the source to look at when constructing a seed
+ *        for the deterministic RNG. These bytes are discarded before passing
+ *        the data to lz4 functions. Every fuzzer initializes the RNG exactly
+ *        once before doing anything else, even if it is unused.
+ *        Default: 4.
+ * @param LZ4_DEBUG:
+ *        This is a parameter for the lz4 library. Defining `LZ4_DEBUG=1`
+ *        enables assert() statements in the lz4 library. Higher levels enable
+ *        logging, so aren't recommended. Defining `LZ4_DEBUG=1` is
+ *        recommended.
+ * @param LZ4_FORCE_MEMORY_ACCESS:
+ *        This flag controls how the zstd library accesses unaligned memory.
+ *        It can be undefined, or 0 through 2. If it is undefined, it selects
+ *        the method to use based on the compiler. If testing with UBSAN set
+ *        MEM_FORCE_MEMORY_ACCESS=0 to use the standard compliant method.
+ * @param FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+ *        This is the canonical flag to enable deterministic builds for fuzzing.
+ *        Changes to zstd for fuzzing are gated behind this define.
+ *        It is recommended to define this when building zstd for fuzzing.
+ */
+
+#ifndef FUZZ_H
+#define FUZZ_H
+
+#ifndef FUZZ_RNG_SEED_SIZE
+#  define FUZZ_RNG_SEED_SIZE 4
+#endif
+
+#include <stddef.h>
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/ossfuzz/fuzz_helpers.h b/ossfuzz/fuzz_helpers.h
new file mode 100644
index 0000000..626f209
--- /dev/null
+++ b/ossfuzz/fuzz_helpers.h
@@ -0,0 +1,89 @@
+/*
+ * Copyright (c) 2016-present, 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).
+ */
+
+/**
+ * Helper functions for fuzzing.
+ */
+
+#ifndef FUZZ_HELPERS_H
+#define FUZZ_HELPERS_H
+
+#include "fuzz.h"
+#include "xxhash.h"
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define MIN(a, b) ((a) < (b) ? (a) : (b))
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+
+#define FUZZ_QUOTE_IMPL(str) #str
+#define FUZZ_QUOTE(str) FUZZ_QUOTE_IMPL(str)
+
+/**
+ * Asserts for fuzzing that are always enabled.
+ */
+#define FUZZ_ASSERT_MSG(cond, msg)                                             \
+  ((cond) ? (void)0                                                            \
+          : (fprintf(stderr, "%s: %u: Assertion: `%s' failed. %s\n", __FILE__, \
+                     __LINE__, FUZZ_QUOTE(cond), (msg)),                       \
+             abort()))
+#define FUZZ_ASSERT(cond) FUZZ_ASSERT_MSG((cond), "");
+
+#if defined(__GNUC__)
+#define FUZZ_STATIC static __inline __attribute__((unused))
+#elif defined(__cplusplus) ||                                                  \
+    (defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) /* C99 */)
+#define FUZZ_STATIC static inline
+#elif defined(_MSC_VER)
+#define FUZZ_STATIC static __inline
+#else
+#define FUZZ_STATIC static
+#endif
+
+/**
+ * Deterministically constructs a seed based on the fuzz input.
+ * Consumes up to the first FUZZ_RNG_SEED_SIZE bytes of the input.
+ */
+FUZZ_STATIC uint32_t FUZZ_seed(uint8_t const **src, size_t* size) {
+    uint8_t const *data = *src;
+    size_t const toHash = MIN(FUZZ_RNG_SEED_SIZE, *size);
+    *size -= toHash;
+    *src += toHash;
+    return XXH32(data, toHash, 0);
+}
+
+#define FUZZ_rotl32(x, r) (((x) << (r)) | ((x) >> (32 - (r))))
+
+FUZZ_STATIC uint32_t FUZZ_rand(uint32_t *state) {
+    static const uint32_t prime1 = 2654435761U;
+    static const uint32_t prime2 = 2246822519U;
+    uint32_t rand32 = *state;
+    rand32 *= prime1;
+    rand32 += prime2;
+    rand32 = FUZZ_rotl32(rand32, 13);
+    *state = rand32;
+    return rand32 >> 5;
+}
+
+/* Returns a random numer in the range [min, max]. */
+FUZZ_STATIC uint32_t FUZZ_rand32(uint32_t *state, uint32_t min, uint32_t max) {
+    uint32_t random = FUZZ_rand(state);
+    return min + (random % (max - min + 1));
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif
diff --git a/ossfuzz/round_trip_fuzzer.c b/ossfuzz/round_trip_fuzzer.c
new file mode 100644
index 0000000..3a66e80
--- /dev/null
+++ b/ossfuzz/round_trip_fuzzer.c
@@ -0,0 +1,50 @@
+/**
+ * This fuzz target performs a lz4 round-trip test (compress & decompress),
+ * compares the result with the original, and calls abort() on corruption.
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "fuzz_helpers.h"
+#include "lz4.h"
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
+{
+    uint32_t seed = FUZZ_seed(&data, &size);
+    size_t const dstCapacity = LZ4_compressBound(size);
+    char* const dst = (char*)malloc(dstCapacity);
+    char* const rt = (char*)malloc(size);
+
+    FUZZ_ASSERT(dst);
+    FUZZ_ASSERT(rt);
+
+    /* Compression must succeed and round trip correctly. */
+    int const dstSize = LZ4_compress_default((const char*)data, dst,
+                                             size, dstCapacity);
+    FUZZ_ASSERT(dstSize > 0);
+
+    int const rtSize = LZ4_decompress_safe(dst, rt, dstSize, size);
+    FUZZ_ASSERT_MSG(rtSize == size, "Incorrect size");
+    FUZZ_ASSERT_MSG(!memcmp(data, rt, size), "Corruption!");
+
+    /* Partial decompression must succeed. */
+    {
+        size_t const partialCapacity = FUZZ_rand32(&seed, 0, size);
+        char* const partial = (char*)malloc(partialCapacity);
+        FUZZ_ASSERT(partial);
+        int const partialSize = LZ4_decompress_safe_partial(
+                dst, partial, dstSize, partialCapacity, partialCapacity);
+        FUZZ_ASSERT(partialSize >= 0);
+        FUZZ_ASSERT_MSG(partialSize == partialCapacity, "Incorrect size");
+        FUZZ_ASSERT_MSG(!memcmp(data, partial, partialSize), "Corruption!");
+        free(partial);
+    }
+
+    free(dst);
+    free(rt);
+
+    return 0;
+}
diff --git a/ossfuzz/standaloneengine.c b/ossfuzz/standaloneengine.c
index 175360e..6afeffd 100644
--- a/ossfuzz/standaloneengine.c
+++ b/ossfuzz/standaloneengine.c
@@ -2,7 +2,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#include "testinput.h"
+#include "fuzz.h"
 
 /**
  * Main procedure for standalone fuzzing engine.
diff --git a/ossfuzz/testinput.h b/ossfuzz/testinput.h
deleted file mode 100644
index 0e50a3c..0000000
--- a/ossfuzz/testinput.h
+++ /dev/null
@@ -1,15 +0,0 @@
-#ifndef TESTINPUT_H_INCLUDED
-#define TESTINPUT_H_INCLUDED
-
-#include <inttypes.h>
-
-#if defined (__cplusplus)
-extern "C" {
-#endif
-
-int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
-
-#if defined(__cplusplus)
-}
-#endif
-#endif