lz4.c: fixed the LZ4_decompress_fast_continue case

The change is very similar to that of the LZ4_decompress_safe_continue
case.  The only reason a make this a separate change is to ensure that
the fuzzer, after it's been enhanced, can detect the flaw in
LZ4_decompress_fast_continue, and that the change indeed fixes the flaw.

1 files changed, 22 insertions, 2 deletions

diff --git a/lib/lz4.c b/lib/lz4.c
index eb3da21..916acf0 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -1752,6 +1752,15 @@ int LZ4_decompress_safe_doubleDict(const char* source, char* dest, int compresse
                                   (BYTE*)dest-prefixSize, (const BYTE*)dictStart, dictSize);
 }
 
+LZ4_FORCE_INLINE
+int LZ4_decompress_fast_doubleDict(const char* source, char* dest, int originalSize,
+                                   size_t prefixSize, const void* dictStart, size_t dictSize)
+{
+    return LZ4_decompress_generic(source, dest, 0, originalSize,
+                                  endOnOutputSize, full, 0, usingExtDict,
+                                  (BYTE*)dest-prefixSize, (const BYTE*)dictStart, dictSize);
+}
+
 /*===== streaming decompression functions =====*/
 
 LZ4_streamDecode_t* LZ4_createStreamDecode(void)
@@ -1831,21 +1840,32 @@ int LZ4_decompress_safe_continue (LZ4_streamDecode_t* LZ4_streamDecode, const ch
     return result;
 }
 
+LZ4_FORCE_O2_GCC_PPC64LE
 int LZ4_decompress_fast_continue (LZ4_streamDecode_t* LZ4_streamDecode, const char* source, char* dest, int originalSize)
 {
     LZ4_streamDecode_t_internal* lz4sd = &LZ4_streamDecode->internal_donotuse;
     int result;
 
-    if (lz4sd->prefixSize == 0 || lz4sd->prefixEnd == (BYTE*)dest) {
+    if (lz4sd->prefixSize == 0) {
+        assert(lz4sd->extDictSize == 0);
         result = LZ4_decompress_fast(source, dest, originalSize);
         if (result <= 0) return result;
+        lz4sd->prefixSize = originalSize;
+        lz4sd->prefixEnd = (BYTE*)dest + originalSize;
+    } else if (lz4sd->prefixEnd == (BYTE*)dest) {
+        if (lz4sd->prefixSize >= 64 KB - 1 || lz4sd->extDictSize == 0)
+            result = LZ4_decompress_fast(source, dest, originalSize);
+        else
+            result = LZ4_decompress_fast_doubleDict(source, dest, originalSize,
+                                                    lz4sd->prefixSize, lz4sd->externalDict, lz4sd->extDictSize);
+        if (result <= 0) return result;
         lz4sd->prefixSize += originalSize;
         lz4sd->prefixEnd  += originalSize;
     } else {
         lz4sd->extDictSize = lz4sd->prefixSize;
         lz4sd->externalDict = lz4sd->prefixEnd - lz4sd->extDictSize;
         result = LZ4_decompress_fast_extDict(source, dest, originalSize,
-                                             (const char*)lz4sd->externalDict, lz4sd->extDictSize);
+                                             lz4sd->externalDict, lz4sd->extDictSize);
         if (result <= 0) return result;
         lz4sd->prefixSize = originalSize;
         lz4sd->prefixEnd  = (BYTE*)dest + originalSize;