fix one UB pointer arithmetic in encoder

Instead of calculating distance between 2 memory objects, which is UB,
we extract the offset from object 1, and transfer it into object 2.

lib/compress/zstd_compress.c

@@ -1819,17 +1819,21 @@ static size_t ZSTD_compressContinue_internal (ZSTD_CCtx* cctx,
         cctx->stage = ZSTDcs_ongoing;
     }
 
+    if (!srcSize) return fhSize;  /* do not generate an empty block if no input */
+
     /* Check if blocks follow each other */
     if (src != cctx->nextSrc) {
         /* not contiguous */
-        ptrdiff_t const delta = cctx->nextSrc - ip;
+        size_t const distanceFromBase = (size_t)(cctx->nextSrc - cctx->base);
         cctx->lowLimit = cctx->dictLimit;
-        cctx->dictLimit = (U32)(cctx->nextSrc - cctx->base);
+        assert(distanceFromBase == (size_t)(U32)distanceFromBase);  /* should never overflow */
+        cctx->dictLimit = (U32)distanceFromBase;
         cctx->dictBase = cctx->base;
-        cctx->base -= delta;
+        cctx->base = ip - distanceFromBase;
         cctx->nextToUpdate = cctx->dictLimit;
         if (cctx->dictLimit - cctx->lowLimit < HASH_READ_SIZE) cctx->lowLimit = cctx->dictLimit;   /* too small extDict */
     }
+    cctx->nextSrc = ip + srcSize;
 
     /* if input and dictionary overlap : reduce dictionary (area presumed modified by input) */
     if ((ip+srcSize > cctx->dictBase + cctx->lowLimit) & (ip < cctx->dictBase + cctx->dictLimit)) {
@@ -1838,17 +1842,13 @@ static size_t ZSTD_compressContinue_internal (ZSTD_CCtx* cctx,
         cctx->lowLimit = lowLimitMax;
     }
 
-    cctx->nextSrc = ip + srcSize;
-
-    if (srcSize) {
-        size_t const cSize = frame ?
+    {   size_t const cSize = frame ?
                              ZSTD_compress_frameChunk (cctx, dst, dstCapacity, src, srcSize, lastFrameChunk) :
                              ZSTD_compressBlock_internal (cctx, dst, dstCapacity, src, srcSize);
         if (ZSTD_isError(cSize)) return cSize;
         cctx->consumedSrcSize += srcSize;
         return cSize + fhSize;
-    } else
-        return fhSize;
+    }
 }
 
 size_t ZSTD_compressContinue (ZSTD_CCtx* cctx,

lib/decompress/zstd_decompress.c

@@ -1225,7 +1225,8 @@ seq_t ZSTD_decodeSequenceLong(seqState_t* seqState, ZSTD_longOffset_e const long
 
     {   size_t const pos = seqState->pos + seq.litLength;
         const BYTE* const matchBase = (seq.offset > pos) ? seqState->dictEnd : seqState->prefixStart;
-        seq.match = matchBase + pos - seq.offset;
+        seq.match = matchBase + pos - seq.offset;  /* note : this operation can overflow when seq.offset is really too large, which can only happen when input is corrupted.
+                                                    * No consequence though : no memory access will occur, overly large offset will be detected in ZSTD_execSequenceLong() */
         seqState->pos = pos + seq.matchLength;
     }
 
@@ -1372,7 +1373,7 @@ static size_t ZSTD_decompressSequencesLong(
             seq_t const sequence = ZSTD_decodeSequenceLong(&seqState, isLongOffset);
             size_t const oneSeqSize = ZSTD_execSequenceLong(op, oend, sequences[(seqNb-ADVANCED_SEQS) & STOSEQ_MASK], &litPtr, litEnd, prefixStart, dictStart, dictEnd);
             if (ZSTD_isError(oneSeqSize)) return oneSeqSize;
-            PREFETCH(sequence.match);
+            PREFETCH(sequence.match);  /* note : it's safe to invoke PREFETCH() on any memory address, including invalid ones */
             sequences[seqNb&STOSEQ_MASK] = sequence;
             op += oneSeqSize;
         }