From 4da336062e5b295c0a54bb3683cdc6ca9ba23ca3 Mon Sep 17 00:00:00 2001
From: Dave Watson <davejwatson@fb.com>
Date: Thu, 24 Jan 2019 13:07:57 -0800
Subject: decompress_generic: Refactor variable length fields

Make a helper function to read variable lengths for literals and
match length.
---
 lib/lz4.c | 47 +++++++++++++++++++++++++++++++++++------------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/lib/lz4.c b/lib/lz4.c
index 53eff2e..4a063eb 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -1434,6 +1434,35 @@ typedef enum { decode_full_block = 0, partial_decode = 1 } earlyEnd_directive;
 #undef MIN
 #define MIN(a,b)    ( (a) < (b) ? (a) : (b) )
 
+/* Read the variable-length literal or match length.
+ *
+ * ip - pointer to use as input.
+ * lencheck - end ip.  Return an error if ip advances >= lencheck.
+ * loop_check - check ip >= lencheck in body of loop.  Returns loop_error if so.
+ * initial_check - check ip >= lencheck before start of loop.  Returns initial_error if so.
+ * error (output) - error code.  Should be set to 0 before call.  
+ */
+typedef enum { loop_error = -2, initial_error = -1, ok = 0} variable_length_error;
+LZ4_FORCE_INLINE unsigned read_variable_length(const BYTE**ip, const BYTE* lencheck, int loop_check, int initial_check, variable_length_error* error) {
+  unsigned length = 0;
+  unsigned s;
+  if (initial_check && unlikely((*ip) >= lencheck)) {    /* overflow detection */
+    *error = initial_error;
+    return length;
+  }
+  do {
+    s = **ip;
+    (*ip)++;
+    length += s;
+    if (loop_check && unlikely((*ip) >= lencheck)) {    /* overflow detection */
+      *error = loop_error;
+      return length;
+    }
+  } while (s==255);
+
+  return length;
+}
+
 /*! LZ4_decompress_generic() :
  *  This generic decompression function covers all use cases.
  *  It shall be instantiated several times, using different sets of directives.
@@ -1536,12 +1565,9 @@ LZ4_decompress_generic(
 
             /* decode literal length */
             if (length == RUN_MASK) {
-                unsigned s;
-                if (unlikely(endOnInput ? ip >= iend-RUN_MASK : 0)) goto _output_error;   /* overflow detection */
-                do {
-                    s = *ip++;
-                    length += s;
-                } while ( likely(endOnInput ? ip<iend-RUN_MASK : 1) & (s==255) );
+              variable_length_error error = ok;
+              length += read_variable_length(&ip, iend-RUN_MASK, endOnInput, endOnInput, &error);
+              if (error == initial_error) goto _output_error;
                 if ((safeDecode) && unlikely((uptrval)(op)+length<(uptrval)(op))) goto _output_error;   /* overflow detection */
                 if ((safeDecode) && unlikely((uptrval)(ip)+length<(uptrval)(ip))) goto _output_error;   /* overflow detection */
             }
@@ -1588,12 +1614,9 @@ LZ4_decompress_generic(
             }   /* note : when partialDecoding, there is no guarantee that at least 4 bytes remain available in output buffer */
 
             if (length == ML_MASK) {
-                unsigned s;
-                do {
-                    s = *ip++;
-                    if ((endOnInput) && (ip > iend-LASTLITERALS)) goto _output_error;
-                    length += s;
-                } while (s==255);
+              variable_length_error error = ok;
+              length += read_variable_length(&ip, iend - LASTLITERALS + 1, endOnInput, 0, &error);
+              if (error != ok) goto _output_error;
                 if ((safeDecode) && unlikely((uptrval)(op)+length<(uptrval)op)) goto _output_error;   /* overflow detection */
             }
             length += MINMATCH;
-- 
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