Fix #8530: Prevent stringlib fastsearch from reading beyond the front of an array.

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 3.2 Alpha 2?
 Core and Builtins
 -----------------
 
+- Issue #8530: Prevent stringlib fastsearch from reading beyond the front
+  of an array.
+
 - Issue #5319: Print an error if flushing stdout fails at interpreter
   shutdown.
 

Objects/stringlib/fastsearch.h

@@ -140,13 +140,13 @@ fastsearch(const STRINGLIB_CHAR* s, Py_ssize_t n,
                     /* got a match! */
                     return i;
                 /* miss: check if previous character is part of pattern */
-                if (!STRINGLIB_BLOOM(mask, s[i-1]))
+                if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1]))
                     i = i - m;
                 else
                     i = i - skip;
             } else {
                 /* skip: check if previous character is part of pattern */
-                if (!STRINGLIB_BLOOM(mask, s[i-1]))
+                if (i > 0 && !STRINGLIB_BLOOM(mask, s[i-1]))
                     i = i - m;
             }
         }