src: fix Boyer-Moore array bounds safety

Refs https://chromium-review.googlesource.com/c/chromium/src/+/6703757

The above CL enabled array bounds sanitization for Electron and therefore Node.js, which triggered
a crash in parallel/test-buffer-indexof.js. Upon a bit more investigation, some Boyer-Moore related functions
function were using biased pointer arithmetic to create array pointers that point outside the actual array bounds.

When start_ is large this creates pointers far outside the valid memory range. The sanitizer detects that we're creating and dereferencing pointers outside array boundaries, even though the final memory access happens to land in valid memory.

This changes the functions to use bounds-safe array indexing.

Author: codebytere

include/nbytes.h

@@ -548,7 +548,11 @@ size_t StringSearch<Char>::BoyerMooreSearch(Vector subject,
   size_t start = start_;
 
   int *bad_char_occurrence = bad_char_shift_table_;
-  int *good_suffix_shift = good_suffix_shift_table_ - start_;
+
+  auto good_suffix_get = [&](size_t idx) -> int {
+    if (idx < start || idx - start > kBMMaxShift) return 0;
+    return good_suffix_shift_table_[idx - start];
+  };
 
   Char last_char = pattern_[pattern_length - 1];
   size_t index = start_index;
@@ -575,7 +579,7 @@ size_t StringSearch<Char>::BoyerMooreSearch(Vector subject,
       index +=
           pattern_length - 1 - CharOccurrence(bad_char_occurrence, last_char);
     } else {
-      int gs_shift = good_suffix_shift[j + 1];
+      int gs_shift = good_suffix_get(j + 1);
       int bc_occ = CharOccurrence(bad_char_occurrence, c);
       int shift = j - bc_occ;
       if (gs_shift > shift) {
@@ -591,22 +595,25 @@ size_t StringSearch<Char>::BoyerMooreSearch(Vector subject,
 template <typename Char>
 void StringSearch<Char>::PopulateBoyerMooreTable() {
   const size_t pattern_length = pattern_.length();
-  // Only look at the last kBMMaxShift characters of pattern (from start_
-  // to pattern_length).
   const size_t start = start_;
   const size_t length = pattern_length - start;
 
-  // Biased tables so that we can use pattern indices as table indices,
-  // even if we only cover the part of the pattern from offset start.
-  int *shift_table = good_suffix_shift_table_ - start_;
-  int *suffix_table = suffix_table_ - start_;
+  auto shift_get = [&](size_t idx) -> int & {
+    if (idx < start) abort();
+    return good_suffix_shift_table_[idx - start];
+  };
+
+  auto suffix_get = [&](size_t idx) -> int & {
+    if (idx < start) abort();
+    return suffix_table_[idx - start];
+  };
 
   // Initialize table.
   for (size_t i = start; i < pattern_length; i++) {
-    shift_table[i] = length;
+    shift_get(i) = length;
   }
-  shift_table[pattern_length] = 1;
-  suffix_table[pattern_length] = pattern_length + 1;
+  shift_get(pattern_length) = 1;
+  suffix_get(pattern_length) = pattern_length + 1;
 
   if (pattern_length <= start) {
     return;
@@ -620,34 +627,35 @@ void StringSearch<Char>::PopulateBoyerMooreTable() {
     while (i > start) {
       Char c = pattern_[i - 1];
       while (suffix <= pattern_length && c != pattern_[suffix - 1]) {
-        if (static_cast<size_t>(shift_table[suffix]) == length) {
-          shift_table[suffix] = suffix - i;
+        if (static_cast<size_t>(shift_get(suffix)) == length) {
+          shift_get(suffix) = suffix - i;
         }
-        suffix = suffix_table[suffix];
+        suffix = suffix_get(suffix);
       }
-      suffix_table[--i] = --suffix;
+      suffix_get(--i) = --suffix;
       if (suffix == pattern_length) {
         // No suffix to extend, so we check against last_char only.
         while ((i > start) && (pattern_[i - 1] != last_char)) {
-          if (static_cast<size_t>(shift_table[pattern_length]) == length) {
-            shift_table[pattern_length] = pattern_length - i;
+          if (static_cast<size_t>(shift_get(pattern_length)) == length) {
+            shift_get(pattern_length) = pattern_length - i;
           }
-          suffix_table[--i] = pattern_length;
+          suffix_get(--i) = pattern_length;
         }
         if (i > start) {
-          suffix_table[--i] = --suffix;
+          suffix_get(--i) = --suffix;
         }
       }
     }
   }
+
   // Build shift table using suffixes.
   if (suffix < pattern_length) {
     for (size_t i = start; i <= pattern_length; i++) {
-      if (static_cast<size_t>(shift_table[i]) == length) {
-        shift_table[i] = suffix - start;
+      if (static_cast<size_t>(shift_get(i)) == length) {
+        shift_get(i) = suffix - start;
       }
       if (i == suffix) {
-        suffix = suffix_table[suffix];
+        suffix = suffix_get(suffix);
       }
     }
   }