vvellanki commented on a change in pull request #11287:
URL: https://github.com/apache/arrow/pull/11287#discussion_r747696180



##########
File path: cpp/src/gandiva/precompiled/string_ops.cc
##########
@@ -2195,4 +2195,22 @@ const char* byte_substr_binary_int32_int32(gdv_int64 
context, const char* text,
   memcpy(ret, text + startPos, *out_len);
   return ret;
 }
+
+FORCE_INLINE
+int32_t instr_utf8(const char* string, int32_t string_len, const char* 
substring,
+                   int32_t substring_len) {
+  if (substring_len == 0) {
+    return 1;
+  }
+
+  int32_t end_idx =
+      (string_len - substring_len) <= 0 ? string_len : string_len - 
substring_len;
+
+  for (int i = 0; i < end_idx; i++) {

Review comment:
       Let the input be: "Hello" with string_len = 5
   substring = "lower" with substring_len = 5
   
   end_idx = 5 in this case
   
   For i = 2, string[i] = 'l' = substring[0]. The memcmp() is going to access 
string + i + substring_len, which is string + 2 + 5 = string + 7. string + 7 is 
out of range of the input. We cannot do this, it will cause issues




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