Vulnerability: Comparison of narrow type with wide type in loop condition

[FeedehC]

I'm working on a project using PubSubClient and decided to implement vulnerability scans with CodeQL in GitHub Actions. It also scans libraries. It is detecting in the file /src/PubSubClient.cpp in line 460 there is a comparison between "i" of type uint16_t and "call to length" of wider type unsigned int.

CodeQL information: In a loop condition, comparison of a value of a narrow type with a value of a wide type may result in unexpected behavior if the wider value is sufficiently large (or small). This is because the narrower value may overflow. This can lead to an infinite loop. Recommendation: Change the types of the compared values so that the value on the narrower side of the comparison is at least as wide as the value it is being compared with.

Example: In this example, bytes_received is compared against max_get in a while loop. However, bytes_received is an int16_t, and max_get is an int32_t. Because max_get is larger than INT16_MAX, the loop condition is always true, so the loop never terminates.

This problem is avoided in the 'GOOD' case because bytes_received2 is an int32_t, which is as wide as the type of max_get.

void main(int argc, char **argv) {
      uint32_t big_num = INT32_MAX;
      char buf[big_num];
      int16_t bytes_received = 0;
      int max_get = INT16_MAX + 1;

      // BAD: 'bytes_received' is compared with a value of a wider type.
      // 'bytes_received' overflows before  reaching 'max_get',
      // causing an infinite loop
      while (bytes_received < max_get)
          bytes_received += get_from_input(buf, bytes_received);
      }

      uint32_t bytes_received = 0;

      // GOOD: 'bytes_received2' has a type  at least as wide as 'max_get'
      while (bytes_received < max_get) {
          bytes_received += get_from_input(buf, bytes_received);
      }

  }

  int getFromInput(char *buf, short pos) {
      // write to buf
      // ...
      return 1;
  }

References: Data type ranges INT18-C. Evaluate integer expressions in a larger size before comparing or assigning to that size Common Weakness Enumeration: CWE-190. Common Weakness Enumeration: CWE-197. Common Weakness Enumeration: CWE-835.