ZABBIX BUGS AND ISSUES
  1. ZABBIX BUGS AND ISSUES
  2. ZBX-2003

arithmetic expressions are evaluated incorrectly

    Details

    • Type: Bug Bug
    • Status: Closed
    • Priority: Critical Critical
    • Resolution: Fixed
    • Affects Version/s: 1.8.1
    • Fix Version/s: 1.8.2, 1.9.0 (alpha)
    • Component/s: Server (S)
    • Labels:
      None

      Description

      The problematic function is evaluate_simple() in src/libs/zbxserver/expression.c.

      The way it works is as follows. When it processes an operator, it searches the expression for that operator from left to right, splits the expression in two, recursively evaluates each part, and, finally, applies the operator.

      This algorithm has problems for both "-" and "/". For instance, evaluating "64/8/8" yields 64, whereas the correct answer is 1. Similarly, "100-10-1" would yield "91", whereas the correct value is 89. Luckily, evaluate_simple() evaluates "100-10-1" to 89, because of the function compress_signs(), which turns "100-10-1" into "100+N10+N1".

      A quick solution to this problem would be to search for "-" and "/" from right to left (using strrchr), instead of left to right (with strchr).

      Another solution would be to completely rewrite evaluate() and evaluate_simple(), as more efficient algorithms for evaluating expressions exist.

      PS: You can also note the statement *p='|'; throughout the function, executed for all the operators. It does not seem to have any adverse effects, however, it seems to be a sign of copy-paste, which, potentially, is a major source of problems of its own.

        Activity

        Hide
        Aleksandrs Saveljevs added a comment -

        Implemented the quick solution in r10198 in ZBX-2003-expression-evaluation branch.

        Show
        Aleksandrs Saveljevs added a comment - Implemented the quick solution in r10198 in ZBX-2003 -expression-evaluation branch.
        Hide
        Alexei Vladishev added a comment -

        Please test it.

        Show
        Alexei Vladishev added a comment - Please test it.
        Hide
        richlv added a comment -

        non-trivial to test from user perspective, but seems to work as expected with changes from the dev branch. probably easier to test by code review

        Show
        richlv added a comment - non-trivial to test from user perspective, but seems to work as expected with changes from the dev branch. probably easier to test by code review
        Hide
        Aleksandrs Saveljevs added a comment -

        Fixed in pre1.8.2, revision 10380.

        Show
        Aleksandrs Saveljevs added a comment - Fixed in pre1.8.2, revision 10380.

          People

          • Assignee:
            Aleksandrs Saveljevs
            Reporter:
            Aleksandrs Saveljevs
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            Dates

            • Created:
              Updated:
              Resolved: