Compiler optimisation bug

[Nick Hibma]

There is a bug in sbin/dhclient.c for large expiry values on 32 bit platforms 
where time_t is a uint32_t (bug #218980, 
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218980). It is caused by a 
compiler optimisation that removes an if-statement. The code below shows the 
following output, clearly showing that the optimised case provides a different 
answer:


        % cc -O2 main.c -o main.a && ./main.a
        no opt: 0x7fffffff
        with opt: 0xfffffffe
        rephrased: 0x7fffffff

The code is as follows:

        % cat main.c
        #include <stdio.h>
        #include <sys/time.h>
        #define TIME_MAX 2147483647

        time_t a = TIME_MAX;
        time_t b = TIME_MAX;

        time_t
        add_noopt(time_t a, time_t b) __attribute__((optnone))
        {
                a += b;
                if (a < b)
                        a = TIME_MAX;
                return a;
        }

        time_t
        add_withopt(time_t a, time_t b)
        {
                a += b;
                if (a < b)
                        a = TIME_MAX;
                return a;
        }

        time_t
        add_rephrased(time_t a, time_t b)
        {
                if (a < 0 || a > TIME_MAX - b)
                        a = TIME_MAX;
                else
                        a += b;
                return a;
        }

        int
        main(int argc, char **argv)
        {
                printf("no opt:    0x%08x\n", add_noopt(a, b));
                printf("with opt:  0x%08x\n", add_withopt(a, b));
                printf("rephrased: 0x%08x\n", add_rephrased(a, b));
        }

Should this be reported to the clang folks? Or is this to be expected when 
abusing integer overflows this way?

Also: The underlying problem is the fact that time_t is a 32 bit signed int. 
Any pointers as to a general method of resolving these time_t issues?

Regards,

Nick Hibma
n...@van-laarhoven.org

-- Open Source: We stand on the shoulders of giants.

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