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▲ sltkr 4 hours ago

From a DoS risk perspective there is no practical difference between an infinite loop, or a finite but arbitrarily large loop, which was always possible.

For example, this doesn't work:

    #define DOUBLE(x) DOUBLE(x) DOUBLE(x)
    DOUBLE(x)
That would only expand once and then stop because of the rule against repeated expansion. But nothing prevents you from unrolling the first few recursive expansions, e.g.:

    #define DOUBLE1(x) x x
    #define DOUBLE2(x) DOUBLE1(x) DOUBLE1(x)
    #define DOUBLE3(x) DOUBLE2(x) DOUBLE2(x)
    #define DOUBLE4(x) DOUBLE3(x) DOUBLE3(x)
    DOUBLE4(x)
This will generate 2^4 = 16 copies of x. Add 60 more lines to generate 2^64 copies of x. While 2^64 is technically a finite number, for all practical purposes it might as well be infinite.