IDIV DoS (INT_MIN / -1) • Hacker News Board

Re: IDIV DoS (INT_MIN / -1)

#21

Post by bdonlan » Mon, Jan 28, 2013, 4:27 AM UTC

loeg wrote: post #20

makomk wrote: post #6

If anyone here has used Second Life's scripting language LSL, they had to work around this issue. Apparently they now calculate x / -1 as -x instead, which gets a counter-intuitive result when x=INT_MIN but doesn't crash. I believe once upon a time this was documented and everything.

But -1*INT_MIN is still 1 greater than INT_MAX in two's complement ... so that's signed overflow and thus undefined behavior in C. Assuming x86 signed overflow behavior, you're just back to INT_MIN again? Or am I crazy?

The language in question isn't C, so they can have whatever semantics they want.

Re: IDIV DoS (INT_MIN / -1)

#22

Post by malkia » Mon, Jan 28, 2013, 4:58 AM UTC

ScottBurson wrote: post #7

Fixed-precision integer arithmetic is evil.

And then a lot of game programmers would disagree :)

I for one just learned to love doubles...

Re: IDIV DoS (INT_MIN / -1)

#23

Post by mikeash » Mon, Jan 28, 2013, 2:55 PM UTC

haberman wrote: post #15

mikeash wrote: post #14

Earlier quoted context omitted.

Regardless of undefined behavior, it should not crash the compiler.

Ah, I missed that it can crash the tcc compiler . Yeah, that's a bug.

Probably easy to miss. One really does not expect the compiler to crash on code like that.

Re: IDIV DoS (INT_MIN / -1)

#24

Post by loeg » Tue, Jan 29, 2013, 3:40 AM UTC

bdonlan wrote: post #21

loeg wrote: post #20

Earlier quoted context omitted.

But -1*INT_MIN is still 1 greater than INT_MAX in two's complement ... so that's signed overflow and thus undefined behavior in C. Assuming x86 signed overflow behavior, you're just back to INT_MIN again? Or am I crazy?

The language in question isn't C, so they can have whatever semantics they want.

Sorry, the use of the nym INT_MIN suggested to me that the implementation under the covers was C.

It looks like it was once an interpreter, but is now compiled down to CLR and run with Mono.