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V8 Garbage Collector

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84 points swah | 1 comments | 14 Nov 25 09:53 UTC | HN request time: 0s | source
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ZeroConcerns ◴[14 Nov 25 11:10 UTC] No.45925770[source]▶
Interesting article! One thing that made me literally LOL was the fact that several exploits were enabled via a Google "style recommendation" that caused on-heap length fields to be signed and thus subject to sign-extension attacks.

The conversation-leading-up-to-that played out a bit like this in my head:

Google Engineer #1: Hey, shouldn't that length field be unsigned? Not like a negative value ever makes sense there?

GE#2: Style guide says no

GE#1: Yeah, but that could easily be exploited, right?

GE#2: Maybe, but at least I won't get dinged on code review: my metrics are already really lagging this quarter

GE#1: Good point! In fact, I'll pre-prepare an emergency patch for that whole thing, as my team lead indicated I've been a bit slow on the turnaround lately...

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dbdr ◴[14 Nov 25 11:59 UTC] No.45925990[source]▶
Quote from their style guide:

> The fact that unsigned arithmetic doesn't model the behavior of a simple integer, but is instead defined by the standard to model modular arithmetic (wrapping around on overflow/underflow), means that a significant class of bugs cannot be diagnosed by the compiler.

Fair enough, but signed arithmetic doesn't model the behavior of a "simple integer" (supposedly the mathematical concept) either. Instead, overflow in signed arithmetic is undefined behavior. Does that actually lead to the compiler being able to diagnose bugs? What's the claimed benefit exactly?

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1. dzaima ◴[14 Nov 25 13:45 UTC] No.45926636[source]▶
A sanitizer or static analysis or any other tool can unconditionally give you a warning/error on signed integer overflow. Whereas that's invalid for unsigned integers as they have well-defined behavior, and things depend on said overflow (hashing, bitwise magic, temporary wrapping that unwraps later, etc).

Ideally there'd be a third type for unsigned-non-wrapping-integer (and llvm even supports a UB-on-unsigned-wrapping flag for arith ops in its IR that largely goes unused for C/C++), but alas such doesn't exist. Half-relatedly, this previously appeared as a discussion point on Linux (though Linus really did not like the concept of multiple unsigned types and as such it didn't go anywhere iirc).