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Rollup merge of #37470 - arthurprs:sip-smaller, r=alexcrichton

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Don't reuse RandomState seeds

cc #36481
This commit is contained in:
Alex Crichton 2016-11-04 16:49:29 -07:00
commit 1d41d5bd01
1 changed files with 12 additions and 14 deletions
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26
src/libstd/collections/hash/map.rs
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@ -11,6 +11,7 @@
use self::Entry::*;
use self::VacantEntryState::*;
use cell::Cell;
use borrow::Borrow;
use cmp::max;
use fmt::{self, Debug};
@ -2049,24 +2050,21 @@ impl RandomState {
// many hash maps are created on a thread. To solve this performance
// trap we cache the first set of randomly generated keys per-thread.
//
// In doing this, however, we lose the property that all hash maps have
// nondeterministic iteration order as all of those created on the same
// thread would have the same hash keys. This property has been nice in
// the past as it allows for maximal flexibility in the implementation
// of `HashMap` itself.
//
// The constraint here (if there even is one) is just that maps created
// on the same thread have the same iteration order, and that *may* be
// relied upon even though it is not a documented guarantee at all of
// the `HashMap` type. In any case we've decided that this is reasonable
// for now, so caching keys thread-locally seems fine.
thread_local!(static KEYS: (u64, u64) = {
// Later in #36481 it was discovered that exposing a deterministic
// iteration order allows a form of DOS attack. To counter that we
// increment one of the seeds on every RandomState creation, giving
// every corresponding HashMap a different iteration order.
thread_local!(static KEYS: Cell<(u64, u64)> = {
let r = rand::OsRng::new();
let mut r = r.expect("failed to create an OS RNG");
(r.gen(), r.gen())
Cell::new((r.gen(), r.gen()))
});
KEYS.with(|&(k0, k1)| RandomState { k0: k0, k1: k1 })
KEYS.with(|keys| {
let (k0, k1) = keys.get();
keys.set((k0.wrapping_add(1), k1));
RandomState { k0: k0, k1: k1 }
})
}
}