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BRIAN_____'s profile
Brian Smith
Brian Smith
Brian Smith
@BRIAN_____

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Brian Smith

@BRIAN_____

Code farmer. Security, crypto, performance, networking, usability. Rust, C++, C, Haskell, DSLs, etc. *ring*, webpki, crypto-bench, mozilla::pkix.

Honolulu & San Francisco
briansmith.org
Joined April 2008

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    1. Alexis Beingessner‏ @Gankro 23 Nov 2016

      Alexis Beingessner Retweeted @nelhage

      This post is really great but also 😭☠️https://twitter.com/nelhage/status/801472446121422848 …

      Alexis Beingessner added,

      @nelhage @nelhage
      Rust's hash tables were accidentally quadratic in some simple usage patterns. http://accidentallyquadratic.tumblr.com/post/153545455987/rust-hash-iteration-reinsertion …
      3 replies 6 retweets 16 likes
    2. Alexis Beingessner‏ @Gankro 23 Nov 2016
      Replying to @Gankro

      This only happened because we recently tried to weaken our seeding strategy, but this is still latently problematic for non-seeded hashers.

      1 reply 0 retweets 3 likes
    3. Alexis Beingessner‏ @Gankro 23 Nov 2016
      Replying to @Gankro

      We long ago discussed tracking run length, and reallocating if it exceeded some constant like 16/32. I'd assume this is the "right" fix.

      2 replies 0 retweets 2 likes
    4. Brian Smith‏ @BRIAN_____ 23 Nov 2016
      Replying to @Gankro

      HashMap insertion is O(n) wost case. Copying a m-element HashMap by inserting each of its elements into another is O(m*n)==O(n**2).

      2 replies 0 retweets 0 likes
    5. Alexis Beingessner‏ @Gankro 23 Nov 2016
      Replying to @BRIAN_____

      this is an excellent example of why unqualified worst-case analysis is often very misleading/uninteresting.

      1 reply 0 retweets 1 like
    6. Alexis Beingessner‏ @Gankro 23 Nov 2016
      Replying to @Gankro @BRIAN_____

      I am sure you know HashMaps are probabilistic data structures. This is akin to pointing out QuickSort takes O(n^2) time.

      3 replies 0 retweets 0 likes
    7. Brian Smith‏ @BRIAN_____ 23 Nov 2016
      Replying to @Gankro

      In general, with attacker-controlled data we always assume by default the attacker can trigger the worst-case behavior.

      1 reply 0 retweets 1 like
    8. Brian Smith‏ @BRIAN_____ 23 Nov 2016
      Replying to @BRIAN_____ @Gankro

      People are putting much effort into making it harder for an attacker to trigger worst case, at significant cost of avg & best case.

      1 reply 0 retweets 0 likes
      Brian Smith‏ @BRIAN_____ 23 Nov 2016
      Replying to @BRIAN_____ @Gankro

      But, the point of using a hash table is really to get the very best best- and average- case behavior. So, what's the point now?

      5:57 PM - 23 Nov 2016
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Alexis Beingessner‏ @Gankro 23 Nov 2016
          Replying to @BRIAN_____

          the defenses in place also protect against accidentally bumbling into the worst case, as was the case for the Rust issue.

          1 reply 0 retweets 0 likes
        3. Brian Smith‏ @BRIAN_____ 23 Nov 2016
          Replying to @Gankro

          If you clone a HashMap then will both HashMaps have equivalent RandomState? Why wasn't Clone impl modified too?

          2 replies 0 retweets 0 likes
        4. Alexis Beingessner‏ @Gankro 23 Nov 2016
          Replying to @BRIAN_____

          sorry I have no idea how they actually addressed the issue; I don't really follow Rust dev anymore (can't contrib).

          1 reply 0 retweets 0 likes
        5. Alexis Beingessner‏ @Gankro 23 Nov 2016
          Replying to @Gankro @BRIAN_____

          it's common for a quick fix to land without thinking about all the issues. e.g. my mem::forget fixes to Rc/Arc missed some funcs

          0 replies 0 retweets 0 likes
        6. End of conversation

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