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taviso's profile
Tavis Ormandy
Tavis Ormandy
Tavis Ormandy
Verified account
@taviso

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Tavis OrmandyVerified account

@taviso

Vulnerability researcher at Google. This is a personal stream, opinions expressed are mine.

California
taviso.decsystem.org
Joined April 2008

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    1. whitequark‏ @whitequark Jan 4
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      took me only 30 minutes of CPU time but now i know if there's a more efficient* way to rotate a multibyte value by 2 bits in 8051 than just rotating by one bit twice (the anwer is no, there isn't**)pic.twitter.com/MbfNCFT1R2

      4 replies 20 retweets 116 likes
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    2. whitequark‏ @whitequark Jan 4
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      * by instruction count ** within a model that considers only these instructions, which is to say, without indirect addressing and without assuming a specific register bankpic.twitter.com/WbmtuLl1iF

      2 replies 2 retweets 22 likes
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    3. whitequark‏ @whitequark Jan 4
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      on the second half-hour of waiting, my SMT solver sent to me: this way to rotate a 16-bit value in R1:R0 by 3 bits. i don't quite understand how it works and i'm not sure i want to.pic.twitter.com/Qq66AJKGp7

      7 replies 9 retweets 52 likes
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    4. whitequark‏ @whitequark Jan 4
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      4 bits but we've seen that before, it's just a bunch of xors and nibble swapspic.twitter.com/602zctvRSu

      1 reply 1 retweet 11 likes
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    5. whitequark‏ @whitequark Jan 4
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      spent around 3 hours of CPU time to get this rotate by 5pic.twitter.com/eYqWmjq9BM

      1 reply 0 retweets 13 likes
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    6. whitequark‏ @whitequark Jan 4
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      enjoying how it rotates by 6 bits with 5 RLC instructionspic.twitter.com/RC1Iy1jyl8

      1 reply 1 retweet 12 likes
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    7. whitequark‏ @whitequark Jan 4
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      i -really- love this one. it made a rotate by 10 by doing two rotates by 2 on -individual bytes- and then doing some sort of XOR swapping of the 2 remaining bits. this is a brilliant solution i wouldn't even consider on my own!pic.twitter.com/SJylCYeDjB

      8 replies 10 retweets 66 likes
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    8. Igor Skochinsky‏ @IgorSkochinsky Jan 4
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      Replying to @whitequark

      You should post this on code golf SE, someone might just come up with something better

      1 reply 0 retweets 1 like
    9. whitequark‏ @whitequark Jan 4
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      Replying to @IgorSkochinsky

      as in, using instructions outside the model I'm using?

      1 reply 0 retweets 1 like
      Tavis Ormandy‏Verified account @taviso Jan 4
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      Replying to @whitequark @IgorSkochinsky

      There could be side effects you weren't modeling, I've seen a superoptimizer use totally unexpected status flags as part of a calculation, for example (I don't know 8051 though, maybe it doesn't apply!).

      12:43 PM - 4 Jan 2020
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      • secmeant Pablo Sole Lolucoca Mark C.
      1 reply 0 retweets 5 likes
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        2. whitequark‏ @whitequark Jan 4
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          Replying to @taviso @IgorSkochinsky

          there are definitely effects I'm not modelling (I'm using only 1/3 of the useful instructions here, omitting any use of direct- or indirect-memory access to registers), but relying on those would result in a different ABI

          1 reply 0 retweets 1 like
        3. whitequark‏ @whitequark Jan 4
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          Replying to @whitequark @taviso @IgorSkochinsky

          there are *also* flags I'm not modelling, but none of those seem any more useful for rotates. if I was doing arithmetics though I would definitely expand the model considerably.

          0 replies 0 retweets 0 likes
        4. End of conversation

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