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RichFelker's profile
Rich Felker
Rich Felker
Rich Felker
@RichFelker

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Rich Felker

@RichFelker

Yeah, I do @musllibc, FOSS & infosec stuff. But now is not the time for a mostly-/only-tech Twitter feed.

musl-libc.org
Joined March 2014

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    1. Patrick Walton‏ @pcwalton 2 Jul 2017

      Someone should write a library that uses seccomp-bpf to intercept blocking syscalls and route them to a user-mode scheduler on Linux…

      3 replies 8 retweets 16 likes
    2. Patrick Walton‏ @pcwalton 2 Jul 2017
      Replying to @pcwalton

      I actually tested this with a proof of concept. Trapping with seccomp/SIGSYS adds only ~0.75µs of overhead to every syscall.

      2 replies 2 retweets 14 likes
      Rich Felker‏ @RichFelker 2 Jul 2017
      Replying to @pcwalton

      Seems hard to believe. How fast is your box? 750ns does not sound plausible for the round-trips needed.

      8:28 AM - 2 Jul 2017
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Patrick Walton‏ @pcwalton 2 Jul 2017
          Replying to @RichFelker

          2.8 GHz Core i7 MacBook Pro. It was in a VM though… Raw read() on /dev/zero was like 0.18µs and the signal handler added like 0.5µs.

          1 reply 0 retweets 1 like
        3. Rich Felker‏ @RichFelker 2 Jul 2017
          Replying to @pcwalton

          I guess 180ns ≃ 500 cycles is achievable for syscalls without much work to do, and ~3x that might be plausible for round trips.

          2 replies 0 retweets 0 likes
        4. Patrick Walton‏ @pcwalton 2 Jul 2017
          Replying to @RichFelker

          To be clear, this is an awful hack, and I don’t love M:N, but IMHO if you’re going to do it you should make it compatible with existing code

          1 reply 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker 2 Jul 2017
          Replying to @pcwalton

          IMO it's more interesting for sandboxing N threads or even processes in one actual thread.

          1 reply 0 retweets 1 like
        6. Patrick Walton‏ @pcwalton 2 Jul 2017
          Replying to @RichFelker

          Sure, that’s what seccomp was designed for. My basic problem is “if people really want M:N, what’s the best way to provide it”.

          1 reply 0 retweets 0 likes
        7. Rich Felker‏ @RichFelker 2 Jul 2017
          Replying to @pcwalton

          https://www.youtube.com/watch?v=7knIi3LGf4M …

          0 replies 0 retweets 0 likes
        8. End of conversation
        1. New conversation
        2. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @RichFelker @pcwalton

          It would only be for compatibility, similar to the Windows implementation, i.e. the performance shouldn't really matter all that much.

          1 reply 0 retweets 0 likes
        3. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @CopperheadOS @RichFelker @pcwalton

          It lets you use legacy code using blocking system calls without it blocking everything, but it wouldn't be the preferred way to do I/O.

          1 reply 0 retweets 0 likes
        4. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @CopperheadOS @RichFelker @pcwalton

          BTW, Go still only inserts yield points in function preludes. You can still block indefinitely by simply having a long loop with no calls.

          1 reply 0 retweets 0 likes
        5. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @CopperheadOS @RichFelker @pcwalton

          https://github.com/golang/go/issues/10958 … since it doesn't support any kind of async preempt to deal with tasks that aren't yielding on their own.

          1 reply 0 retweets 0 likes
        6. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @CopperheadOS @RichFelker @pcwalton

          seccomp-bpf wouldn't know if read is going to be non-blocking since it's a setting on the file rather than a parameter to read/write though.

          1 reply 0 retweets 0 likes
        7. CopperheadOS‏ @CopperheadOS 2 Jul 2017
          Replying to @CopperheadOS @RichFelker @pcwalton

          So doing it that way is going to hurt performance of proper idiomatic code too. Can't see a way to tag read/write calls for seccomp to see.

          3 replies 0 retweets 0 likes
        8. Patrick Walton‏ @pcwalton 2 Jul 2017
          Replying to @CopperheadOS @RichFelker

          By how much though? You’d just restart the syscall in the signal handler and BPF would let it through by checking rip register.

          0 replies 0 retweets 0 likes
        9. End of conversation

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