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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. Jonas Termansen‏ @sortiecat 28 Jul 2017
      Replying to @sortiecat

      On Linux, a static program has to open its own ELF executable to initialize thread local storage. On #Sortix, the kernel does that.

      1 reply 0 retweets 0 likes
    2.  🎃 unsafe { mem::transmute(@erincandescent) }  🎃‏ @erincandescent 28 Jul 2017
      Replying to @sortiecat

      "has to open its' own ELF executable" hmm? Isn't the thread segment just loaded somewhere into memory to be copied by startup code?

      1 reply 0 retweets 1 like
    3. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @erincandescent @oshepherd

      Hmm looks like you're right according to a quick strace. I am not sure where I got this idea then.

      1 reply 0 retweets 0 likes
    4. Rich Felker‏ @RichFelker 29 Jul 2017
      Replying to @sortiecat @oshepherd

      It has to process its own program headers to find the TLS image and copy it to main thread's TLS, but that doesn't involve file loading.

      2 replies 0 retweets 2 likes
    5. Rich Felker‏ @RichFelker 29 Jul 2017
      Replying to @RichFelker @sortiecat @oshepherd

      A possible alt design would be having the kernel responsible for making a TLS image copy for main thread & setting thread ptr to pt to it.

      1 reply 0 retweets 0 likes
    6. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @RichFelker @oshepherd

      That is what Sortix does. It also loads a master copy that can be used for new threads.

      1 reply 0 retweets 0 likes
    7. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @sortiecat @RichFelker @oshepherd

      __thread int errno; is online the instant _start is run

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker 29 Jul 2017
      Replying to @sortiecat @oshepherd

      This is problematic for dynamic linking because the kernel won't know how much TLS space libraries need (assuming userspace ldso).

      1 reply 0 retweets 0 likes
    9. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @RichFelker @oshepherd

      I'm yet to look into how dynamic link TLS works. If the dynamic linker uses static TLS, maybe it can upgrade the process to dynamic TLS.

      1 reply 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker 29 Jul 2017
      Replying to @sortiecat @oshepherd

      Main program refs to TLS in DT_NEEDED libs are accessed at a ldso-time constant (can't vary per thread) offset from thread pointer.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker 29 Jul 2017
      Replying to @RichFelker @sortiecat @oshepherd

      This requires a contiguous VA range whose size is sum of all library TLS sizes.

      11:55 AM - 29 Jul 2017
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Rich Felker‏ @RichFelker 29 Jul 2017
          Replying to @RichFelker @sortiecat @oshepherd

          Actual dynamic TLS access goes thru a function call so you can make the lookup as complex as you want, if you're happy paying.

          1 reply 0 retweets 0 likes
        3. Jonas Termansen‏ @sortiecat 29 Jul 2017
          Replying to @RichFelker @oshepherd

          Ah so it's not a nice constant time memory access relative to the segment. OK, I start to see how this could work.

          1 reply 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker 29 Jul 2017
          Replying to @sortiecat @oshepherd

          Yes. The offset is assigned by ldso and stored with GOT. Then the program just loads the offset from GOT and applies it.

          0 replies 0 retweets 0 likes
        5. End of conversation

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