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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.  🎃 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
    2. 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
    3. 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
    4. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @RichFelker @oshepherd

      Aha - so the kernel loads the program headers for static programs? libc then iterates them and locates TLS?

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

      The program headers are part of one of the PT_LOAD maps since they're needed at runtime for various reasons including TLS.

      2 replies 0 retweets 1 like
    6. Jonas Termansen‏ @sortiecat 29 Jul 2017
      Replying to @RichFelker @oshepherd

      How does the process locate the headers? What other reasons besides TLS?

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

      Via auxv slot AT_PHDR. Dwarf unwinding/exception handling also needs program headers for .eh_frame.

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

      You can either let kernel loader complexity & interface surface to userspace grow O(n) in these sort of features, or let ELF handle it. :-)

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

      I'm trying out the tradeoff where the kernel is more helpful within reason. I might change stuff once I do dynamic linking.

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

      One way to do the tradeoff without attack surface is have the kernel map the code that does this stuff to userspace ala vdso...

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

      Then the actual kernel need not do anything but open the ELF file; the code it mapped in userspace can handle the mmaps, etc.

      12:05 PM - 29 Jul 2017
      1 reply 0 retweets 0 likes
        1. New conversation
        2. Jonas Termansen‏ @sortiecat 29 Jul 2017
          Replying to @RichFelker @oshepherd

          I think this is an interesting approach and I have considered it. Maybe I'll do it down the road. It's like privsepping the kernel.

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

          Of course with the whole program loader, the TLS parts is just a couple percent of fairly trivial code, not worth splitting out on its own.

          0 replies 0 retweets 1 like
        4. End of conversation

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