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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. Rich Felker‏ @RichFelker Aug 31

      I finally worked out how to make abort() conforming when SIGABRT's disposition is under aggressive alteration by other threads, and it's gloriously simple. Especially if you've ever read the (utterly wrong) glibc monstrosity: https://sourceware.org/git/?p=glibc.git;a=blob;f=stdlib/abort.c;h=9bb97c10552223a65f2a423cb6d5ad184fad5438;hb=HEAD …

      2 replies 0 retweets 5 likes
      Show this thread
    2. Jonas Termansen‏ @sortiecat Sep 1
      Replying to @RichFelker

      How?

      1 reply 0 retweets 1 like
    3. Rich Felker‏ @RichFelker Sep 1
      Replying to @sortiecat

      Put a lock on it.

      1 reply 0 retweets 1 like
    4. Jonas Termansen‏ @sortiecat Sep 1
      Replying to @RichFelker

      You mean adding a lock to sigaction, so abort() can prevent concurrent sigaction calls?

      1 reply 0 retweets 1 like
    5. Rich Felker‏ @RichFelker Sep 1
      Replying to @sortiecat

      Yep.

      2 replies 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker Sep 1
      Replying to @RichFelker @sortiecat

      Oddly the glibc abort() does take lock, but for synchronizing its fallback steps against concurrent calls to abort, not against sigaction. And as far as I can tell, the lock is not AS-safe...

      2 replies 0 retweets 1 like
    7. Jonas Termansen‏ @sortiecat Sep 1
      Replying to @RichFelker

      What is supposed to happen if two threads run abort() concurrently? The interesting case would be if a signal handler is installed. It runs potentially twice concurrently? Is the program supposed to mask SIGABRT except in one thread?

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker Sep 1
      Replying to @sortiecat

      abort is specified to behave as if it performs raise(SIGABRT), and not to perform any termination if SIGABRT is caught and the handler does not return. I read that as allowing well-defined concurrent calls as long as the handler does not return (each thread enters a handler)...

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker Sep 1
      Replying to @RichFelker @sortiecat

      Once a handler returns (observable by raise returning in the abort implementation), though, it's mandated that program termination happen as if by SIGABRT, so there's really no concurrency issue at this point. Another thread calling abort is not sequenced wrt the sigreturn, so...

      8:13 AM - 1 Sep 2018
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Rich Felker‏ @RichFelker Sep 1
          Replying to @RichFelker @sortiecat

          ...it's possible that a concurrent abort from another thread may still have a small window to run, but this situation is not observably different from signal handler still being about to return in the first (or abort not yet entered, if first has SIGABRT blocked).

          1 reply 0 retweets 0 likes
        3. Jonas Termansen‏ @sortiecat Sep 1
          Replying to @RichFelker

          So I think we're in an agreement that a program handling SIGABRT, which may have multiple threads (perhaps through libraries), needs to synchronize in the signal handler if it can't safely be run concurrently/multiple times.

          1 reply 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker Sep 1
          Replying to @sortiecat

          Yes, but normally a library making threads that are supposed to be transparent to the application needs to block all signals in those threads anyway, and of course not do things that would terminate the process, which is rather non-transparent. :-)

          1 reply 1 retweet 1 like
        5. Jonas Termansen‏ @sortiecat Sep 1
          Replying to @RichFelker

          I need to write that libraries(7) manual page for Sortix that documents these best practices. I forgot what the that page was called on Linux.

          0 replies 1 retweet 1 like
        6. End of conversation
        1. New conversation
        2. Jonas Termansen‏ @sortiecat Sep 1
          Replying to @RichFelker

          Hmm. If thread A raises a signal that it has blocked, but thread B does not have it blocked; and a signal handler is installed; does the signal handler run in thread B and does raise() only complete when thread B is done?

          2 replies 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker Sep 1
          Replying to @sortiecat

          No. raise() makes a signal pending for thd calling thread not the process.

          1 reply 0 retweets 0 likes
        4. Jonas Termansen‏ @sortiecat Sep 1
          Replying to @RichFelker

          Yeah that's where my research took me. I need to brush up on all the POSIX requirements and do some full testing of my implementation (which I think is mostly correct, but lacks features) and existing systems. Thanks for the answer!

          0 replies 0 retweets 0 likes
        5. End of conversation

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