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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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    Rich Felker‏ @RichFelker Apr 12

    TIL another awful thing about Linux procfs behavior; top-level /proc only reports processes (thread-groups) via getdents/readdir, but opening /proc/$tid where $tid is any thread id (not necessarily also a process id) succeeds.

    8:50 AM - 12 Apr 2018
    • 8 Retweets
    • 29 Likes
    • Mukilan Thiagarajan Emanuele Aina Kan-Ru Chen ██████████████████████████████████████████████████ Sean Hinchee Emin Uğur mr. waddlesplash Erika *censored*
    5 replies 8 retweets 29 likes
      1. Rich Felker‏ @RichFelker Apr 12

        My guess: this was some awful compromise with the historical zealots who didn't want Linux to support real threads at all because they thought the "processes sharing memory" model of LinuxThreads was sufficient...

        1 reply 2 retweets 4 likes
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      1. New conversation
      2. Jed Davis  🏳️‍🌈‏ @xlerb Apr 12
        Replying to @RichFelker

        The second part seems reasonable to me in that it matches everything else (e.g., kill(2) a non-main thread by tid). The first part is backwards compat for stuff like ps(1), I assume.

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Apr 12
        Replying to @xlerb

        The argument to kill is a pid, not a tid; it sends the signal to the process not a particular thread of the process. It makes no sense to call it with a tid.

        1 reply 0 retweets 0 likes
      4. Rich Felker‏ @RichFelker Apr 12
        Replying to @RichFelker @xlerb

        The first part is correct. /proc is supposed to list processes; there's no reason for threads to be there.

        0 replies 0 retweets 0 likes
      5. End of conversation
      1. New conversation
      2. mork  👽‏ @corkmork Apr 12
        Replying to @RichFelker

        IIRC in earlier versions of Linux all tasks (processes, threads) were in /proc and when threads got moved (tasks without CLONE_VM?) the open behaviour was kept for backwards compatibility. At one point ps and top showed threads by default.

        1 reply 1 retweet 0 likes
      3. Rich Felker‏ @RichFelker Apr 12
        Replying to @corkmork

        Rich Felker Retweeted Rich Felker

        Yes I'm aware. See the other branch of this thread. Also it's CLONE_THREAD; you can use CLONE_VM without CLONE_THREAD, e.g. as vfork.https://twitter.com/RichFelker/status/984465290489749504 …

        Rich Felker added,

        Rich Felker @RichFelker
        My guess: this was some awful compromise with the historical zealots who didn't want Linux to support real threads at all because they thought the "processes sharing memory" model of LinuxThreads was sufficient...
        Show this thread
        4 replies 0 retweets 0 likes
      4. mork  👽‏ @corkmork Apr 12
        Replying to @RichFelker

        Oh that’s right. Early on they only had CLONE_VM and no concept of tid. I think that is why CLONE_THREAD got added, so that proc and procps could differentiate processes from threads. I remember in the 2.4.x days a period where ps showed all threads as processes. Linus relented

        1 reply 0 retweets 1 like
      5. Rich Felker‏ @RichFelker Apr 12
        Replying to @corkmork

        Showing them in /proc was the least of the problems with this naive and bone-headed approach. Signal semantics, child process relationships, exec behavior, etc. were all wrong and could not be fixed just by emulating pthreads in userspace.

        0 replies 0 retweets 1 like
      6. End of conversation
      1. mork  👽‏ @corkmork Apr 12
        Replying to @RichFelker

        You have to remember Linux internally has “tasks” and there is a task struct. A process is a group of tasks that share the same mm struct. There are so many different CLONE_ flags nowadays, a task can be many thinking including a new namespace (is that a zone?)

        0 replies 0 retweets 1 like
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      1. Sean Hinchee‏ @H3n3sy Apr 12
        Replying to @RichFelker

        Sounds like the perfect time to plug Plan 9’s /proc in all its glory

        0 replies 0 retweets 0 likes
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