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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 Oct 1

    Anyone want to take a stab at why the following is really problematic? fcntl(0, F_SETFL, O_NONBLOCK);

    9:27 AM - 1 Oct 2018
    • 5 Likes
    • Erika Mischan Toosarani-Hausberger aldebaran James Wu
    5 replies 0 retweets 5 likes
      1. New conversation
      2. @landley‏ @landley Oct 1
        Replying to @RichFelker

        Who else is using that tty?

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Oct 1
        Replying to @landley

        The shell that you invoked the program from.

        1 reply 0 retweets 0 likes
      4. Rich Felker‏ @RichFelker Oct 1
        Replying to @RichFelker @landley

        And all future commands/programs you run on the tty.

        1 reply 0 retweets 0 likes
      5. @landley‏ @landley Oct 1
        Replying to @RichFelker

        And stdin/stdout tend to be the same tty...

        0 replies 0 retweets 0 likes
      6. End of conversation
      1. New conversation
      2. davmac‏ @davmac314 Oct 1
        Replying to @RichFelker

        I guess because the file description is likely shared between child and parent processes? And so if the child unexpectedly terminates (or backgrounds) then the parent is left with stdin in non-blocking mode, which it doesn't know how to deal with?

        1 reply 0 retweets 5 likes
      3. Rich Felker‏ @RichFelker Oct 1
        Replying to @davmac314

        Yep, exactly. It's unexpectedly (due to programmer confusion about file descriptors vs descriptions) modifying shared state. Try in an interactive shell, and "read x" after the program exits.

        0 replies 0 retweets 1 like
      4. End of conversation
      1. Petr Skocik‏ @pskocik Oct 1
        Replying to @RichFelker

        It can make your cat sick. $cat cat: -: Resource temporarily unavailable

        0 replies 1 retweet 7 likes
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      1. New conversation
      2. Jonas Termansen‏ @sortiecat Oct 3
        Replying to @RichFelker

        I've been considering the ability to create a copy of a file description pointing to the same underlying file. The Sortix kernel can do it, no problem, just not exposed in any syscall. An open question is whether it will have unintended consequences.

        2 replies 0 retweets 0 likes
      3. Petr Skocik‏ @pskocik Oct 3
        Replying to @sortiecat @RichFelker

        Linux does just that if you `open()` the filedescriptor's file in `/dev/fd`.

        1 reply 0 retweets 1 like
      4. Jonas Termansen‏ @sortiecat Oct 3
        Replying to @pskocik @RichFelker

        That's a good point, I didn't realize that was possible. It's non-portable though and I don't really like to magic filesystem features (though admittedly convenient now and then).

        1 reply 0 retweets 1 like
      5. Jonas Termansen‏ @sortiecat Oct 3
        Replying to @sortiecat @pskocik @RichFelker

        I wonder if there's any subtleties in how /dev/fd is implemented. E.g. imagine a file open for reading, but the user cannot open it anymore (unlinked / parent directory access denied), but the file mode allows writing, so /dev/fd lets you reopen for writing. Expected? Probably?

        1 reply 0 retweets 1 like
      6. Jonas Termansen‏ @sortiecat Oct 3
        Replying to @sortiecat @pskocik @RichFelker

        What if the unlinked file descriptor was sent over a Unix domain socket, and was intended to contain immutable data? But then both ends would be running as the same user. And I guess the sender could fchmod it 000 after opening it. Interesting cases to consider.

        1 reply 0 retweets 0 likes
      7. Jonas Termansen‏ @sortiecat Oct 3
        Replying to @sortiecat @pskocik @RichFelker

        Well I guess the receiver could fchmod it 700 again unless the sender fchown'd it (but then sender has to be root).

        0 replies 0 retweets 0 likes
      8. End of conversation
      1. Michael Foukarakis‏ @mfukar Oct 1
        Replying to @RichFelker

        because any duplication of 0 to other fds or processes breaks?

        0 replies 0 retweets 0 likes
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