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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 Sep 16

    Is an uncatchable SIGABRT better than a[n unintentionally] catchable SIGSEGV/SIGILL (which varies by arch) when dangerous/corrupted process state is detected? https://www.openwall.com/lists/musl/2018/09/17/2 …

    8:32 PM - 16 Sep 2018
    • 1 Retweet
    • Rich Felker
    6 replies 1 retweet 0 likes
      1. New conversation
      2. Josiah Worcester‏ @pikhq Sep 16
        Replying to @RichFelker

        I feel like the intent of your a_crash semantics are "no, really, crash no matter what", and using possibly-catchable signals don't quite match that.

        1 reply 0 retweets 1 like
      3. Rich Felker‏ @RichFelker Sep 16
        Replying to @pikhq

        Right now it *is* catchable, just because we didn't have a good way to make it non-catchable. Now the tail of abort() provides a way to do so, if we want to.

        1 reply 0 retweets 0 likes
      4. Josiah Worcester‏ @pikhq Sep 16
        Replying to @RichFelker

        I know, it just always felt like an accident that really violated intent.

        1 reply 0 retweets 1 like
      5. Rich Felker‏ @RichFelker Sep 16
        Replying to @pikhq

        Yeah I'd even considered SIGKILL before, but it's unfriendly to debugging.

        0 replies 0 retweets 0 likes
      6. End of conversation
      1. New conversation
      2. Alexander Monakov‏ @_monoid Sep 16
        Replying to @RichFelker

        Other: work with kernel to introduce an interface to terminate the process "here and now". I think @sortiecat has the right idea with scram(2). (and I raise eyebrows at the "uncatchable", nothing is stopping the program from installing a handler via raw syscalls)

        1 reply 0 retweets 1 like
      3. Rich Felker‏ @RichFelker Sep 16
        Replying to @_monoid @sortiecat

        Installing via raw syscalls alone doesn't do it. You still have to successfully race against the thread that's trying to terminate. See https://git.musl-libc.org/cgit/musl/commit?id=9b14ad541068d4f7d0be9bcd1ff4c70090d868d3 …

        1 reply 0 retweets 0 likes
      4. Rich Felker‏ @RichFelker Sep 16
        Replying to @RichFelker @_monoid @sortiecat

        It would be nice to have a syscall to do it all faily-safe and atomically, but in the absence of one I was pretty happy to find a solution that works this well for abort, and it still seems better than the status quo for where a_crash() is currently used.

        1 reply 0 retweets 0 likes
      5. Jonas Termansen‏ @sortiecat Sep 17
        Replying to @RichFelker @_monoid

        Sortix currently implements abort() with exit_thread() (backend of _exit) that can atomically unconditionally exit and fake the exit code as if by a signal. I have considered doing it with scram(2). Question is how much alerting I want to go off on abort().

        1 reply 0 retweets 1 like
      6. Jonas Termansen‏ @sortiecat Sep 17
        Replying to @sortiecat @RichFelker @_monoid

        Neither exit_thread (undocumented) / scram(2) are necessarily long term interfaces, they were designed to reasonably solve the problem of the day and do it right. We shouldn't be afraid of adding new system calls to Linux if there's no reliable & simple way to do stuff in libc.

        1 reply 0 retweets 1 like
      7. Jonas Termansen‏ @sortiecat Sep 17
        Replying to @sortiecat @RichFelker @_monoid

        (Yes -- Linux ABI is long term, but there it's worth to discuss stuff through and come up with a long term good design from the start, and keep total kernel + userland complexity in mind)

        0 replies 0 retweets 3 likes
      8. End of conversation
      1. New conversation
      2. Nathan McSween‏ @nwmcsween Sep 17
        Replying to @RichFelker

        Why would you ever want bad state to be catchable?

        1 reply 0 retweets 1 like
      3. Rich Felker‏ @RichFelker Sep 17
        Replying to @nwmcsween

        Because you don't understand that introspective crash reporting/debugging simply gives attackers a vector to turn a DoS into an RCE, and are trying to auto-report crashes or get users to report them without running gdb...

        0 replies 0 retweets 0 likes
      4. End of conversation
      1. New conversation
      2. Ed Maste‏ @ed_maste Sep 17
        Replying to @RichFelker

        My answer may have been different if you didn't include "[n unintentionally]"

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Sep 17
        Replying to @ed_maste

        Hope it made sense. The sense I intended was that a_crash() being catchable in the past was not an intended property, just a consequence of it being hard to prevent.

        1 reply 0 retweets 0 likes
      4. Ed Maste‏ @ed_maste Sep 17
        Replying to @RichFelker

        Yep. The parenthetical comment resulted in a more considered choice.

        0 replies 0 retweets 1 like
      5. End of conversation
      1. New conversation
      2. Michael Foukarakis‏ @mfukar Sep 17
        Replying to @RichFelker

        No, please keep it. In a system, we'd like SIGABRT when errors are being detected (internally), and masking e.g. SIGSEGV with it would cause a lot of time wasted investigating red herrings.

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Sep 17
        Replying to @mfukar

        I don't understand the "no, please keep it" aspect. Current behavior is crashing with a SIGSEGV or SIGILL (arch dependent) that could be caught just like any other one. Proposal is generating an uncatchable SIGABRT instead.

        1 reply 0 retweets 0 likes
      4. Michael Foukarakis‏ @mfukar Sep 17
        Replying to @RichFelker

        I meant option #2

        0 replies 0 retweets 0 likes
      5. End of conversation
      1. Arvid E. Picciani‏ @arvidep Sep 16
        Replying to @RichFelker

        Both Abort and segv is sometimes cought by crashhandlers for statistics and helpful feedback to the user. Should be catchable. Corrupt libc state should not be catchable.

        0 replies 0 retweets 0 likes
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