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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 2

    *sigh* gcc <8 UBSan fails to catch this serious, trivial-to-make error:https://godbolt.org/z/kNil_u 

    9:44 AM - 2 Sep 2018
    • 1 Retweet
    • 3 Likes
    • vierito5 Dexter M Haslem Shafik Yaghmour Jonas Termansen
    2 replies 1 retweet 3 likes
      1. Rich Felker‏ @RichFelker Sep 2

        Original test case was slightly wrong and gratuitously distracting. I've now filed a bug report with a much clearer one: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87191 … and it fits in a tweet: void bar(void *); int foo() { char a[10]; bar(&a+2); }

        1 reply 1 retweet 1 like
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      2. Alexander Monakov‏ @_monoid Sep 2
        Replying to @RichFelker

        oh ffs. post a proper explanation (for those of 1.5k+ that follow you who are there to learn, not blindly hate), delete this crap, or I'll unfollow

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

        I love gcc, contribute to it, and don't understand your reaction here, but I'd like to try to. Can you DM or msg me on IRC when it's convenient?

        1 reply 0 retweets 0 likes
      4. Alexander Monakov‏ @_monoid Sep 2
        Replying to @RichFelker

        I've read your clarifications in the other subthread. Prototype mismatch and irrelevant ipv6 struct usage completely obfuscate your point, as did the ubsan mention. What do you need ubsan's runtime checks for here? This is diagnosable at compile time, as I know you're aware.

        1 reply 0 retweets 0 likes
      5. Rich Felker‏ @RichFelker Sep 2
        Replying to @_monoid

        Indeed, I would love a compile-time warning to catch it (not 100% false-positive free since it could happen in unreachable code, but good enough for me), but couldn't find one, so I tried UBSan to catch it, and also failed.

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker Sep 2
        Replying to @RichFelker @_monoid

        Then I started trying things on godbolt, and UBSan did seem to catch some alternate formulations (which I'd have to try to figure out again now) so I went back to the original bug and minimized the test case to that.

        1 reply 0 retweets 0 likes
      7. Rich Felker‏ @RichFelker Sep 2
        Replying to @RichFelker @_monoid

        struct sockaddr_in6 is not critical to the issue, but it's a good real-world example of a structure containing an array where you could accidentally write &s->a+k instead of s->a+k.

        1 reply 0 retweets 0 likes
      8. Rich Felker‏ @RichFelker Sep 2
        Replying to @RichFelker @_monoid

        OK, here's an example with no sockaddr_in6: https://godbolt.org/z/kvw5zU  The situation is actually worse than I thought. gcc 8 and clang *don't* catch it. They just put in a wrong/useless overflow check for overflow past end of addr space (can't happen) not past object size.

        1 reply 0 retweets 0 likes
      9. Rich Felker‏ @RichFelker Sep 2
        Replying to @RichFelker @_monoid

        As I understand, -fsanitize=object-size is included in -fsanitize=undefined and supposed to catch this, but doesn't... ?

        0 replies 0 retweets 0 likes
      10. End of conversation

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