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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 Jul 17

    Optimization I'd like to see compilers be able to make: if (strlen(s)>100) -> if (strnlen(s,101)>100)

    8:35 AM - 17 Jul 2018
    • 6 Retweets
    • 25 Likes
    • scherzo Enno Boland Jed Davis 🏳️‍🌈 Gregor Glawitsch Ryan Salsamendi Clifford Wolf Joachim Nilsson Erika Arvid Gerstmann
    6 replies 6 retweets 25 likes
      1. Rich Felker‏ @RichFelker Jul 17

        Actually strnlen is only POSIX, not plain C, so -> if (!memchr(s, 0, 100))

        0 replies 0 retweets 3 likes
        Show this thread
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      1. New conversation
      2. m4c0‏ @m4c0 Jul 17
        Replying to @RichFelker

        That requires a compiler with knowledge over specific fn calls. I can see that easily done on LLVM, but it feels like bloating the compile process. I’d rather see some form of “lint” doing the same thing.

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Jul 17
        Replying to @m4c0

        The std functions are part of the language and gcc and clang already know about them.

        1 reply 0 retweets 0 likes
      4. m4c0‏ @m4c0 Jul 17
        Replying to @RichFelker

        The compiler and the stdlib are independently provided, and the stdlib is technically optional - specially important when dealing with embedded platforms - or an OS kernel.

        1 reply 0 retweets 0 likes
      5. Rich Felker‏ @RichFelker Jul 17
        Replying to @m4c0

        Most is "optional" (not included in the spec) in a freestanding implementation. GCC and clang already honor -ffreestanding for this and omit special treatment of std functions then.

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker Jul 17
        Replying to @RichFelker @m4c0

        If you're compiling a kernel or libc without -ffreestanding that's a bug; they'll already make transformations that can break your code (like recursive self-calls).

        1 reply 0 retweets 0 likes
      7. reabstraction thunk golem‏ @jckarter Jul 18
        Replying to @RichFelker @m4c0

        -ffreestanding may suppress optimizations that introduce new stdlib calls, but isn’t it still UB for a function to be defined with the same name as a stdlib function and not have the same semantics? Dunno if gcc or clang exercise that even if so though

        0 replies 0 retweets 2 likes
      8. End of conversation
      1. New conversation
      2. John Regehr‏ @johnregehr Jul 18
        Replying to @RichFelker

        it's a good shortcut. you know of codes where this would make a noticeable difference?

        1 reply 0 retweets 0 likes
      3. Rich Felker‏ @RichFelker Jul 18
        Replying to @johnregehr

        It's mainly hardening against extra linear factors (increased polynomial order) via untrusted inputs.

        2 replies 0 retweets 2 likes
      4. Gok‏ @Gok Jul 18
        Replying to @RichFelker @johnregehr

        Wonder if there's code that relies on the trailing memory getting touched...

        1 reply 0 retweets 0 likes
      5. Steve Canon‏ @stephentyrone Jul 18
        Replying to @Gok @RichFelker @johnregehr

        😢

        1 reply 0 retweets 0 likes
      6. Rich Felker‏ @RichFelker Jul 18
        Replying to @stephentyrone @Gok @johnregehr

        It's possible there is, but if so this code is broken. The compiler is free to optimize out dead loads, which is ultimately what this is (if you think of it as inlining and unrolling the strlen).

        2 replies 0 retweets 0 likes
      7. Steve Canon‏ @stephentyrone Jul 18
        Replying to @RichFelker @Gok @johnregehr

        Cut to 2028: committee fixes this “bug” by making all string functions take volatile pointers.

        1 reply 0 retweets 2 likes
      8. John Regehr‏ @johnregehr Jul 18
        Replying to @stephentyrone @RichFelker @Gok

        2029: volatile pointers to volatile contents

        1 reply 1 retweet 3 likes
      9. Fabian Giesen‏ @rygorous Jul 18
        Replying to @johnregehr @stephentyrone and

        static volatile functions will solve this problem

        1 reply 0 retweets 3 likes
      10. 2 more replies
      1. New conversation
      2. Clifford Wolf‏ @oe1cxw Jul 17
        Replying to @RichFelker

        If "s" is between 1 and 100 bytes longer than the range of size_t, is this defined as true, or is strlen() itself undefined for this case? (I don't see this case explicitly covered in C99 std, but strlen returns size_t, not ssize_t, so I would assume overflow is well defined?)

        1 reply 0 retweets 0 likes
      3. Koorogi‏ @koorogi Jul 17
        Replying to @oe1cxw @RichFelker

        there's no way to create an object larger than the range of size_t in C, and how would you access all of it if there were? And some implementations (musl) only allow allocations in the range of ssize_t.

        1 reply 0 retweets 1 like
      4. Clifford Wolf‏ @oe1cxw Jul 18
        Replying to @koorogi @RichFelker

        I don't think size_t is required to be the same size as a pointer. You might not be able to create such a large string yourself on the heap, but that doesn't mean it can't already exist...

        3 replies 0 retweets 0 likes
      5. Rich Felker‏ @RichFelker Jul 18
        Replying to @oe1cxw @koorogi

        size_t is an unsigned integer type for representing the size of an object. The standard is poorly vague about what requirements that imposes, but any interpretation allowing objects whose size it can't represent is a critically broken one.

        0 replies 0 retweets 2 likes
      6. End of conversation
      1. Leandro Pereira‏ @lafp Jul 18
        Replying to @RichFelker

        That's an easy Coccinelle script, though.

        0 replies 0 retweets 0 likes
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