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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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    1. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @CopperheadOS

      @RichFelker @ch3root The pointer used to access an object has to be derived from a pointer to that same object. Not a different one.

      2 replies 0 retweets 0 likes
    2. Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @ch3root I don't see that language anywhere in the spec. Certainly cast to uintptr_t and back should be able to bypass.

      2 replies 0 retweets 0 likes
    3. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @RichFelker

      @RichFelker @ch3root Still seems like it's undefined unless the uintptr_t was cast from a pointer to the object that's being accessed.

      1 reply 0 retweets 0 likes
    4. Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @ch3root Integrrs don't have "memory" of where they came from. Imagine printf, user types it back, scanf.

      1 reply 0 retweets 0 likes
    5. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @RichFelker

      @RichFelker @ch3root That's undefined under the interpretation of the C standard used by GCC and LLVM.

      2 replies 0 retweets 0 likes
    6. Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @ch3root If so their models are inconsistent with the standard.

      1 reply 0 retweets 0 likes
    7. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @RichFelker

      @RichFelker @ch3root This interpretation is based on rules involving things like aliasing rather than integer -> pointer casts themselves.

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @ch3root It's still wrong. printf 2 equal-as-uintptr_t ptrs. User types one back in. Which object is it allowed to access?

      2 replies 0 retweets 0 likes
    9. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @RichFelker

      @RichFelker @ch3root It's going to be allowed in practice if it comes from scanf since it can't track the origin of the pointer.

      1 reply 0 retweets 0 likes
    10. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @CopperheadOS

      @RichFelker @ch3root But it still seems technically wrong per their interpretation if it was calculated from a different object.

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @CopperheadOS

      @CopperheadSec @ch3root It's wrong and inconsistent for integers to carry any notion of "calculated from".

      4:11 AM - 23 Nov 2015
      1 reply 0 retweets 0 likes
        1. New conversation
        2. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @ch3root The standard doesn't directly allow it but their rationale is that it's allowed under the as-if rule.

          1 reply 0 retweets 0 likes
        3. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @CopperheadOS

          @RichFelker @ch3root i.e. if all other rules of the standard are followed, then this Clang/GCC rule is implicitly followed too.

          2 replies 0 retweets 0 likes
        4. Rich Felker‏ @RichFelker 23 Nov 2015
          Replying to @CopperheadOS

          @CopperheadSec @ch3root But its not. The rules about integer to pointer conversions and uintptr_t are not respected by this.

          1 reply 0 retweets 0 likes
        5. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @ch3root The logic is that valid code constructing a pointer this way must derive it from an existing pointer in practice.

          2 replies 0 retweets 0 likes
        6. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @CopperheadOS

          @RichFelker @ch3root It might not be true though... one example is the incorrect __attribute__((malloc)) on strdup in glibc.

          1 reply 1 retweet 1 like
        7. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @CopperheadOS

          @RichFelker @ch3root It tells the compiler that the returned data (not just the outer pointer) cannot alias an existing pointer.

          2 replies 0 retweets 0 likes
        8. Rich Felker‏ @RichFelker 23 Nov 2015
          Replying to @CopperheadOS

          @CopperheadSec @ch3root Ouch, that sounds very bad.

          1 reply 0 retweets 0 likes
        9. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @ch3root Yeah, look at the description in https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes …. The glibc usage on strdup appears to be completely wrong.

          1 reply 1 retweet 0 likes
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