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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. Alexander Cherepanov‏ @ch3root 23 Nov 2015

      int a[20], b[20], *p = a, i; if ((uintptr_t)(a + 20) == (uintptr_t)b) { b[5] = 1; for (i = 0; i < 40; i++) *p++ = 0; printf("%d\n", b[5]); }

      1 reply 0 retweets 1 like
    2. Alexander Cherepanov‏ @ch3root 23 Nov 2015
      Replying to @ch3root

      gcc 4.9 and 5.2 with -O2 print 1 while the right result is 0. How much black magic do you need, to explain that the code is invalid C?

      4 replies 0 retweets 0 likes
    3. Rich Felker‏ @RichFelker 23 Nov 2015
      Replying to @ch3root

      @ch3root Are you sure? I think the code has UB. Does p1==p2 evaluating true imply they are the same/interchangeable?

      2 replies 0 retweets 0 likes
    4. Alexander Cherepanov‏ @ch3root 23 Nov 2015
      Replying to @RichFelker

      @RichFelker Isn't interchangability of equal values is one of the fundamentals of C?

      2 replies 0 retweets 0 likes
    5. CopperheadOS‏ @CopperheadOS 23 Nov 2015
      Replying to @ch3root

      @RichFelker @ch3root The code does have UB because you aren't allowed to access objects via a pointer to another.

      1 reply 0 retweets 0 likes
    6. 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
      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.

      3:58 AM - 23 Nov 2015
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Rich Felker‏ @RichFelker 23 Nov 2015
          Replying to @RichFelker

          @CopperheadSec @ch3root If the values as uintptr_t are equal, casting back should yield a pointer which could access either object.

          1 reply 0 retweets 0 likes
        3. Alexander Cherepanov‏ @ch3root 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @CopperheadSec This is wrong in practice. gcc tracks pointer's origin through casts to integers (and even floats).

          2 replies 0 retweets 0 likes
        4. CopperheadOS‏ @CopperheadOS 23 Nov 2015
          Replying to @ch3root

          @ch3root @RichFelker Yes, it's difficult to interpret the standard. But it's objectively true that GCC and LLVM do it this way.

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

          @CopperheadSec @ch3root Do you have an example to demonstrate the behavior I claim is a bug?

          1 reply 0 retweets 0 likes
        6. Alexander Cherepanov‏ @ch3root 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @CopperheadSec Sure. It's too big for a tweet. Sent by email.

          1 reply 0 retweets 0 likes
        7. Rich Felker‏ @RichFelker 23 Nov 2015
          Replying to @ch3root

          @ch3root @CopperheadSec Thanks. I think this should be a bug report.

          1 reply 0 retweets 0 likes
        8. Alexander Cherepanov‏ @ch3root 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @CopperheadSec Wait, you said that equality doesn't imply interchangability. What's the problem then? :-)

          1 reply 0 retweets 0 likes
        9. Rich Felker‏ @RichFelker 23 Nov 2015
          Replying to @ch3root

          @ch3root @CopperheadSec For integers it does. The requirements of an implementation-defined conversion and faithfulness for uintptr_t...

          0 replies 0 retweets 0 likes
        10. End of conversation
        1. New conversation
        2. 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
        3. 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
        4. 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
        5. 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
        6. 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
        7. 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
        8. Alexander Cherepanov‏ @ch3root 23 Nov 2015
          Replying to @RichFelker

          @RichFelker @CopperheadSec Yes, this is one to strip history. Another one is pass it through a volatile variable.

          1 reply 0 retweets 0 likes
        9. Alexander Cherepanov‏ @ch3root 23 Nov 2015
          Replying to @ch3root

          @RichFelker @CopperheadSec If a compiler cannot "see" history it has to be cautious. The problem is when it can see it.

          0 replies 0 retweets 0 likes
        10. End of conversation

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