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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 2 Jun 2016

      Ever seen an object larger than SIZE_MAX bytes? How about SIZE_MAX * SIZE_MAX bytes on x86-64?pic.twitter.com/AncfvROab0

      6 replies 20 retweets 30 likes
    2. Solar Designer‏ @solardiz 3 Jun 2016
      Replying to @ch3root

      What libc? Looks like there's still/again an integer overflow in calloc(), or am I reading this wrong?

      2 replies 0 retweets 0 likes
    3. Rich Felker‏ @RichFelker 3 Jun 2016
      Replying to @solardiz @ch3root

      A highly invalid "optimization" by clang. That compiler needs -fno-builtin to be usable.

      2 replies 1 retweet 3 likes
    4. Alexander Cherepanov‏ @ch3root 3 Jun 2016
      Replying to @RichFelker @solardiz

      Why don't you like this optimization?

      1 reply 0 retweets 0 likes
    5. Rich Felker‏ @RichFelker 3 Jun 2016
      Replying to @ch3root @solardiz

      Because it violates the as-if rule. The calloc call is required to fail; transformation makes it succeed.

      1 reply 0 retweets 1 like
    6. Alexander Cherepanov‏ @ch3root 3 Jun 2016
      Replying to @RichFelker @solardiz

      What requires this calloc call to fail? C11? POSIX? For all sizes > PTRDIFF_MAX? Or > SIZE_MAX?

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker 3 Jun 2016
      Replying to @ch3root @solardiz

      Multiple things. The most important is the compiler's inability to handle objects > PTRDIFF_MAX in a consistent manner.

      9:48 AM - 3 Jun 2016
      2 replies 0 retweets 0 likes
        1. New conversation
        2. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @RichFelker @solardiz

          I consider this a bug in compilers. Which would be nice to fix. This is not a requirement of some standard.

          2 replies 0 retweets 0 likes
        3. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @ch3root @RichFelker @solardiz

          But I don't expect compilers to be fixed to deal with objects larger than SIZE_MAX. Broken toopic.twitter.com/VphageihwR

          $ cat test.c
#include <stdint.h>
#include <stdlib.h>
#include <stdio.h>

int main()
{
  char *p = calloc(SIZE_MAX, SIZE_MAX);
  if (p) {
    *p = 123;
    printf("%d\n", *(p + SIZE_MAX + 1));
  }
}

$ clang -std=c11 -Weverything -O3 test.c && ./a.out
123

$ clang --version | head -n 1
clang version 3.9.0 (trunk 271312)
          0 replies 0 retweets 0 likes
        4. End of conversation
        1. New conversation
        2. Rich Felker‏ @RichFelker 3 Jun 2016
          Replying to @RichFelker @ch3root @solardiz

          But C also reqs an obj of size n*m be produced, or calloc fail. Since the former is not possible, latter is mandatory.

          1 reply 0 retweets 1 like
        3. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @RichFelker @solardiz

          Why the former is not possible?

          1 reply 0 retweets 0 likes
        4. friend void‏ @volatile_void 3 Jun 2016
          Replying to @ch3root

          because of the definitions of size_t and of SIZE_MAX

          2 replies 0 retweets 0 likes
        5. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @volatile_void

          You cannot apply sizeof to a memory block so there is no problem here, right?

          1 reply 0 retweets 0 likes
        6. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @ch3root

          Then, yes, you cannot apply sizeof to objects larger than SIZE_MAX. Does it mean that such objects cannot exist? No.

          1 reply 0 retweets 0 likes
        7. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @ch3root

          It just means that such sizeof is UB. There is no a filed issue in tis-interpreter for this specifically only due to #20 and #22.

          1 reply 0 retweets 0 likes
        8. Alexander Cherepanov‏ @ch3root 3 Jun 2016
          Replying to @ch3root

          See also https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68065#c7 ….

          0 replies 0 retweets 0 likes
        9. End of conversation

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