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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. friend void‏ @volatile_void 24 Apr 2017

      Poll: does this violate strict aliasing? struct b { int x; } S; int *p = (int*)&S; *p = 1;

      8 replies 1 retweet 8 likes
    2. Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @volatile_void

      No. 6.7.2.1 ¶15

      1 reply 1 retweet 7 likes
    3. Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @RichFelker

      Found in < 1 min knowing the right phrase to grep for: "initial member".

      2 replies 1 retweet 3 likes
    4.  🎃 p-zombie Damien Miller‏ @damienmiller 24 Apr 2017
      Replying to @RichFelker

      oh, ha. So this is what allows struct sockaddr casting not to be UB...

      1 reply 0 retweets 1 like
    5. Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @damienmiller

      That's different and it is UB if you use sockaddr.

      1 reply 0 retweets 0 likes
    6.  🎃 p-zombie Damien Miller‏ @damienmiller 24 Apr 2017
      Replying to @RichFelker

      how? if you check af_family and only used subsequent fields for same type, then this seems to fall under initial member rules

      1 reply 0 retweets 1 like
    7. Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @damienmiller

      (*(struct sockaddr *)sin).sa_family is not the same as *(sa_family_t *)sa. Latter is well-defined C but assumes sa_family is first member.

      1 reply 0 retweets 0 likes
    8. Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @RichFelker @damienmiller

      The former is an aliasing violation because it accesses the object pointed to by sin with the wrong type, struct sockaddr.

      1 reply 0 retweets 0 likes
    9.  🎃 p-zombie Damien Miller‏ @damienmiller 24 Apr 2017
      Replying to @RichFelker

      ah, but accessing the family directly as (sa_family_t*)sockaddr would be okay I guess.

      1 reply 0 retweets 0 likes
      Rich Felker‏ @RichFelker 24 Apr 2017
      Replying to @damienmiller

      Yes, but I don't think POSIX imposes a requirement that sa_family be the first member...

      6:20 AM - 24 Apr 2017
      • 1 Like
      • 🎃 p-zombie Damien Miller
      1 reply 0 retweets 1 like
        1. New conversation
        2. Rich Felker‏ @RichFelker 24 Apr 2017
          Replying to @RichFelker @damienmiller

          You could do *(sa_family_t*)((char *)p+offsetof(struct sockaddr, sa_family)) :-)

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker 24 Apr 2017
          Replying to @RichFelker @damienmiller

          The right solution is treating "struct sockaddr *" as an abstract/opaque type and using it just with getaddrinfo/getnameinfo/bind/connect.

          1 reply 0 retweets 0 likes
        4.  🎃 p-zombie Damien Miller‏ @damienmiller 24 Apr 2017
          Replying to @RichFelker

          That's not an option if you need to e.g. extract a port number from a sockaddr or manipulate/compare an address sadly.

          1 reply 0 retweets 0 likes
        5. Rich Felker‏ @RichFelker 24 Apr 2017
          Replying to @damienmiller

          Sure it is. getnameinfo (use NI_NUMERICHOST|NI_NUMERICSERV) converts them to string forms you can manipulate and match.

          0 replies 0 retweets 0 likes
        6. End of conversation

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