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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. John Regehr‏ @johnregehr 9 Nov 2017

      here's a malloc optimization (from Ralf Jung) that isn't as cute as the one from yesterdaypic.twitter.com/tXtvTrtva7

      5 replies 7 retweets 38 likes
    2. ɐɯǝʇǝʞ uǝoɹǝſ (華天馬・ユルン)‏ @TerminateThread 9 Nov 2017
      Replying to @johnregehr

      I can’t find anything in C11 that forbids the max value of size_t being a lot smaller than my address space? If so, than we could build a system on which malloc(-1) can succeed?

      3 replies 0 retweets 1 like
    3. John Regehr‏ @johnregehr 9 Nov 2017
      Replying to @TerminateThread

      on a segmented machine where segments are smaller than RAM this will definitely happen (and did, not so long ago...)

      2 replies 0 retweets 1 like
    4. You and 52 others‏ @kragen 9 Nov 2017
      Replying to @johnregehr @TerminateThread

      It still seems like it's bad to turn *possible* UB (which won't happen if your memory is full) into incorrect code. I mean, that doesn't even comply with the shitty ANSI standard, does it?

      1 reply 0 retweets 1 like
    5. John Regehr‏ @johnregehr 9 Nov 2017
      Replying to @kragen @TerminateThread

      well, the question is whether this malloc is allowed to succeed. cases like this are not really addressed in the standard.

      3 replies 0 retweets 0 likes
    6. Josiah Worcester‏ @pikhq 9 Nov 2017
      Replying to @johnregehr @kragen @TerminateThread

      The optimization, though, assumes it succeeds. Which is clearly bogus, even if malloc could in some cases succeed here.

      1 reply 0 retweets 2 likes
    7. John Regehr‏ @johnregehr 9 Nov 2017
      Replying to @pikhq @kragen @TerminateThread

      you've got it slightly backwards -- the optimization works because malloc is allowed to succeed (for smaller allocations there isn't much to argue about here)

      2 replies 0 retweets 2 likes
    8. Josiah Worcester‏ @pikhq 9 Nov 2017
      Replying to @johnregehr @kragen @TerminateThread

      But this invokes UB iff it succeeds? If malloc fails there is no UB reached. The compiler is assuming success, not just permitting it.

      1 reply 0 retweets 0 likes
    9. John Regehr‏ @johnregehr 9 Nov 2017
      Replying to @pikhq @kragen @TerminateThread

      well, it's a dodgy optimization for sure. not one I'd have put into a compiler I wrote...

      1 reply 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker 9 Nov 2017
      Replying to @johnregehr @pikhq and

      Note that this kind of optimization can break security properties of real legitimate code.

      2 replies 0 retweets 0 likes
      Rich Felker‏ @RichFelker 9 Nov 2017
      Replying to @RichFelker @johnregehr and

      For example, saturating multiply then letting malloc fail, assuming mul didn't saturate if malloc succeeded.

      3:16 PM - 9 Nov 2017
      2 replies 0 retweets 0 likes
        1. New conversation
        2. John Regehr‏ @johnregehr 9 Nov 2017
          Replying to @RichFelker @pikhq and

          well, this particular optimization only fires when the return value of malloc is only tested for null, it doesn't fire if it is used for real

          4 replies 0 retweets 0 likes
        3. John Regehr‏ @johnregehr 9 Nov 2017
          Replying to @johnregehr @RichFelker and

          making it (1) probably fairly harmless (2) probably fairly useless

          0 replies 0 retweets 2 likes
        4. End of conversation
        1. New conversation
        2. Rich Felker‏ @RichFelker 9 Nov 2017
          Replying to @RichFelker @johnregehr and

          Similarly (better example I think), calling calloc(n,m) and assuming n*m doesn't overflow if calloc succeeded.

          1 reply 0 retweets 3 likes
        3. friend void‏ @volatile_void 9 Nov 2017
          Replying to @RichFelker @johnregehr and

          Ah yes that one. We had to put “requires n*m < PTRDIFF_MAX” as pre-condition of calloc when we realized what was going on and that felt stupid. (NB “*” means mathematical multiplication in ACSL specifications)

          0 replies 0 retweets 2 likes
        4. End of conversation

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