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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 Jan 8
      Replying to @volatile_void @dekisu

      The problem is that the * in the format string consumes an int argument. If we were source code pedants, we'd say it's UB to pass a size_t and leave it there. But with common architectures and ABIs, could this bug be exploited?

      1 reply 0 retweets 2 likes
    2. friend void‏ @volatile_void Jan 8
      Replying to @volatile_void @dekisu

      On a 32-bit arch where int and size_t are both 32-bit, the size 0x80000000 is likely to be interpreted as a negative value. But look at the last sentence of this clause: https://port70.net/~nsz/c/c11/n1570.html#7.21.6.1p5 … (negative precisions are ignored)

      1 reply 0 retweets 1 like
    3. friend void‏ @volatile_void Jan 8
      Replying to @volatile_void @dekisu

      But since I take the informal specification to implicitly state that src and dst must not overlap, I can't build an example (it would require dst >2GiB and src larger than dst).

      1 reply 0 retweets 0 likes
    4. dx � 🎃‏ @dekisu Jan 8
      Replying to @volatile_void

      fun! what happens in 64 bit archs? i have no idea what *really* happens when you pass a 64 bit size_t to an int parameter that also happens to be a vararg. does that case just work because of an implementation detail?

      1 reply 0 retweets 0 likes
    5. friend void‏ @volatile_void Jan 8
      Replying to @dekisu

      On the only 64-bit architecture ABI I am familiar with (macOS on x86-64), the value would be passed as the same 32/64-bit register as either int or size_t. So funny things may happen when the size_t argument is interpreted as an int… … … wait, you're right, this might work.

      2 replies 0 retweets 1 like
    6. friend void‏ @volatile_void Jan 8
      Replying to @volatile_void @dekisu

      It really depends on how the code of sprintf is written/translated to assembly, but in theory this may work on some 64-bit architectures. It doesn't on mine.https://pastebin.com/yAWKZ4Yp 

      3 replies 0 retweets 0 likes
    7. Tavian Barnes‏ @tavianator Jan 8
      Replying to @volatile_void @dekisu

      Related: is it UB to overflow the return value of *printf()?

      1 reply 0 retweets 0 likes
    8. friend void‏ @volatile_void Jan 8
      Replying to @tavianator

      Well @RichFelker has convinced me that the following interpretation is the only one possible: it would be incorrect for printf to print any number of character other than the nonnegative number it returns, so if the format and arguments mandate more than INT_MAX characters, …

      1 reply 0 retweets 1 like
    9. friend void‏ @volatile_void Jan 8
      Replying to @volatile_void @tavianator @RichFelker

      - printf cannot print anything other than the characters specified by the format and arguments - printf cannot return the nonnegative length of this result, and therefore - printf must return a negative value and print nothing.

      1 reply 0 retweets 0 likes
    10. Rich Felker‏ @RichFelker Jan 8
      Replying to @volatile_void @tavianator

      Surely printf can legitimately return an error (e.g.EILSEQ) after some output.

      1 reply 0 retweets 1 like
      Rich Felker‏ @RichFelker Jan 8
      Replying to @RichFelker @volatile_void @tavianator

      The obligation to return # of chars produced only exists when printf succeeds.

      11:50 AM - 8 Jan 2018
      • 2 Likes
      • Steve Canon friend void
      1 reply 0 retweets 2 likes
        1. friend void‏ @volatile_void Jan 8
          Replying to @RichFelker @tavianator

          Ah, thanks for clarifying that aspect.

          0 replies 0 retweets 0 likes
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