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Date: Thu, 6 Jul 2017 14:15:25 +0000
From: Pascal Cuoq <cuoq@...st-in-soft.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Documentation of memcpy and undefined behavior in memset

Hello all,

when I started testing parts of musl with TIS Interpreter, I made sure to use TIS Interpreter versions of low-level functions such as memcpy and memset, while testing higher-level functions. Musl's functions can provide guarantees beyond the standard, and it is fair game to rely on these guarantees elsewhere in musl since musl's versions of these functions are called, but I thought it would be interesting to know that musl provides additional guarantees and relies on them.

That was informative. It turned out that musl's implementation of fwrite() can call memcpy() with a length of 0 and a pointer one-past, inside __fwritex:

https://git.musl-libc.org/cgit/musl/tree/src/stdio/fwrite.c?id=a08910fc2cc739f631b75b2d09b8d72a0d64d285#n23

It can be argued that C11 does not define the behavior of memcpy in this case: https://stackoverflow.com/questions/25390577/is-memcpya-1-b-1-0-defined-in-c11

For this reason, it may be worth documenting that musl's memcpy does not require valid pointers when invoked with a size of 0, and any future memcpy implementation (e.g. in assembly) should continue to do so.


Changing course and using musl's implementation of memcpy and memset to analyse higher-level functions, we found what I think is an undefined behavior in memset. The following line in the implementation of memset can be reached with n = 1:


s[0] = s[n-1] = c;

https://git.musl-libc.org/cgit/musl/tree/src/string/memset.c?id=a08910fc2cc739f631b75b2d09b8d72a0d64d285#n14

I think this is undefined because i = i++;, which is equivalent to i = i = i + 1;, is the canonical example for the “unsequenced side-effect in expression” undefined behavior(*), and what makes this latter example undefined is the “i = i =” part, not the “i + 1” part. Musl's “s[0] = s[n-1] =” is identical to that when n == 1.
The same problem occurs in the next lines of memset for other values of n.

Pascal

(*) https://en.wikipedia.org/wiki/Sequence_point#Examples_of_ambiguity

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