[Bug libc/16004] memcpy/strcpy: detect memory overlap and crash when error is detected

["allachan at au1 dot ibm.com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16004

paxdiablo <allachan at au1 dot ibm.com> changed:

           What    |Removed                     |Added
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                 CC|                            |allachan at au1 dot ibm.com

--- Comment #2 from paxdiablo <allachan at au1 dot ibm.com> ---
Hmmm.

"Sometimes people do silly things". I think the fix here may be to convince
people to stop doing those silly things :-)

ISO C11 (and earlier) specifically call out the possibility of overlap
problems, for both memcpy() and strcpy():

"If copying takes place between objects that overlap, the behavior is
undefined."

What you seem to be proposing is to slow down all copy operations to handle
things people shouldn't be doing in the first place. I'm of the opinion that C
bods who need this level of hand-holding should consider moving to C++ (I'm not
dissing C++ here but people should learn the tools they use, or get tools
better suited to them). Putting "safety" into one clib implementation will not
help you when you have to use another.

C already has a perfectly good library function for overlapping memory copies,
to wit, memmove(). I'd be more inclined to ask ISO WG14 to introduce a new
strmove() call for doing the same for C strings, or just use memmove() with
strlen() to achieve the same result, something like:

char *myStrMove (
  char * restrict s1,
  char * restrict s2)
{
  memmove (s1, s2, strlen (s2) + 1);
  return s1;
}

Don't take this as an attack, you've done exactly the right thing in your
layered approach for solutions ("if too heavy, do something else"). I just
don't think this should be changed for ALL XXXcpy() calls since it'll have a
performance hit.

By all means, if there's a different XXXcpy() we can use in those fortify
modes, I'd be happy. Though it's still going to crash, at least it'll fail fast
hopefully before corrupting data. Not sure how good that's going to make users
feel since they'll still have lost their unsaved work :-)

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