Comment 30 for bug 305901

Anders Kaseorg noticed that the use of _FORTIFY_SOURCE breaks a specific use of
sprintf (see attached):

$ gcc -O0 -o foo foo.c && ./foo
not fail
$ gcc -O2 -o foo foo.c && ./foo
not fail
$ gcc -O2 -D_FORTIFY_SOURCE=2 -o foo foo.c && ./foo
fail

The original report was filed in Ubuntu, where -D_FORTIFY_SOURCE=2 is enabled by
default: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/305901

C99 states:
The sprintf function is equivalent to fprintf, except that the output is written
into an array (specified by the argument s) rather than to a stream. A null
character is written at the end of the characters written; it is not counted as
part of the returned value. If copying takes place between objects that overlap,
the behavior is undefined.

The man page does not mention this limitation, and prior to the use of
__sprintf_chk, this style of call worked as expected. As such, a large volume
of source code uses this style of call:
http://web.mit.edu/andersk/Public/sprintf-results

It seems that it would make sense to fix __sprintf_chk, or very loudly mention
the C99-described overlap-is-undefined behavior in sprintf documentation.