[Bug middle-end/80807] Improve FORTIFY_SOURCE protection for sprintf

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80807

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-05-18
          Component|c                           |middle-end
      Known to work|                            |7.1.0
     Ever confirmed|0                           |1
      Known to fail|                            |5.4.0, 6.3.0

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I get with GCC 7:

> gcc-7 t.c -O3 -D_FORTIFY_SOURCE=2 -Wall -S
t.c: In function ‘test’:
t.c:6:29: warning: ‘__builtin___sprintf_chk’ writing a terminating nul past the
end of the destination [-Wformat-overflow=]
           sprintf(buf, "%c%c", a, b);
                             ^
In file included from /usr/include/stdio.h:936:0,
                 from t.c:1:
/usr/include/bits/stdio2.h:33:10: note: ‘__builtin___sprintf_chk’ output 3
bytes into a destination of size 2
   return __builtin___sprintf_chk (__s, __USE_FORTIFY_LEVEL - 1,
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       __bos (__s), __fmt, __va_arg_pack ());
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

confirmed with GCC 5 and 6.