[Bug c/79554] Zero length format string passed to fprintf under if statement causes error message

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79554

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2017-02-17
                 CC|                            |msebor at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |msebor at gcc dot 
gnu.org
     Ever confirmed|0                           |1

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
Unfortunately, because of the inherent limitations of the warning being
implemented in the front end, using a const char* const doesn't help.  See the
test case below.  I think this warning might be better handled in the
gimple-ssa-sprintf.c pass where trusted strings can be more reliably
distinguished from potentially tainted ones.  Let me see if I can do this in
GCC 8.

$ cat t.c && gcc -O2 -S -Wall -Wformat -Wformat-security t.c
void f (char *d)
{
  const char* fmt = "";
    __builtin_sprintf (d, fmt);
}

void g (char *d)
{
  const char* const fmt = "";
  if (*fmt)
    __builtin_sprintf (d, fmt);
}

t.c: In function ‘f’:
t.c:4:5: warning: format not a string literal and no format arguments
[-Wformat-security]
     __builtin_sprintf (d, fmt);
     ^~~~~~~~~~~~~~~~~
t.c: In function ‘g’:
t.c:9:27: warning: zero-length gnu_printf format string [-Wformat-zero-length]
   const char* const fmt = "";
                           ^~