[Bug tree-optimization/82945] New: add warning for passing non-strings to functions that expect string arguments

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82945

            Bug ID: 82945
           Summary: add warning for passing non-strings to functions that
                    expect string arguments
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The -Wstringop-truncation warning new in GCC 8.0 warns about calls to strncpy
and strncat that may non nul-terminate the copied string.  Attribute nonstring
was also added to make it possible to suppress the warning for character arrays
that are not meant to be nul-terminated.  However, using such arrays as
arguments to functions that expect nul-terminated string arguments is accepted
with no warning.  To detect such misuses of the non-nul terminated arrays GCC
should issue a warning when they are passed to string functions like strlen.

$ cat c.c && gcc -O2 -S -Wall c.c
char a[4];

int f (void)
{
  __builtin_strncpy (a, "1234", 4);   // -Wstringop-truncation (good)
  return __builtin_strlen (a);        // because this is undefined
}

char b[4] __attribute__ ((nonstring));

int g (void)
{
  __builtin_strncpy (b, "1234", 4);   // no warning because of nonstring
  return __builtin_strlen (b);        // but this is also undefined
}

c.c: In function ‘f’:
c.c:5:3: warning: ‘__builtin_strncpy’ output truncated before terminating nul
copying 4 bytes from a string of the same length [-Wstringop-truncation]
   __builtin_strncpy (a, "1234", 4);   // -Wstringop-truncation
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~