[Bug c/81117] New: Improve buffer overflow checking in strncpy

["bugzilla at poradnik-webmastera dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81117

            Bug ID: 81117
           Summary: Improve buffer overflow checking in strncpy
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugzilla@poradnik-webmastera.com
  Target Milestone: ---

Code:
#include <string.h>

char buf[2];

void test(const char* str)
{
    strncpy(buf, "12345", sizeof("12345")); // 1
    strncpy(buf, "12345", strlen("12345")); // 2
    strncpy(buf, str, sizeof(str));         // 3
    strncpy(buf, str, strlen(str));         // 4
}

Compile command:
gcc -c -o test.o -Wall -Wextra -O2 test.c -D_FORTIFY_SOURCE=2

When above code is compiled using gcc 4.8.5 on Linux RHEL 7 x86_64, gcc prints
warning about line "3" (-Wsizeof-pointer-memaccess), plus there are two
warnings for lines "1" and "3" detected by -D_FORTIFY_SOURCE=2. There are no
warnings about buffer overflow in lines "2" and "4", where strlen of source is
used instead of buffer size.

What is interesting, gcc 5.4.0 from Cygwin x86_64 does not print warnings from
-D_FORTIFY_SOURCE=2, only -Wsizeof-pointer-memaccess one.

Please improve these checks, to detect cases when user will try to use sizeof
or strlen of source string instead of target buffer size.