[Bug c++/60792] New: bogus buffer overflow warning and abort on static flexible array member in a child object

[abalint21 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60792

            Bug ID: 60792
           Summary: bogus buffer overflow warning and abort on static
                    flexible array member in a child object
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: abalint21 at gmail dot com

g++ emits a bogus warning on the program below which then aborts at runtime.
The strange thing is that if I get the reference of the child object and then
get the address of the str field then everything is OK. It seems that g++
cannot handle the inner child object's str if it is accessed via
parent->child.str but it is ok when a reference is taken from the child and
then accessed via child.str.


$ cat main.cpp && g++ -D_FORTIFY_SOURCE=2 -O2 main.cpp && ./a.out 
#include <cstdlib>
#include <cstring>
#include <iostream>

struct Parent
{

    struct Child
    {
        int a;
        char b;
        char str[0]; ///< ASCIIZ
    } child;
};

//#define DONT_CRASH

int main(int argc, char** argv)
{
    char* buffer = new char[32768];

    Parent* parent = (Parent*) buffer;

    parent->child.a = 1;
    parent->child.b = 'a';

#ifdef DONT_CRASH
    Parent::Child& child = parent->child;
    char* childStr = child.str;
#else
    char* childStr = parent->child.str;
#endif

    std::cout << __USE_FORTIFY_LEVEL << std::endl;
    std::cout << __bos(childStr) << std::endl;

    size_t strLen = 4;
    std::strncpy(childStr, "test", strLen);
    if (childStr[strLen] not_eq '\0')
    {
        childStr[strLen] = '\0';
    }

    return 0;
}

In file included from /usr/include/string.h:640:0,
                 from /usr/include/c++/4.8.2/cstring:42,
                 from main.cpp:2:
In function ‘char* strncpy(char*, const char*, size_t)’,
    inlined from ‘int main(int, char**)’ at main.cpp:38:43:
/usr/include/bits/string3.h:120:71: warning: call to char*
__builtin___strncpy_chk(char*, const char*, long unsigned int, long unsigned
int) will always overflow destination buffer [enabled by default]
   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
                                                                       ^
2
0
*** buffer overflow detected ***: ./a.out terminated
...
Aborted