Language: C
Label: vulnerable
CWE: CWE-190 \u2014 Integer Overflow or Wraparound
The product performs a calculation that can produce an integer overflow or wraparound, when the logic assumes that the resulting value will always be larger than the original value. This can introduce other weaknesses when the calculation is used for resource management or execution control.

Code:
regtilde(char_u *source, int magic)
{
    char_u	*newsub = source;
    char_u	*tmpsub;
    char_u	*p;
    int		len;
    int		prevlen;

    for (p = newsub; *p; ++p)
    {
	if ((*p == '~' && magic) || (*p == '\\' && *(p + 1) == '~' && !magic))
	{
	    if (reg_prev_sub != NULL)
	    {
		// length = len(newsub) - 1 + len(prev_sub) + 1
		prevlen = (int)STRLEN(reg_prev_sub);
		tmpsub = alloc(STRLEN(newsub) + prevlen);
		if (tmpsub != NULL)
		{
		    // copy prefix
		    len = (int)(p - newsub);	// not including ~
		    mch_memmove(tmpsub, newsub, (size_t)len);
		    // interpret tilde
		    mch_memmove(tmpsub + len, reg_prev_sub, (size_t)prevlen);
		    // copy postfix
		    if (!magic)
			++p;			// back off backslash
		    STRCPY(tmpsub + len + prevlen, p + 1);

		    if (newsub != source)	// already allocated newsub
			vim_free(newsub);
		    newsub = tmpsub;
		    p = newsub + len + prevlen;
		}
	    }
	    else if (magic)
		STRMOVE(p, p + 1);	// remove '~'
	    else
		STRMOVE(p, p + 2);	// remove '\~'
	    --p;
	}
	else
	{
	    if (*p == '\\' && p[1])		// skip escaped characters
		++p;
	    if (has_mbyte)
		p += (*mb_ptr2len)(p) - 1;
	}
    }

    // Store a copy of newsub  in reg_prev_sub.  It is always allocated,
    // because recursive calls may make the returned string invalid.
    vim_free(reg_prev_sub);
    reg_prev_sub = vim_strsave(newsub);

    return newsub;
}