Forwarding variadic template parameter to printf-like function - c++

Question

I have the following log function:
template<typename... Arguments>
void Log(const char* file, const int line, int level, const char* fmt, Arguments... args)
{
std::string formattedFile;
if (file)
{
boost::filesystem::path p(file);
formattedFile = p.filename().string();
}
std::string message{boost::str(boost::format("%1%:%2% [%3%] - %s") % formattedFile % line % m_uxid % fmt)};
__android_log_print(level, m_tag.c_str(), message.c_str(), args...);
}
This application is run on Android using NDK, so this is the logging system for that platform. The problem is that __android_log_print() fails to compile with:
error: format not a string literal and no format arguments [-Werror=format-security]
__android_log_print(level, m_tag.c_str(), message.c_str(), std::forward<Arguments>(args)...);
^
I'm not sure what this means. Am I not using the variadic template argument correctly?

Answer 1

Untrusted input into printf can be a security problem. Enforcing the format by using a string literal is one way of improving security
Turning warnings into errors will cause the build to fail so you are forced to address the warning.
GCC's warning options have this to say
-Werror:
Make all warnings into errors.
-Wformat-security:
Warn about uses of format functions that represent possible security problems.
At present, this warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments
This may be a security hole if the format string came from untrusted input and contains %n.
What is generally advised is to create a std::string in your function and pass this with a %s format string literal to your logging function
__android_log_print(level, m_tag.c_str(), "%s", message.c_str());
Where message is built from processing args..., typically using something like boost::format or a std::stringstream.
If you want to use your provided fmt string and the variadic args, you can parse the arguments using a custom printf style function which produces a std::string
std::string va_string_printf(const char* format, va_list ap)
{
char stack_buf[256];
char* buf = stack_buf;
int buf_size = sizeof(stack_buf);
std::string out_str;
while(true)
{
va_list ap1;
va_copy(ap1, ap);
int min_buf_size = vsnprintf(buf, buf_size, format, ap1) + 1;
va_end(ap1);
if (min_buf_size > buf_size)
{
if (buf != stack_buf) // allocate a bigger buffer
delete[] buf;
buf = new char[min_buf_size];
buf_size = min_buf_size;
continue;
}
out_str = buf;
break;
}
if (buf != stack_buf)
delete[] buf;
return out_str;
}
std::string string_printf(const char* format, ...)
{
va_list ap;
va_start(ap, format);
std::string str = va_string_printf(format, ap);
va_end(ap);
return str;
}