descr

The error indicator errno shall not be used.

Motivation
==========

Using errno as an indicator of an error condition is fragile and may lead to an undefined behavior. 

Exceptions
==========

It is fine to use `errno` for diagnostic messages and in rare cases when it is the only 
and well-documented way to detect whether an error has occured. We will not try to handle 
the latter exception. 

Implementation Details
======================

Given that `errno` is an implementation-defined macro, it may not appear in binary's symbol table,
and could be implemented either as an externally defined symbol, e.g., `__libc_errno` or a function 
returning a pointer to the `errno` location. Therefore while `errno` is a standard symbol in the
source code, it is ABI specific, and we will gradually add support for different ABIs. Currently,
we support the following ABI:

  - Linux SysV ABI (which uses the `__errno_location` function)

The current implementation only checks if this symbol is imported. If it is, then `errno` is potentially
used. It could be a benign diagnostic message, though. Later, we will extend the analysis to check
if the `errno` variable actually induces any control dependencies.