Don't use addition to wrap low-level error codes by high-level error codes

[hanno-becker]

Issue: If a high-level function (e.g. X.509) calls a low-level function (e.g. ASN.1) and the latter fails, the high-level function usually wraps the low-level error code by something like

ret = low_level();
if( ret != 0 )
   return( HIGH_LEVEL + ret );

Functionally, that's OK because low-level and high-level error codes don't have bits in common, but it has the following deficiencies:

• It leads to larger code: If the high-level function is in turn called from somewhere else where do something like ret = high_level(); if( ret != 0 ) return( ret );, then the compiler cannot squash the two return statements into one because it cannot argue that HIGH_LEVEL + ret doesn't lead to 0 being returned.
• It's fragile: If we ever make the mistake of returning a high-level error code from a low-level module, the addition + could in theory overflow and lead to unintended behaviour.
• Static analyzers might notice the first point and complain.

Task: Switch to a different idiom, e.g. returning HIGH_LEVEL | ret instead of HIGH_LEVEL + ret, throughout the library.

[ciarmcom]

ARM Internal Ref: IOTSSL-2793

[irwir]

* It's fragile: If we ever make the mistake of returning a high-level error code from a low-level module, the addition `+` could in theory overflow and lead to unintended behaviour.

Still fragile in that sense.
Logical addition might not cure unintended behaviour; though it should prevent overfow and related undefined behaviour for arithmetic addition.

[gstrauss]

In development branch, it looks like this has been consolidated into include/mbedtls/error.h inline function mbedtls_error_add() which is used in macro MBEDTLS_ERROR_ADD(). If the names stay the same, changing to bitwise-or the results is a one line (one char) change:

--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -163,7 +163,7 @@ static inline int mbedtls_error_add( int high, int low,
     (void)file;
     (void)line;
 
-    return( high + low );
+    return( high | low );
 }
 
 /**