Typed Words

Jonathan S. Shapiro shap at eros-os.com
Sat Apr 28 18:02:13 CEST 2007

This is a very small issue.

If I understand the L4 specifications correctly, typed words provided by
the sender are copied to the recipient. This allows the recipient to
know what was sent.

An observation:

The transfer of typed words exposes sender-side address values to the
receiver. This is a small security violation, since there is no valid
use that the receiver can make of this information. It is probably not
an important issue, except that we noticed an interaction with
cross-model calls.

If the IPC transport replaces these addresses with zero, it becomes
possible to re-encode a 64-bit typed item word into a 32-bit typed item
word without losing any useful information.

This appears (to us) to be useful in the implementation of IPCs that
cross from (e.g.) 64-bit sender to 32-bit receiver.

It does not appear to us that the receiver loses any useful information
if the sender address is zeroed or truncated. Have we missed something?

Jonathan S. Shapiro, Ph.D.
Managing Director
The EROS Group, LLC

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