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Re: Codezero v0.2 Capabilities

From: Bahadir Balban
Subject: Re: Codezero v0.2 Capabilities
Date: Sun, 13 Dec 2009 14:16:00 +0200
User-agent: Thunderbird (X11/20090817)

Sam Mason wrote:
On Tue, Dec 08, 2009 at 02:08:18PM +0200, Bahadir Balban wrote:
To your ambient authority argument, wikipedia reads:

The authority is "ambient" in the sense that it exists in a broadly visible environment (often, but not necessarily a global environment) where any subject can request it by name.

This is not true for this case, since designation, authorization and ownership information is all bundled in the capability structure and gets checked on each operation.

It depends on the level of abstraction you're thinking about.  Within
codezero a single process can exercise all authority in error because
the kernel checks which capabilities determine whether an operation has
enough authority to proceed.  When the capabilities are directly exposed
to the process it's "harder" for it to go wrong because the code is
directly naming the authority needed for every operation.

Admittedly this is a qualitative appeal rather than a quantitative one,
but I don't possess the experience to argue the point in any other way.

I thought that once a process has a certain capability, it doesn't matter how it is invoked. It may be explicitly by a capid, or implicitly. Keeping the existing L4 interface was highly desirable for design reasons so I implemented it this way.

I didn't get how a process would exercise authority in error. If you start the process with a single capability, it may try all it wants with the syscalls. It will always end up with an -ENOCAP error unless it is that very operation that corresponds to that possessed capability.

When you say the code is directly naming the authority you are approaching it with a pure capability way of thinking. It is doing that because it comes natural to name entities with their direct name when you get to use them. I believe this is more practical. While this might be an issue on a capability way of thinking, I think it is an aesthetical one, security-wise it seems to me that they are no different.

Bahadir Balban

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