More system layer stuff
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@ -12,3 +12,55 @@ at the correct level to effectively capture the concepts intrinsic to
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a system layer.
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- Protocols capturing a synthesis of system layer behaviours, based on the analysis
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The theory of object
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capabilities (“ocaps”), exemplified in languages such as E and
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programming models such as Actors, offers a fine-grained approach that
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can be made to scale further than a single machine. However, ocaps
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only control access to shared programs. Access controls for shared
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data are left implicit. In addition, ideas of location and system
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boundary are left implicit in ocap systems.
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I will adapt ocaps to syndicated actors. Because the Syndicated Actor model includes a
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first-class notion of shared data as well as a layered conception of locations and location
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boundaries, syndicated capabilities will reflect these ideas directly. I will generalize the
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Syndicated Actor model’s existing notions of place, connecting capabilities not to individual
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actors but to individual places and the data held therein. I will draw on existing ocap
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literature, including in particular the recent notion of Macaroons ([Birgisson et al 2014][])
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and older ideas from SPKI/SDSI ([Ylonen et al 1999][]; [Ellison 1999][]).
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**Q. How do you feel dataspaces would most enhance privacy or trust?**
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Capability technology offers strong, flexible control over access to any given dataspace
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without getting lost in the weeds of identity management: identity is an application-local,
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application-private concern.
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Dataspaces default to being closed, "invite-only" networks, meaning casual observation of
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activity in a dataspace is not possible. But the necessary extension of the capability model to
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handle the data-sharing aspects of dataspaces gives benefit in terms of privacy and trust that
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goes beyond the already considerable benefits a traditional capability model offers.
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Traditional capabilities directly control access to behavioural objects, and only indirectly
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control access to data held within such objects. Syndicated capabilities, by contrast, directly
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control access to shared data held within a space - changes to which may trigger activity in
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"objects" participating in the dataspace.
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In other words, traditional capabilities encode data access controls in terms of object access
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controls; syndicated capabilities, vice versa.
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This ability to directly express access to shared data gives system designers a powerful tool
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for thinking about permitted information flows, including questions of privacy. Furthermore,
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*attenuating* the authority of syndicated capabilities before passing them on to some other
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principal allows for strong partitioning of access within a dataspace, offering fine-grained,
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local, compositional decisions about access to shared data. Finally, it becomes possible to
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expose capabilities to end-users (roughly analogous to URLs), putting that power in their hands
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also.
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I should also mention that dataspaces can scale from managing activity within a single OS
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process up to coordinating activity between machines around the world. A distributed dataspace
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could be an excellent foundation for collaborative applications, where privacy concerns come to
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the forefront. In effect, a dataspace can become a richly-structured "VPN", containing
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application-specific shared data and with application- or schema-specific access controls.
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@ -9,7 +9,7 @@ importance lies in its role as the technical foundation for many qualities[^qual
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to system security, resilience, connectivity, maintainability and usability.
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In the Linux world, existing system layer realizations cross-cut many, many projects:
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NetworkManager, GNOME, DBus, systemd, OpenRC, apt, apk, and so on. Each project has its own
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NetworkManager, GNOME, D-Bus, systemd, OpenRC, apt, apk, and so on. Each project has its own
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role in the overall system layer, and none takes a strong stance on the overall architecture
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that results from their combination. However, there are a group of basic concepts involved in a
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system layer that transcend individual subprojects, relating to issues of IPC, discovery, and
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@ -124,9 +124,9 @@ These tasks can be broken down into smaller, but still general, pieces:
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Concrete examples include:
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- starting services in response to statically-configured runlevels (OpenRC, systemd, SysV init, etc.)
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- starting dependencies before dependent services (OpenRC, systemd, SysV init, etc.)
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- starting dependencies before dependent services (OpenRC, systemd, SysV init, etc.), including readiness-detection and -signalling
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- restarting terminated or failed services in a supervision hierarchy (daemontools, s6, etc.; Erlang/OTP)
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- starting services by service name on demand (DBus, etc.)
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- starting services by service name on demand (D-Bus, etc.)
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- starting services by socket activation (systemd, etc.)
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- virtual-machine and container lifecycles, including supervision and restart of containers (docker, docker-compose, etc.)
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- reacting to hotplugging of a device by installing a driver or starting a program (udevd, etc.)
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@ -141,14 +141,22 @@ distributions](https://skarnet.com/projects/service-manager.html).
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## Higher-level perspectives on and control over system state and resources
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An essential system layer task is to give users and applications higher-level perspectives on
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system state, resource availability and resource consumption than those offered by the kernel.
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An essential system layer task is to give users and applications **higher-level perspectives**
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on system state, resource availability and resource consumption than those offered by the
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kernel. This has two parts: refining low-level information about system state into higher-level
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knowledge, and reflecting user (or application) preferences expressed in terms of the
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higher-level perspective back into concrete actions to perform at the lower level.
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For example, the kernel's [`NETLINK_ROUTE`](https://en.wikipedia.org/wiki/Netlink) sockets
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allow processes to observe changes in network interface and routing configuration, but
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applications often do not need the fine detail on offer: instead, they need higher-level
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knowledge such as "a usable default route for IPv4 exists", or "IPv4 connectivity is available,
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but metered".
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As an example of the first, the kernel's
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[`NETLINK_ROUTE`](https://en.wikipedia.org/wiki/Netlink) sockets allow processes to observe
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changes in network interface and routing configuration, but applications often do not need the
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fine detail on offer: instead, they need higher-level knowledge such as "a usable default route
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for IPv4 exists", or "IPv4 connectivity is available, but metered".
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As an example of the second, NetworkManager allows users to set policy for wifi connection
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establishment in terms of a priority ordering over SSIDs and conditions for when and whether to
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use a particular network. NetworkManager's job is to translate this into a sequence of
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low-level wifi scans, associations and disconnections.
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Breaking this task down into smaller pieces yields:
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@ -156,6 +164,8 @@ Breaking this task down into smaller pieces yields:
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- ability to either poll for or subscribe to changes in such state
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- ability to compute relevant higher-level perspectives on the state
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- a medium for communicating such changes to users and applications
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- a medium for retrieving preferences and actions from users and applications
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- ability to perform actions on low-level system resources
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Concrete examples include:
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@ -163,7 +173,7 @@ Concrete examples include:
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- use of `NETLINK_KOBJECT_UEVENT` by udev to configure and expose hotplugged devices to userland
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- interrogation of disk devices and partition tables to provide views on and control over available filesystems (gnome-disks, etc.)
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- interrogation of audio devices and audio routing options to provide high-level views and control over audio setup (pipewire, pulseaudio, etc.), e.g. volume level display and volume controls, mute, select input/output channel, play/pause, skip, rewind etc.
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- high-level perspectives on devices such as displays, printers, mice, keyboards, touchpads, accelerometers, proximity sensors, temperature monitors and so on (GNOME, XFCE4, KDE, cups, etc.), communicated via DBus and friends
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- high-level perspectives on devices such as displays, printers, mice, keyboards, touchpads, accelerometers, proximity sensors, temperature monitors and so on (GNOME, XFCE4, KDE, cups, etc.), communicated via D-Bus and friends
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- system configuration databases (`/etc`, Windows' Registry, GNOME configuration databases)
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- location services mapping from low-level GPS and wifi information to medium-level concrete location coordinates to high-level "you are at home", "you are in the office"-style knowledge about location
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- telephony services exposing high-level call management interfaces backed by low-level modem operations
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@ -180,147 +190,187 @@ system services are analogous parts of the system layer.
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## Access control mechanisms and policies, security, and privacy
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- offer access control mechanisms and enforce access control policies
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- offer a coherent, system-wide approach to security and privacy
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An inescapable concern when composing software across trust domains is **access control**.
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System layers provide mechanisms for controlling access to software resources and data, allow
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users and applications to specify access control policies, and enforce those policies on their
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behalf.
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- *access control*
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- resource allocation services
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- ACL-based access control for system services and DBus objects
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Given the increasingly blurry lines between local and cloud-based personal computing, the scope
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of access controls can be broad, including confidentiality and integrity protections for user
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data and careful control over user privacy.
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### Security and privacy
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Multiple trust domains appear even in a single-user personal computing system: the kernel is
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its own trust domain; its daemon representatives within the system layer are at least one
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other; the user is a trust domain, and its system-layer representatives another; and each
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application is a trust domain, particularly when it is a third-party application acting on
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behalf of a user, perhaps bringing cloud services into the picture. Moving from a single- to a
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multiple-user system then adds only minor complexity.
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Existing system layers rely on single-machine approaches to security
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and securability that do not scale well: for example, Unix ACLs and
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user- and group-ID-based permissions. The theory of object
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capabilities (“ocaps”), exemplified in languages such as E and
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programming models such as Actors, offers a fine-grained approach that
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can be made to scale further than a single machine. However, ocaps
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only control access to shared programs. Access controls for shared
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data are left implicit. In addition, ideas of location and system
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boundary are left implicit in ocap systems.
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Existing system layer realizations, at least within the Linux world, tend to address access
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control, security and particularly privacy at a relatively primitive level, relying on
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single-machine approaches to security and securability that do not scale well: for example,
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Unix [ACLs](https://en.wikipedia.org/wiki/Access-control_list) and user- and group-ID-based
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permissions.
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I will adapt ocaps to syndicated actors. Because the Syndicated Actor model includes a
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first-class notion of shared data as well as a layered conception of locations and location
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boundaries, syndicated capabilities will reflect these ideas directly. I will generalize the
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Syndicated Actor model’s existing notions of place, connecting capabilities not to individual
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actors but to individual places and the data held therein. I will draw on existing ocap
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literature, including in particular the recent notion of Macaroons ([Birgisson et al 2014][])
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and older ideas from SPKI/SDSI ([Ylonen et al 1999][]; [Ellison 1999][]).
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- Debian, Alpine, and other Unix-like Linux distributions offer little or no access controls
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other than those provided by the kernel
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**Q. How do you feel dataspaces would most enhance privacy or trust?**
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- Android uses the kernel user ID mechanism in a different way, giving an effective
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improvement in separation between trust domains when compared to traditional Unix approaches
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Capability technology offers strong, flexible control over access to any given dataspace
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without getting lost in the weeds of identity management: identity is an application-local,
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application-private concern.
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Dataspaces default to being closed, "invite-only" networks, meaning casual observation of
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activity in a dataspace is not possible. But the necessary extension of the capability model to
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handle the data-sharing aspects of dataspaces gives benefit in terms of privacy and trust that
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goes beyond the already considerable benefits a traditional capability model offers.
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Traditional capabilities directly control access to behavioural objects, and only indirectly
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control access to data held within such objects. Syndicated capabilities, by contrast, directly
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control access to shared data held within a space - changes to which may trigger activity in
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"objects" participating in the dataspace.
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In other words, traditional capabilities encode data access controls in terms of object access
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controls; syndicated capabilities, vice versa.
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This ability to directly express access to shared data gives system designers a powerful tool
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for thinking about permitted information flows, including questions of privacy. Furthermore,
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*attenuating* the authority of syndicated capabilities before passing them on to some other
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principal allows for strong partitioning of access within a dataspace, offering fine-grained,
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local, compositional decisions about access to shared data. Finally, it becomes possible to
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expose capabilities to end-users (roughly analogous to URLs), putting that power in their hands
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also.
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I should also mention that dataspaces can scale from managing activity within a single OS
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process up to coordinating activity between machines around the world. A distributed dataspace
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could be an excellent foundation for collaborative applications, where privacy concerns come to
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the forefront. In effect, a dataspace can become a richly-structured "VPN", containing
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application-specific shared data and with application- or schema-specific access controls.
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- D-Bus authenticates each connection separately, usually mapping principal identities onto
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Unix user IDs; within the scope of a connection, it uses ACLs to make authorization
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decisions
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- Some isolation among trust domains can be achieved with careful use of [kernel
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namespaces](https://en.wikipedia.org/wiki/Linux_namespaces); however, namespaces are not
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fine-grained and are awkward to use for privacy-protection purposes. They see use primarily
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for resource isolation in containerization systems.
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## Inter-process communication and networking
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- offer inter-process communication media
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> Networking is interprocess communication.
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> *—Robert Metcalfe, 1972, quoted in [Day 2008][]*
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- *inter-process communication*
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- DBus as a program-to-program communication bus
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- email for use by system services
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A key part of an operating system is the selection of communications media it offers its
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applications. The kernel itself offers a plethora of communication channels, from the file
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system itself through SysV IPC, shared memory, and pipes up to sockets in multiple flavours.
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X11 for IPC
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System layers need richer facilities in order to handle the reactivity, publish-subscribe,
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name-discovery and -management and access control needs previously discussed. In addition, the
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concept of an "address" within a system layer is often more complex than the low-level endpoint
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addresses on offer by the kernel: for example, D-Bus object names, email addresses and aliases,
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and Docker container names do not fit easily into kernel constructs, and this applies double
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for the addresses of fine-grained resources (e.g. single objects) within a process.
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- Traditional Unix-like system layers configure *email* for use by system services, primarily
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for system-to-user communication but also in principle for program-to-program communication.
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- D-Bus is a coarse-grained, ACL-based message bus with an ad-hoc object model and
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publish-subscribe mechanism. It has been used as the foundation for a lot of system layer
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software such as the components in the GNOME desktop environment and the building-blocks of
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NetworkManager and similar services.
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- X11 offers multiple methods by which clients can communicate with each other. Primary
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applications include shared clipboard management and window management, but the selection
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and property change notification mechanisms are general-purpose and could in principle form
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an interesting substrate for organising software components.
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- <span id="binder"></span>Android IPC is (if I understand correctly!) primarily based around
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[binder](https://elinux.org/Android_Binder) and layers a number of communication
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"personalities" on top of it (such as
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[AIDL](https://developer.android.com/guide/components/aidl),
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[Broadcasts](https://developer.android.com/guide/components/broadcasts), and
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[Messenger](https://developer.android.com/reference/android/os/Messenger)s). Binder is
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apparently ([1](https://elinux.org/Android_Binder), [2](https://lkml.org/lkml/2009/6/25/3),
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[3](https://lwn.net/Articles/466304/)) a (mostly) object-capability ("ocap") system, with
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fine-grained object passing, failure-signalling (a "link to death" facility, much like
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Erlang's [links and
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monitors](https://www.erlang.org/docs/22/reference_manual/processes.html#links)), and
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distributed garbage-collection[^binder-vs-syndicate] that is extremely widely used in
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Android.
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From a [2009 email from Dianne Hackborne](https://lkml.org/lkml/2009/6/25/3):
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<q>For a rough idea of the scope of the binder's use in Android, here is a list of the basic
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system services that are implemented on top of it: package manager, telephony manager, app
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widgets, audio services, search manager, location manager, notification manager,
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accessibility manager, connectivity manager, wifi manager, input method manager, clipboard,
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status bar, window manager, sensor service, alarm manager, content service, activity
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manager, power manager, surface compositor.</q>
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## Name-binding, name-resolution, and namespaces
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- provide name-binding and name resolution services
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udev - /dev namespace
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- *naming services*
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- publishing names for intra-machine services on this system
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- publishing names for LAN services on this system
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- resolving names of intra-machine services on this system
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- resolving names of services on other systems[^libc-resolver]
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Many of the services offered by a system layer involve management and querying of mappings
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between high-level *names* and (zero or more) lower-level *addresses* ([Day 2008][]). These
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appear in many different guises, from the directories in the file system, to DNS names (mDNS
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services like [avahi](https://www.avahi.org/); the libc resolver; services like dnsmasq), to
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device names (managed by udev), to object names (DBus), to service names, to preconfigured
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connection settings (NetworkManager), to user and group names and so on. Namespace management
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is a core feature of a system layer.
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## Job queueing and job scheduling
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- provide job-queueing and -scheduling services, including calendar-like and time-based scheduling
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System layers frequently provide job-queueing and -scheduling services, including calendar-like
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and time-based scheduling. As a corollary, they also provide job- and schedule-management
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interfaces.
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cron
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at
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systemd timers
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- Traditional Unix has `cron` and `at` for job scheduling.
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cups, lpd
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- Android has system [alarm services](https://developer.android.com/reference/android/app/AlarmManager).
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mail queue management?
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- systemd has [timers](https://www.freedesktop.org/software/systemd/man/systemd.timer.html) as
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a replacement for `cron`.
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- systemd also has a [job
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engine](https://www.freedesktop.org/software/systemd/man/systemd-run.html) (see also
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[here](https://www.freedesktop.org/software/systemd/man/systemctl.html#Job%20Commands) and
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[here](https://bl33pbl0p.github.io/systemd.html)) for decoupling work in space and time.
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- print queues like `lpd` and `cups` are job management engines at heart
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- you can even see the mail queue as a kind of job queue (and if you squint *very* hard, you
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can see all the intermediate buffers in a networking or IPC system as job queues; cf [Day
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2008][]).
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## User interface
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- provide user interface facilities
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The user interface is a classic example of a system facility that cross-cuts individual
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applications and tasks. A system layer must provide some kind of user interface service to
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applications (and to its own system services).
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(TO APPLICATIONS but I guess also for the system layer itself)
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- At a minimum, Unix-like kernels offer `tty`s. Access to a system via `ssh` is a natural next
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step.
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- provide system-wide "cut-and-paste" services for user-controlled IPC
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- X11 is the traditional Unix user interface, with its own IPC protocol and ad-hoc object
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model; wayland is a recent entrant into a similar space, also with its own IPC protocol and
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ad-hoc object model. Android offers [SurfaceFlinger and
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WindowManager](https://source.android.com/docs/core/graphics/surfaceflinger-windowmanager)
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along with a large library of user interface widgets; the underlying IPC is presumably
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binder ([see above](#binder)).
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email for talking to users
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notifications - system tray
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- In Smalltalk-80-derived systems (like [squeak](https://squeak.org/)), the user interface is
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tightly integrated with the multiprocessing and IPC facilities (such as they are). Squeak
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also offers simple, quick-and-dirty "alert" and "prompt" APIs to applications, similar to
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the
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[`alert`](https://developer.mozilla.org/en-US/docs/Web/API/Window/alert)/[`prompt`](https://developer.mozilla.org/en-US/docs/Web/API/Window/prompt)/[`confirm`](https://developer.mozilla.org/en-US/docs/Web/API/Window/confirm)
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functions included in web browsers.
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- ui facilities
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- the thing that asks for user input during apt configuration
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- the alert/prompt boxes in a web browser (?)
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- notifications
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- system tray, applets
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- Many, but not all, system layers provide a system-wide "cut and paste" service as part of
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their user interface, for *user-controlled* IPC. X11 applications have a clipboard
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convention; Mac OS, Windows, Android etc. have a standard clipboard.
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## System configuration and user preferences
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- System-level *email* can be seen as a form of user interface for reaching users (system
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administrators).
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- provide system configuration and user preference databases
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- Many desktop environments include *notifications* and some form of *system tray* giving
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quick reference to high-level perspectives on system status as previously discussed.
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- system configuration database
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- system settings manager
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- Some system-layer administration tasks require user interface: for example, user input
|
||||
during `apt` package configuration.
|
||||
|
||||
## Software management
|
||||
|
||||
- support software package installation, upgrade, and removal
|
||||
|
||||
cc
|
||||
apt
|
||||
apk
|
||||
System management involves upgrade of system code and installation, management and removal of
|
||||
application code. Android has a solid story around software management. Linux distributions
|
||||
tend to have package management tools (e.g. `apt`, `apk`, `yum` etc.). Stretching a little
|
||||
further, one might include the system programming language and its development environment as
|
||||
part of the software management portion of a system layer: for example, many Unix-like systems
|
||||
include `cc`, and Smalltalk systems make the system programming language (Smalltalk) available
|
||||
from any text input field.
|
||||
|
||||
## State replication and data backup
|
||||
|
||||
- offer state replication services
|
||||
- provide data backup facilities
|
||||
The notion of state replication appears in many different contexts. For example, user
|
||||
contact/address databases must often be replicated and accessible across devices. System
|
||||
configuration data is often shared across servers in a cloud deployment (ansible, puppet). Many
|
||||
add-on applications like Dropbox, NextCloud, Syncthing etc. add file replication to a system.
|
||||
Applications like Google Keep, to-do list applications, and other sticky-notes/reminder apps
|
||||
replicate their databases across machines. Very few system layer realizations offer a coherent
|
||||
data replication facility, despite its clear cross-application utility.
|
||||
|
||||
- state replication services
|
||||
- contact book, address book
|
||||
- file replication across machines
|
||||
- sticky-notes, google keep
|
||||
- todo list
|
||||
|
||||
- backup facilities
|
||||
- Time Machine
|
||||
Relatedly, preserving user data in case of calamity is a core operating system feature. Despite
|
||||
this, few whole systems offer a coherent data backup facility. Exceptions include Apple's Time
|
||||
Machine and Google's Android backup support libraries.
|
||||
|
||||
## Synthesis, or, Toward a Complete Vision of a System Layer
|
||||
|
||||
|
@ -354,6 +404,8 @@ to be good IPC and state-management and -introspection.
|
|||
| GNOME | | ✓ | ✓ | ✓ | | | | | ✓ | ✓ | | |
|
||||
| Android | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ | | |
|
||||
|
||||
- ideally, a system layer's security mechanisms would offer a coherent, system-wide approach
|
||||
to security and privacy. few do so
|
||||
|
||||
|
||||
## References
|
||||
|
@ -377,6 +429,10 @@ Klein. Evaluating Software Architectures: Methods and Case Studies. Addison-Wesl
|
|||
[**Corbet 2019**] <span id="ref:corbet19"> Corbet, Jonathan. “Systemd as Tragedy.” LWN.Net,
|
||||
January 28, 2019. <https://lwn.net/Articles/777595/>.</span>
|
||||
|
||||
[Day 2008]: #ref:day08
|
||||
[**Day 2008**] <span id="ref:day08"> Day, John. Patterns in Network Architecture: A Return to
|
||||
Fundamentals. Prentice Hall, 2008.</span>
|
||||
|
||||
[Ellison 1999]: #ref:ellison99
|
||||
[**Ellison 1999**] <span id="ref:ellison99"> Ellison, Carl. SPKI Requirements. Request for
|
||||
Comments 2692. RFC Editor, 1999. <https://doi.org/10.17487/RFC2692>.</span>
|
||||
|
@ -413,3 +469,6 @@ Comments 2693. RFC Editor, 1999. <https://doi.org/10.17487/RFC2693>.</span>
|
|||
[^libc-resolver]: The resolver built in to libc plays the major part in this; but things like
|
||||
dnsmasq play a role too, especially when combined with virtual machines running within a
|
||||
host.
|
||||
|
||||
[^binder-vs-syndicate]: Looking at binder, I see *strong* similarities with the [Syndicated
|
||||
Actor Model](syndicated-actor-model.md) and its [protocol](protocol.md)!
|
||||
|
|
Loading…
Reference in New Issue