This patch largely reverts the commit "base: lay groundwork for
base-linux caps change" because the use of 'epoll' instead of 'select'
alleviated the need to allocate large FD sets, which motivated the
introduction of the 'Native_context' hook.
Related to issue #3581
On Linux, Genode used to represent each RPC object by a socket
descriptor of the receiving thread (entrypoint) and a globally-unique
value that identifies the object. Because the latter was transferred as
plain message payload, clients had to be trusted to not forge the
values. For this reason, Linux could not be considered as a productive
Genode base platform but remained merely a development vehicle.
This patch changes the RPC mechanism such that each RPC object is
represented by a dedicated socket pair. Entrypoints wait on a set of
the local ends of the socket pairs of all RPC objects managed by the
respective entrypoint. The epoll kernel interface is used as the
underlying mechanism to wait for a set of socket descriptors at the
server side.
When delegating a capability, the remote end of the socket pair is
transferred to the recipient along with a plaintext copy of the
socket-descriptor value of the local end. The latter value serves as a
hint for re-identifiying a capability whenever it is delegated back to
its origin. Note that the client is not trusted to preserve this
information. The integrity of the hint value is protected by comparing
the inode values of incoming and already present capablities at the
originating site (whenever the capability is invoked or presented to the
owner of the RPC object).
The new mechanism effectively equips base-linux with Genode's capablity
model as described in the Chapter 3 of the Genode Foundations book.
That said, the sandboxing of components cannot be assumed at this point
because each component has still direct access to the Linux system-call
interface.
This patch is based on the extensive exploration work conducted by
Stefan Thoeni who strongly motivated the inclusion of this feature into
Genode.
Issue #3581
This patch allows core's 'Signal_transmitter' implementation to sidestep
the 'Env::Pd' interface and thereby adhere to a stricter layering within
core. The 'Signal_transmitter' now uses - on kernels that depend on it -
a dedicated (and fairly freestanding) RPC proxy mechanism for signal
deliver, instead of channeling signals through the 'Pd_session::submit'
RPC function.
This patch removes possible ambiguities with respect to the naming of
kernel-dependent binaries and libraries. It also removes the use of
kernel-specific global side effects from the build system. The reach of
kernel-specific peculiarities has thereby become limited to the actual
users of the respective 'syscall-<kernel>' libraries.
Kernel-specific build artifacts are no longer generated at magic places
within the build directory (like okl4's includes, or the L4 build
directories of L4/Fiasco and Fiasco.OC, or the build directories of
various kernels). Instead, such artifacts have been largely moved to the
libcache. E.g., the former '<build-dir>/l4/' build directory for the L4
build system resides at '<build-dir>/var/libcache/syscall-foc/build/'.
This way, the location is unique to the kernel. Note that various tools
are still generated somewhat arbitrarily under '<build-dir>/tool/' as
there is no proper formalism for building host tools yet.
As the result of this work, it has become possible to use a joint Genode
build directory that is usable with all kernels of a given hardware
platform. E.g., on x86_32, one can now seamlessly switch between linux,
nova, sel4, okl4, fiasco, foc, and pistachio without rebuilding any
components except for core, the kernel, the dynamic linker, and the timer
driver. At the current stage, such a build directory must still be
created manually. A change of the 'create_builddir' tool will follow to
make this feature easily available.
This patch also simplifies various 'run/boot_dir' plugins by removing
the option for an externally hosted kernel. This option remained unused
for many years now.
Issue #2190
