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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 938
SEC. 11.3
kernel and services implemented in user-mode processes.
Both the kernel and
process provide private address spaces where data structures can be protected and
service requests can be scrutinized.
However, there can be significant performance differences between services in
the kernel vs. services in user-mode processes.
Entering the kernel from user mode
is slow on modern hardware, but not as slow as having to do it twice because you
are switching back and forth to another process.
Also cross-process communica-
tion has lower bandwidth.
Kernel-mode code can (carefully) access data at the user-mode addresses pas-
sed as parameters to its system calls.
With user-mode services, either
those data
must be copied to the service process, or some games be played by mapping mem-
ory back and forth (the ALPC facilities in Windows handle this under the covers).
In the future it is possible that the hardware costs of crossing between address
spaces and protection modes will be reduced, or perhaps even become irrelevant.
The Singularity project in Microsoft Research (Fandrich et al., 2006) uses run-time
techniques, like those used with C# and Java, to make protection a completely soft-
ware issue.
No hardware switching between address spaces or protection modes is
Windows makes significant use of user-mode service processes to extend the
functionality of the system.
Some of these services are strongly tied to the opera-
tion of kernel-mode components, such as
which is the local security
authentication service which manages the token objects that represent user-identity,
as well as managing encryption keys used by the file system.
The user-mode plug-
and-play manager is responsible for determining the correct driver to use when a
new hardware device is encountered, installing it, and telling the kernel to load it.
Many facilities provided by third parties, such as antivirus and digital rights man-
agement, are implemented as a combination of kernel-mode drivers and user-mode
The Windows
has a tab which identifies the services running on
the system.
Multiple services can be seen to be running in the same process
). Windows does this for many of its own boot-time services to reduce
the time needed to start up the system.
Services can be combined into the same
process as long as they can safely operate with the same security credentials.
Within each of the shared service processes, individual services are loaded as
DLLs. They normally share a pool of threads using the Win32 thread-pool facility,
so that only the minimal number of threads needs to be running across all the resi-
dent services.
Services are common sources of security vulnerabilities in the system because
they are often accessible remotely (depending on the TCP/IP firewall and IP Secu-
rity settings), and not all programmers who write services are as careful as they
should be to validate the parameters and buffers that are passed in via RPC.
The number of services running constantly in Windows is staggering.
Yet few
of those services ever receive a single request, though if they do it is likely to be

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