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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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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 829
798
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
one of them modifies it. When the other one reads the block, it gets the old (stale)
value. The cache is not coherent.
Given the potential severity of this problem, the NFS implementation does sev-
eral things to mitigate it. For one, associated with each cache block is a timer.
When the timer expires, the entry is discarded. Normally, the timer is 3 sec for data
blocks and 30 sec for directory blocks.
Doing this reduces the risk somewhat. In
addition, whenever a cached file is opened, a message is sent to the server to find
out when the file was last modified. If the last modification occurred after the local
copy was cached, the cache copy is discarded and the new copy fetched from the
server. Finally, once every 30 sec a cache timer expires, and all the dirty (i.e., mod-
ified) blocks in the cache are sent to the server. While not perfect, these patches
make the system highly usable in most practical circumstances.
NFS Version 4
Version 4 of the Network File System was designed to simplify certain opera-
tions from its predecessor.
In contrast to NSFv3, which is described above, NFSv4
is a
stateful
file system. This permits
open
operations to be invoked on remote
files, since the remote NFS server will maintain all file-system-related structures,
including the file pointer. Read operations then need not include absolute read
ranges, but can be incrementally applied from the previous file-pointer position.
This results in shorter messages, and also in the ability to bundle multiple NFSv3
operations in one network transaction.
The stateful nature of NFSv4 makes it easy to integrate the variety of NFSv3
protocols described earlier in this section into one coherent protocol. There is no
need to support separate protocols for mounting, caching, locking, or secure opera-
tions. NFSv4 also works better with both Linux (and UNIX in general) and Win-
dows file-system semantics.
10.7 SECURITY IN LINUX
Linux, as a clone of MINIX and UNIX, has been a multiuser system almost
from the beginning. This history means that security and control of information
was built in very early on.
In the following sections, we will look at some of the
security aspects of Linux.
10.7.1 Fundamental Concepts
The user community for a Linux system consists of some number of registered
users, each of whom has a unique
UID
(
User ID
). A UID is an integer between 0
and 65,535.
Files (but also processes and other resources) are marked with the
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