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Modern Operating Systems by Herbert Bos and Andrew...
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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 729
to draw certain curves and then fill them in, but it can do anything else it wants to
as well. Applets, agents, and PostScript files are just three examples of
, but there are many others.
Given the long discussion about viruses and worms earlier, it should be clear
that allowing foreign code to run on your machine is more than a wee bit risky.
Nevertheless, some people do want to run these foreign programs, so the question
arises: ‘‘Can mobile code be run safely’’? Theshort answer is: ‘‘Yes, but not easi-
ly.’’ The fundamental problem is that when a process imports an applet or other
mobile code into its address space and runs it, that code is running as part of a
valid user process and has all the power the user has, including the ability to read,
write, erase, or encrypt the user’s disk files, email data to far-away countries, and
much more.
Long ago, operating systems developed the process concept to build walls be-
tween users. The idea is that each process has its own protected address space and
its own UID, allowing it to touch files and other resources belonging to it, but not
to other users. For providing protection against one part of the process (the applet)
and the rest, the process concept does not help. Threads allow multiple threads of
control within a process, but do nothing to protect one thread against another one.
In theory, running each applet as a separate process helps a little, but is often
infeasible. For example, a Web page may contain two or more applets that interact
with each other and with the data on the Web page. The Web browser may also
need to interact with the applets, starting and stopping them, feeding them data,
and so on.
If each applet is put in its own process, the whole thing will not work.
Furthermore, putting an applet in its own address space does not make it any
harder for the applet to steal or damage data.
If anything, it is easier since nobody
is watching in there.
Various new methods of dealing with applets (and mobile code in general)
have been proposed and implemented. Below we will look at two of these meth-
ods: sandboxing and interpretation.
In addition, code signing can also be used to
verify the source of the applet. Each one has its own strengths and weaknesses.
The first method, called
, confines each applet to a limited range of
virtual addresses enforced at run time (Wahbe et al., 1993).
It works by dividing
the virtual address space up into equal-size regions, which we will call sandboxes.
Each sandbox must have the property that all of its addresses share some string of
high-order bits. For a 32-bit address space, we could divide it up into 256 sand-
boxes on 16-MB boundaries so that all addresses within a sandbox have a common
upper 8 bits. Equally well, we could have 512 sandboxes on 8-MB boundaries,
with each sandbox having a 9-bit address prefix. The sandbox size should be cho-
sen to be large enough to hold the largest applet without wasting too much virtual
address space. Physical memory is not an issue if demand paging is present, as it

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