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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 932
SEC. 11.3
SYSTEM STRUCTURE
901
object is closed it is important to delete the exclusive access at that point rather
than wait for any incidental kernel references to eventually go away (e.g., after the
last flush of data from memory).
Otherwise closing and reopening a file from user
mode may not work as expected because the file still appears to be in use.
Though the object manager has comprehensive mechanisms for managing ob-
ject lifetimes within the kernel, neither the NT APIs nor the Win32 APIs provide a
reference mechanism for dealing with the use of handles across multiple concur-
rent threads in user mode.
Thus, many multithreaded applications have race condi-
tions and bugs where they will close a handle in one thread before they are finished
with it in another.
Or they may close a handle multiple times, or close a handle
that another thread is still using and reopen it to refer to a different object.
Perhaps the Windows APIs should have been designed to require a close API
per object type rather than the single generic
NtClose
operation. That would have
at least reduced the frequency of bugs due to user-mode threads closing the wrong
handles. Another solution might be to embed a sequence field in each handle in
addition to the index into the handle table.
To help application writers find problems like these in their programs, Win-
dows has an
application verifier
that software developers can download from
Microsoft. Similar to the verifier for drivers we will describe in Sec. 11.7, the ap-
plication verifier does extensive rules checking to help programmers find bugs that
might not be found by ordinary testing.
It can also turn on a FIFO ordering for the
handle free list, so that handles are not reused immediately (i.e., turns off the bet-
ter-performing LIFO ordering normally used for handle tables).
Keeping handles
from being reused quickly transforms situations where an operation uses the wrong
handle into use of a closed handle, which is easy to detect.
The device object is one of the most important and versatile kernel-mode ob-
jects in the executive.
The type is specified by the I/O manager, which along with
the device drivers, are the primary users of device objects.
Device objects are
closely related to drivers, and each device object usually has a link to a specific
driver object, which describes how to access the I/O processing routines for the
driver corresponding to the device.
Device objects represent hardware devices, interfaces, and buses, as well as
logical disk partitions, disk volumes, and even file systems and kernel extensions
like antivirus filters. Many device drivers are given names, so they can be accessed
without having to open handles to instances of the devices, as in UNIX.
We will
use device objects to illustrate how the
Parse
procedure is used, as illustrated in
Fig. 11-20:
1. When an executive component, such as the I/O manager imple-
menting the native system call
NtCreateFile
, calls
ObOpenObjectBy-
Name
in the object manager, it passes a Unicode path name for the
NT namespace, say
\ ?? \ C: \ foo \ bar
.
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