All filesystems rely on the VFS to enable them not only to coexist, but also to interoperate.
— Robert Love, Linux Kernel Development, 3rd Edition, Chapter 13, pp. 261.
The virtual filesystem (VFS) layer allows a wide range of filesystems to be used within Linux, even if their implementation details vary significantly. Each filesystem is required to implement a common set of abstractions, which in turn allows Linux to handle them in a uniform manner. This also means that how a process views a filesystem is also standardized, allowing even kernel threads and other specialized processes to interact with different filesystem abstractions in a common and portable (at least within Linux) manner.
In this studio, you will:
Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete. We encourage you to please work in groups of 2 or 3 people on each studio (and the groups are allowed to change from studio to studio) though if you would prefer to complete any studio by yourself that is allowed.
As you work through these exercises, please record your answers, and when finished upload them along with the relevant source code to the appropriate spot on Canvas. If you work in a group with other people, only one of you should please upload the answers (and any other files requested) for the studio, and if you need to resubmit (e.g., if a studio were to be marked incomplete when we grade it) the same person who submitted the studio originally should please do that.
Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.
currentmacro to access its own process descriptor (
struct task_structdeclared in
include/linux/sched.h) and print out the values (i.e., the addresses they contain) of three of its
task_struct's fields to the system log:
These fields give a process direct access into the virtual filesystem.
Respectively, these fields are pointers to the process's filesystem
struct fs_struct, declared in
include/linux/fs_struct.h), its open file table
struct files_struct, declared in
include/linux/fdtable.h), and its namespace proxy
struct nsproxy, a struct declared in
include/linux/nsproxy.h that wraps the pointer to the
mnt_namespace struct described in the text book).
Compile your module and load it on your Raspberry Pi, examine the system log to see your module's output, and then unload it. As the answer to this exercise, please show the lines of the system log that contain your module's output, including the values of the three pointers.
fsfield of its process descriptor to access two fields of the process' filesystem structure (
These fields are path structures (
struct path, declared in
include/linux/path.h) for the process' current working directory
and the root directory, respectively. Each of these path structures
contains two fields:
mnt which points to a VFS mount structure
struct vfsmount, declared in
dentry which points to a directory entry structure
struct dentry, declared in
Modify your module so that its kernel thread prints out the values (the
addresses of the locations they point to) of both of those path structures'
Your module should also check if the values of those pointers differ;
if so, print out the strings in the
d_iname fields of the
directory entry structures to which they each point.
Otherwise, if they point to the same directory entry structure,
print out the string for its
Compile your module and load it on your Raspberry Pi, examine the system log to see your module's output, and then unload it. As the answer to this exercise, please explain, based on your module's output to the system log, whether or not (and why or why not) you think the process' current working directory is the same as its root directory.
d_subdirsfield of the directory entry structure to which the path struct for the root directory points. Because
d_subdirspoints to a child entry, once you have reached the first child directory entry, you will then have to traverse the list of its siblings. This list's head is
d_childfield of the directory entry structure.
Special functions are needed to traverse Linux kernel data structures,
as described in Chapter 6 of the LKD course text book.
Review the discussion and examples in that chapter, and use the appropriate
functions in your module's kernel thread to iterate over each child entry.
To do so, provide the
d_subdirs field as the pointer to the
list head, then use
d_child as the name of the subsequent member list to traverse.
For each child entry, your kernel thread should print the value of its
d_iname to the kernel log.
Compile your module and load it on your Raspberry Pi, examine the system log to see your module's output, and then unload it. Then, in a terminal window on your Raspberry Pi, list the contents of the root directory using the command:
ls -l /
As the answer to this exercise, please show the output from your module that
contains the names of the entries in the root directory.
Does this differ from the output of
d_subdirsfield of the root directory entry, it only prints the value of an entry's
d_inamefield if that entry's
d_subdirslist is non-empty.
Compile your module and load it on your Raspberry Pi, examine the system log to see your module's output, and then unload it. Again, compare the output to the output of the command:
ls -l /
As the answer to this exercise,
please show the output from your module.
This time, does it differ from the output of
Please explain why you think there is, or is not, a difference.