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C语言代写 | CMPT 300 with Brian Fraser

C语言代写 | CMPT 300 with Brian Fraser


CMPT 300 with Brian Fraser
Assignment 2: Syscalls1
1. Preparation
In order to complete this assignment, it is vital that you have carefully completed and understood the content in the following guides which are posted on the course website:
Custom Kernel Guide: how to download, build, and run a custom Linux kernel.
Guide to Linux Syscalls: how to create and test a simple Linux system call (syscall).
In this assignment you’ll be coding in both user space and kernel space. Since it takes a couple of minutes to recompile and re-run a new kernel, you should code carefully!
2. Array Statistics Syscall
First, you’ll add a new system call that computes some basic statistics on an array of data. In practice it makes little sense to have this as a syscall; however, it allows us to become familiar with accessing memory between user and kernel space before accessing other kernel data structures.
In your .c file which implements the syscall, you’ll need:
struct array_stats_s {
long min;
long max;
long sum;
array_stats, /* syscall name */
struct array_stats_s*, stats, /* where to write stats */
long*, data, /* data to process */
long, size) /* # values in data */
// your code here…
array_stats: The first argument to the macro defines the name of the syscall function.
The macro will make this into the function sys_array_stats(…).
stats: A pointer to one array_stats_s structure which was allocated by the user-space application. Structure will be written to by the syscall to store the minimum, maximum, and
sum of all values in the array pointed to by data.
data: An array of long values passed in by the user-space application.
size: The number of elements in data. Must be > 0.
The array_stats syscall must:
Set the three fields of the stats structure based on the data array. The values in data are signed (positive or negative). Nothing special need be done if the sum of all the data will overflow/underflow a long.
Return 0 when successful.
Returns -EINVAL if size <= 0 (in which case, nothing is written to stats).
Returns -EFAULT if unable to access stats or data (such as invalid pointers) (in which case,nothing is written to stats).
You must not allocate or use a large amount of kernel memory to copy the entire input data array into.
The Guide to Linux Syscalls (on course website) has some hints on how to write your own syscall.
2.2 Test Application Requirements
You must create a user-space test application to test your syscall.
Code your application in a folder outside of the kernel’s build folders. For example, put it where ever you normally put your code for assignments.
Your application should execute a few calls to your syscall to prove it works. Try some of the
edge cases.
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Check with different size arrays, and different values.
Check with invalid pointers and array sizes to ensure your syscall handles bad values correctly.
We will test your code with our extensive testing program.
3. Process Ancestor Syscall
In this section you’ll implement a syscall which returns information about the current process, plus its ancestors (its parent process, it’s grandparent process, and so on).
3.1 Syscall Requirements
Create a new syscall named process_ancestors:
Make its syscall number 550 (in syscall_64.tbl):
550 common process_ancestors sys_process_ancestors
The syscall will look at the current process, and each of its ancestors, all the way back to the init process (usually PID 1). For each process it will record some information about the process. This infromation will be written into an array of structs (one per process). Our syscall will need:
a pointer to an array of structs for recording the process information, allocated by the userspace application.

the number of structs in the array a pointer to a long, which the syscall will write to, to record how many processes the syscall analyzed (i.e., how many elements in the array of stucts did the syscall fill).
In your syscall’s .c file you’ll need:
struct process_info {
long pid; /* Process ID */
char name[ANCESTOR_NAME_LEN]; /* Program name of process */
long state; /* Current process state */
long uid; /* User ID of process owner */
long nvcsw; /* # voluntary context switches */
long nivcsw; /* # involuntary context switches */
long num_children; /* # children process has */
long num_siblings; /* # sibling process has */
process_ancestors, /* syscall name for macro */
struct process_info*, info_array, /* array of process info strct */
long, size, /* size of the array */
long*, num_filled) /* # elements written to array */
// your code here…
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process_ancestors: the first argument to the macro defines the name of the syscall function.
The macro will make this into the function sys_process_ancestors(…).
info_array: array of process_info structs, alloctaed by the user-space application, into
which the syscall will write information about each process.
size: the size of info_array (number of process_info structs), passed in by the user-space
application. This is the maximum number of structs that the syscall will write into the array
(starting with the current process as the first entry and working up from there). The size may
be larger or smaller than the actual number of ancestors of the current process: larger means
some entries are left unused; smaller means information about some processes will not be
written into the array.
num_filled: a pointer to a long, which will be written to by the syscall to record how many
structs in the info_array array were filled in by the syscall. num_filled will be at most
equal to size; but could be less than size when there are fewer ancestors processes than the
size of the passed in array.
The process_ancestors syscall must:
Starting at the current process, fill the elements in info_array[] with the correct values.
Ordering: the current process’s information goes into info_array[0]; the parent of the
current process into info_array[1]; grandparent into info_array[2]; and so on.
If there are extra structs in info_array[], they are left unmodified.
Return 0 when successful.
Return -EINVAL if the syscall’s size argument is <= 0.
Return -EFAULT if there are any problems access info_array or num_filled (such as bad
pointers, …)
You must not allocate or use a large amount of kernel memory to copy/store large arrays into.
However, you may allocated on the stack, inside the syscall, up to one process_info struct if
3.2 Test Application Requirements
You must create a user-space test application to test your syscall.
Place the test apps for both of your assignment’s system calls in one folder, built with the same
You must do at least some testing to show the syscall works correctly, and that it generates
correct error values in at least a few of the failure conditions (suc has bad pointers, …).
Hint: Use asserts or the CHECK() macro in your test code.
We will run your test apps to ensure they work.
We will test your syscall with our extensive testing program.
3.3 Hints
You will make extensive use of the kernel’s task_struct structures: each process has a
task_struct. You can access the task_struct for the current process using the macro
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For example, the PID for the currently running process on this CPU can be found with:
long pid = current->pid;
Basic algorithm sketch:
1) Fill in fields for info_array[0] for the current process.
2) Move to the parent of this process (current->parent), and copy its info into info_array[1].
3) Repeat until the parent of the process you are working on is itself
(i.e: cur_task->parent == cur_task).
The first task spawned by Linux (often called init) is its own parent, so hence its parent
pointer points to itself. This process usually has PID 1 and is the idle task (named swapper or
I recommend that you:
first write test values (like 42) into the process_info struct to show you can correctly
write to it in the syscall, and read from it in your user-space test application.
next get the info on the current process and print it to the screen (printk) to ensure you
have the correct values.
then write the process’s values into the process_info struct and handling ancestors.
Hints on filling the fields of process_info:
Quite a few of the values can be pulled directly out of the task_struct structure. Look for
fields with a matching name.
task_struct is defined in include/linux/sched.h (in your kernel folder). To include
this in your syscall implementation use: #include <linux/sched.h>
Here is a good online site to navigate the kernel source;
browse into include → linux → sched.h; search for tast_struct
The name of the program for a process is stored in the task_struct.comm field.
The user ID for the owner of a process can be found inside the process’s credentials (cred
field). Inside cred, you want to look at the uid field.
For counting the number of children and siblings, you’ll want to start with the following linked
list nodes: task_struct.children, and task_struct.sibling.
These are nodes in circular linked lists. Linux uses the struct list_head for a node because
in a circular linked list, each node can be thought of as the head of the list.
You can follow the next field of a node in the list (a list_head) to get the node
(list_head) of the next element in the list.
It is a circular linked list, so you’ll have to determine how to count the number of elements
(think of how you know when to stop following next pointers).
Note that Linux has some clever (complicated) ways of taking a node in the list (which just
has a next and prev field pointing to other list_head structures) and accessing the full
structure that contains the node. For example, given a list_head struct that is in a
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task_struct, the kernel includes macros to give you the full task_struct. However, you
have (mercifully) been spared having to do this. If you are interested, for fun try printing
out the PID of each of the sibling processes.
Safe memory access is critical. Apply all the suggestions from the syscall guide for safe memory
You can use the ps command in your QEMU virtual machine to display information on running
processes and verify the syscall output. Run ‘man ps’ to see how to select the information it
4. Deliverables
In CourSys, create a group for your submission. Even if you worked on your own, you need to be in a
group (of 1 in this case) in order to submit. Submit the following to CourSys:
1. syscalls.tar.gz: An archive file of your kernel code folder.
Required files:
◦ makefile (executed by kernel’s build system to build your syscalls’ .c files)
◦ array stat syscall’s .c
◦ process ancestor syscall’s .c
◦ OK to include cs300_test.c
◦ OK to include any extra .h files (if any)
To create this archive, from inside your linux-5.7/cs300/ folder, execute:
$ tar -czvf syscalls.tar.gz *