Homework 6 Solution

Academic Honesty




All work submitted in this course must be your own. Cheating and plagiarism will not be tolerated. If you have any questions about a specific case, please ask me. We will be checking for this!




NYU Poly’s Policy on Academic Misconduct: http://engineering.nyu.edu/files/SACCofC2-2-16.pdf




General Notes:




Read the assignment carefully, including what files to include.
Don't assume limitations unless they are explicitly stated.
Treat provided examples as just that, not exhaustive list of cases that should work.



When in doubt regarding what needs to be done, ask. Another option is test it in the real UNIX operating system. Does it behave the same way?



TEST your solutions, make sure they work. It's obvious when you didn't test the code.









Adding Timestamps in the XV6 FileSystem




In this assignment you will add support for tracking when a file was created to the xv6 filesystem. We will also add support to the `ls` and `mkfs` utilities so that they display and create timestamps, respectively.







Getting the code from Github




As usual, we'll be working off of a slightly modified version of xv6. The only difference is that the `date.h` header, which you will need in order to get the current time, has been modified so that it can be included multiple times without causing a compile error.

If you still have your xv6 directory from last time, remove or rename it first. Then get the base xv6 code for this assignment. Yes we know the branch below is hw7, but we are doing hw6.




$ git clone https://github.com/moyix/xv6-public.git Cloning into 'xv6-public'...




remote: Counting objects: 4517, done.

remote: Compressing objects: 100% (5/5), done.

remote: Total 4517 (delta 1), reused 0 (delta 0), pack-reused 4512

Receiving objects: 100% (4517/4517), 11.67 MiB | 6.31 MiB/s, done.

Resolving deltas: 100% (1825/1825), done.

Checking connectivity... done.

$ cd xv6-public/




$ git checkout hw7

Branch hw7 set up to track remote branch hw7 from origin.

Switched to a new branch 'hw7'













Part 1: Tracking Creation Time




In xv6, each file and directory is associate with an *i-node*, remember the i -node data structure we saw in class, which keep keeps track of details such as the file's type (file, directory, or device), reference count, and list of disk blocks. i-nodes are stored both on disk (in a `struct dinode`) and in memory (`struct inode`), and xv6 copies data between the two structures as needed.




Modify the filesystem code so that on-disk and in-memory i-nodes both contain a field that keeps track of the time the i-node was created. Make sure that you modify both the on-disk and in-memory i-node structures, and that when xv6 copies data between them you also copy the creation time.




You can verify that you did this part correctly by using `gdb` to examine a newly created i-node and making sure that it contains the correct timestamp (or just do part 2, which modifies the `ls` command to print out timestamps as well).







Hints




All files are created using the `create()` function in `sysfile.c`.
You can use the `cmostime()` function to get the current date and time from inside of xv6.



Because they need to fit neatly into one disk block, the size of the `struct dinode` must divide the block size evenly. So when you add in the timestamp field, you will have to pad out the structure somehow (perhaps by increasing the number of direct block pointers).
Part 2: Displaying Timestamps
Modify the `ls` command so that it prints out the creation time of each file or directory.




sb: size 1000 nblocks 916 ninodes 200 nlog 30 logstart 2 inodestart




32

bmap start 83










init: starting sh








$ ls










.
1
1 512 0/0/0 00:00:00
..
1
1 512 0/0/0 00:00:00
README
2
2 1972 0/0/0 00:00:00
cat
2
3 13420
0/0/0 00:00:00
echo
2
4 12525
0/0/0 00:00:00
forktest
2
5 8257 0/0/0 00:00:00
grep
2
6 15088
0/0/0 00:00:00
init
2
7 13190
0/0/0 00:00:00
kill
2
8 12677
0/0/0 00:00:00
ln
2
9 12531
0/0/0 00:00:00
ls
2
10
15259
0/0/0 00:00:00
mkdir
2
11
12678
0/0/0 00:00:00
rm
2
12
12659
0/0/0 00:00:00
sh
2
13
23639
0/0/0 00:00:00
stressfs
2
14
13401
0/0/0 00:00:00
usertests
2
15
58688
0/0/0 00:00:00
wc
2
16
13938
0/0/0 00:00:00
zombie
2
17
12295
0/0/0 00:00:00
console
3
18
0 11/20/2015 23:08:00
$ echo hello file.txt




$ ls file.txt










file.txt
2
19
3 11/21/2015 23:14:02



Hints:




`ls` uses the `fstat` call to retrieve information about the file or directory; you will need to modify the implementation of `fstat` so that it returns information about the creation time as well.



Don't bother making sure that leading zeroes in the time are printed correctly. You can just use the "%d" format character.






Part 3: timestamps in mkfs




You may have noticed that many of the files on the filesystem have a creation time of 0/0/0 00:00:00. This is because the initial filesystem

image (`fs.img`) is not created from within xv6, but rather by the `mkfs` program, which is run outside of xv6 by the Makefile.




To finish up the implementation, modify `mkfs.c` so that it writes the current timestamp into the on-disk i-node when the initial filesystem image is created. Verify that when you boot xv6, you now get a directory listing that looks like:







$ ls










.
1
1 512 11/22/2015 00:00:29
..
1
1 512 11/22/2015 00:00:29
README
2
2 1972 11/22/2015 00:00:29
cat
2
3 13420
11/22/2015 00:00:29
echo
2
4 12525
11/22/2015 00:00:29
forktest
2
5 8257 11/22/2015 00:00:29
grep
2
6 15088
11/22/2015 00:00:29
init
2
7 13190
11/22/2015 00:00:29
kill
2
8 12677
11/22/2015 00:00:29
ln
2
9 12531
11/22/2015 00:00:29
ls
2
10
15631
11/22/2015 00:00:29
mkdir
2
11
12678
11/22/2015 00:00:29
rm
2
12
12659
11/22/2015 00:00:29
sh
2
13
23639
11/22/2015 00:00:29
stressfs
2
14
13401
11/22/2015 00:00:29
usertests
2
15
58688
11/22/2015 00:00:29
wc
2
16
13938
11/22/2015 00:00:29
zombie
2
17
12295
11/22/2015 00:00:29
console
3
18
0 11/22/2015 00:00:32






Hints:




You will likely find the `gmtime()` function helpful.
You may need to run `make clean` in order to remove the old `fs.img` and re-run `make` before the timestamps show up correctly (or just run `rm fs.img`).






Submitting




As with previous assignments, you will use git to create a patch and then submit the patch to NYU Classes.




Commit your changes:




$ git commit --all --message="Add filesystem timestamps"




[hw7 b778e43] Add filesystem timestamps

7 files changed, 31 insertions(+), 3 deletions(-)







(Note: if you added any new files, you will also have to use `git add <filename` before you run `git commit`.)




Now create the patch file:




$ git format-patch hw7.unmodified




0001-Add-filesystem-timestamps.patch




The command creates a file, `0001-Add-filesystem-timestamps.patch`, containing the changes you've made. Submit this file on **NYU Classes**.




Submission Notes




Don't try to edit the patch file after creating it. Doing so will most likely corrupt the patch file and make it impossible to apply. Instead, change the original file, commit your changes, and run `git format-patch` again:




$ git commit --all --message="Description of your change here"




[hw7 7cc4977] Description of your change here

file changed, 1 insertion(+), 1 deletion(-) $ git format-patch hw7.unmodified
0001-Add-filesystem-timestamps.patch

0002-Description-of-your-change-here.patch




Then submit *both* patch files.