15-213 Recitation: Data Lab Sep 13, 2021
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● Introduction
● Course Details ● Office Hours
● Data Lab
○ Getting started
○ Running your code ○ ANSI C
○ Reminders
● Floating Point
Introduction
● Welcome to 15-213/14-513/15-513! ● Recitations are for…
○ Reviewing lectures
○ Discussing homework problems ○ Interactively exploring concepts
○ Previewing future lecture material
● Please, please ask questions!
Course Details
● How do I get help?
○ Course website: http://cs.cmu.edu/~213
○ Office hours
○ Definitely consult the course textbook
○ Carefully read the assignment writeups!
● All labs are submitted on Autolab.
● All labs should be worked on using the shark machines.
Office Hours
● 6PM-10PM on Zoom and in-person (Sun-Fri) ● 11:30AM – 1:30PM (Wed, Fri)
● Queue link: https://cmqueue.xyz/ NEW LINK
● Please locate the TA in the specified location: https://piazza.com/class/kr9vqwncw253c4?cid=284
OH Etiquette
● Office hours are for getting ideas on how to debug or better approach your homework- conceptual OH coming soon as well so look out for that!
● Please try to narrow down your problem area as much as possible to help TAs help you!
● Write a description! If you don’t have a description, you may be frozen/removed from the queue.
● We will close the queue early so everyone can be helped by around 9:30pm so please keep this in mind!
● Please find the TAs at the carrels. TAs should not need to find you
Data Lab: Getting Started
● Download the handout from autolab
○ scp
○ cd to the datalab.tar file
○ tar -xf datalab.tar
● Upload bits.c file to Autolab for submission
Data Lab: Running your code
● dlc: a modified C compiler that interprets ANSI C only ● btest: runs your solutions on random values
● bddcheck: exhaustively tests your solutions
○ Checks all values, formally verifying the solution ● driver.pl: Runs both dlc and bddcheck
○ Exactly matches Autolab’s grading script
○ You will likely only need to submit once ● For more information, read the writeup
○ Available under autolab as “View writeup” ○ Read the writeup please!
Data Lab: What is ANSI C?
Within two braces, all declarations must go before any expressions.
This is not ANSI C.
unsigned int foo(unsigned int x)
x = x * 2;
int y = 5;
if (x > 5) {
x = x * 3;
int z = 4;
x = x * z;
return x * y; }
Data Lab: What is ANSI C?
This is ANSI C. This is not ANSI C.
unsigned int foo(unsigned int x) unsigned int foo(unsigned int x) {{
int y = 5;
x = x * 2;
if (x > 5) {
int z = 4;
x = x * 3;
x = x * 2;
int y = 5;
if (x > 5) {
x = x * 3;
int z = 4;
x = x * z;
return x * y; return x * y; }}
x = x * z; }}
Data Lab: Reminders
● Casting between int and long is ok, in either direction ● Be aware of operations and their types!
○ ! returns an int even if the argument is a long
● Good idea to append “L” suffix to every integer constant
○ (1L << 63) is not the same as 1 << 63
○(!x << 63) is not the same as ((long)!x) << 63
Form Groups of 3 - 4
■ Series of exercises ■Operators
Questions?
■ Remember, data lab is due this Thursday! ■ You really should have started already!
■ Read the lab writeup!
Looking Ahead... Bomblab!!
What is Bomb Lab?
• An exercise in reading x86-64 assembly code.
• A chance to practice using GDB (a debugger).
• x86 assembly is low level machine code. Useful for understanding security exploits or tuning performance.
• GDB can save you days of work in future labs (Malloc) and can be helpful long after you finish this class.
Downloading Your Bomb
•Here are some highlights of the write-up:
• Bombs can only run on the shark machines. They fail if you run them locally or on another CMU server.
• Each bomb is unique - if you download a second bomb, bad things can happen! Stick to only one bomb.
• Bombs have six phases which get progressively harder.
• Make sure to read the writeup for more tips and common mistakes you might make.
Detonating Your Bomb
• Blowing up your bomb automatically notifies Autolab
• Dr. Evil deducts 0.5 points each time the bomb explodes.
• It’s very easy to prevent explosions using break points in GDB. More information on that soon.
• Inputting the correct string moves you to the next phase.
• Don’t tamper with the bomb. Skipping or jumping between phases detonates the bomb.
• You have to solve the phases in order they are given. Finishing a phase also notifies Autolab automatically.
Bomb Hints
• Dr. Evil may be evil, but he isn’t cruel. You may assume
that functions do what their name implies
• i.e. phase_1() is most likely the first phase. printf() is just printf(). If
there is an explode_bomb() function, it would probably help to set a breakpoint there!
• Use the man pages for library functions.
• Although you can examine the assembly for snprintf(), we assure you that it’s easier to use the man pages ($ man snprintf) than to decipher assembly code for system calls.
• Most cryptic function calls you’ll see (e.g. callq ... are also calls to C library functions.
• You can safely ignore the @plt as that refers to dynamic linking.
S21 Bomblab Slides
https://docs.google.com/presentation/d/1c9IVmK69sVnd zX5_rZYaL-jj5uk_d3Gl/edit?usp=sharing&ouid=1050562 71954280155624&rtpof=true&sd=true
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