Final Answer:
The completed shell script runs a loop executing the `vprog` program until it detects a modification in `/etc/passwd`. It calculates the MD5 checksum and compares it with the original checksum, breaking out of the loop upon successful modification. This approach addresses the probabilistic nature of race condition attacks by allowing repeated attempts until success.
The completed shell script is as follows:
```bash
#!/bin/bash
while true; do
./vprog
if [[ $(md5sum /etc/passwd | cut -d ' ' -f 1) != "<original_checksum>" ]]; then
echo "Attack successful! /etc/passwd has been modified."
break
fi
done
```
Step-by-step explanation:
In the provided shell script, a `while` loop is used to continuously execute the `vprog` program. Within the loop, the script calculates the MD5 checksum of the `/etc/passwd` file using `md5sum` and extracts the checksum value using `cut`. The obtained checksum is then compared to the original checksum ("<original_checksum>"). The original checksum should be replaced with the known checksum of `/etc/passwd` before the attack.
The `if` condition checks whether the current checksum differs from the original one. If they are not equal, it indicates that the `/etc/passwd` file has been modified, and the script prints a message declaring the success of the attack before breaking out of the loop.
This script is designed to run the attack repeatedly until a modification in the `/etc/passwd` file is detected. The probabilistic nature of race condition attacks is addressed by the loop, allowing the attacker to make multiple attempts until a successful modification is achieved. The script terminates upon successful modification, providing a reliable indication of a successful race condition attack.