Security layer
- C course
- Expert
- Lesson 46/47
- 6 min
Write buffer overflow protection and security checks.
Security layer
In the fourth lesson you add the security layer. The security_check function checks a buffer for overflow risk: if the length is too large or the pointer is NULL, it returns 0.
Buffer overflow is exceeding an array's bounds; it is the most common source of security vulnerabilities. security_check verifies the size before processing data. len < 0 || len > 16 catches negative or too-large values; buf == NULL catches a null pointer. If both pass, it returns 1 (safe). This is a basic example of defensive programming.
Defensive programming
Defensive programming catches potential errors in advance. security_check does three checks: is the length negative, too large, or is the pointer NULL. If any check fails, it returns 0.
This approach is similar to a canary: an error is caught before it occurs. A real canary is placed next to a function's return address to detect stack smashing; if it changes, the program stops. security_check checks before data entry. Both methods prevent bad data from entering the system. Secure code checks every input.
Practice Add the security_check function
Below the existing definitions, write int security_check(const char *buf, int len). Check overflow with if (len < 0 || len > 16) return 0;. Check NULL with if (buf == NULL) return 0;. Return safe with return 1;.
Starter code
#include <stdio.h>
#include <string.h>
void kernel_init(void);
int parse_command(const char *cmd);
int security_check(const char *buf, int len);
void kernel_init(void) {
printf("Çekirdek başladı\n");
}
int syscall(int num) {
return num * 2;
}
int parse_command(const char *cmd) {
if (strcmp(cmd, "help") == 0) return 1;
if (strcmp(cmd, "exit") == 0) return 2;
return 0;
}
int main(void) {
printf("Proje başladı\n");
kernel_init();
printf("Komut: %d\n", parse_command("help"));
return 0;
}
// security_check fonksiyonunu yaz:
// len 0-16 arası ve buf NULL değilse 1, yoksa 0Sample solution
#include <stdio.h>
#include <string.h>
void kernel_init(void);
int parse_command(const char *cmd);
int security_check(const char *buf, int len);
void kernel_init(void) {
printf("Çekirdek başladı\n");
}
int syscall(int num) {
return num * 2;
}
int parse_command(const char *cmd) {
if (strcmp(cmd, "help") == 0) return 1;
if (strcmp(cmd, "exit") == 0) return 2;
return 0;
}
int security_check(const char *buf, int len) {
if (len < 0 || len > 16) return 0;
if (buf == NULL) return 0;
return 1;
}
int main(void) {
printf("Proje başladı\n");
kernel_init();
printf("Komut: %d\n", parse_command("help"));
return 0;
}Expected output
Proje başladı Çekirdek başladı Komut: 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does security_check prevent?
- Type errors
- Buffer overflow Correct answer
- Compile errors
- Memory leaks
Explanation: security_check checks for buffer overflow risk.
2 Complete the length check
if (len < 0 || len > ____) return 0;if (len < 0 || len > 16) return 0;Explanation: len > 16 catches overflow risk; 16 bytes is the limit.
3 Match security concept
| Term | Meaning |
|---|---|
Overflow check |
len > 16 |
NULL check |
buf == NULL |
Canary |
Protection value |
Safe |
return 1 |