bad2.c

/*
 * this demonstrates a consequence of failing to check the length of the input string
 * if you enter more than 24 chara cters, the buffer input will overflow, changing
 * the value in either above or below (depending on how the computer works); it
 * may also cause a crash
 *
 * this differs from bad.c as it prints values *before* input as well as after
 *
 * NOTE: buffer overflow causes an abort if compiled with gcc
 * to demonstrate what happens, use -fno-stack-protector
 *
 * Matt Bishop, ECS 36A
 * May 28, 2024	from a buffer overflow program
 */

#include <stdio.h>

char *gets(char *);	/* gcc gives a warning if this isn't here */

int main(void)
{
	int above = 100;	/* before the buffer */
	char input[24];		/* the input buffer */
	int below = 200;	/* after the buffer */

	/* print the values of the variables surrounding the buffer */
	printf("BEFORE INPUT: above = %#010x; below =  %#010x\n", above, below);

	/*
	 * read the input
	 */
	if (gets(input) == NULL){
		fprintf(stderr, "Unexpected EOF\n");
		return(1);
	}

	/*
	 * print out result; on buffer overflow, above or below may
	 * change (or both, depending on where the compiler puts them)
	 */
	printf(" AFTER INPUT: above = %#010x; below = %#010x\n", above, below);

	/* that's it! */
	return(0);
}

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 36A, Programming & Problem Solving Version of April 2, 2024 at 12:13PM

You can get the raw source code here.