Simple buffer overflow example program

example.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void display_args(int size, char *start);

int main(int argc, char **argv)
{
	char pass[16] = "password123";
	char user[8] = "root";
	char buffer[8] = {};
	
	if (argc < 2) {
		fprintf(stderr,"You must enter a single argument!\n");
		return -1;
	}

	strncpy(buffer, argv[1], strlen(argv[1]));


#ifdef DEBUG
	if (argc > 2) {
		display_args(atoi(argv[2]), buffer);
		fprintf(stderr,
				"WARNING: Debug mode is on and arguments"
				" were passed to dump memory.\n");
		return 0;
	}
#endif

		printf("You entered: %s\n", buffer);	
		printf("You are user: %s\n", user);

		return 0;
}

void display_args(int size, char *start)
{
	int i = 0;
	while (i < size) {
		putchar(start[i++]);
	}
}

Makefile

CC:=$(shell which cc)
CC_WIN_32:=$(shell which i686-w64-mingw32-gcc)
CC_WIN_64:=$(shell which x86_64-w64-mingw32-gcc)
CFLAGS=
DEFINES=-DDEBUG
CFILES=example.c
OUTPUT=example

BIN_DIR=bin

.PHONY: all clean winbins posixbins

all: winbins posixbins

clean:
	@rm -r $(BIN_DIR)

winbins: $(BIN_DIR)/example32.exe $(BIN_DIR)/example64.exe 

posixbins: $(BIN_DIR)/example32 $(BIN_DIR)/example64 

$(BIN_DIR)/example32:
	@mkdir -p $(BIN_DIR)
	$(CC) $(DEFINES) $(CFLAGS) $(CFILES) -o $@ -m32
	
$(BIN_DIR)/example64:
	@mkdir -p $(BIN_DIR)
	$(CC) $(DEFINES) $(CFLAGS) $(CFILES) -o $@ -m64

$(BIN_DIR)/example32.exe:
	@mkdir -p $(BIN_DIR)
	$(CC_WIN_32) $(DEFINES) $(CFLAGS) $(CFILES) -o $@ -m32
	
$(BIN_DIR)/example64.exe:
	@mkdir -p $(BIN_DIR)
	$(CC_WIN_64) $(DEFINES) $(CFLAGS) $(CFILES) -o $@ -m64

output

$ ./bin/example32 0123456789ABCDEF 64 | hexdump -C
WARNING: Debug mode is on and arguments were passed to dump memory.
00000000  30 31 32 33 34 35 36 37  38 39 41 42 43 44 45 46  |0123456789ABCDEF|
00000010  70 61 73 73 77 6f 72 64  31 32 33 00 00 00 00 00  |password123.....|
00000020  c0 86 04 08 00 00 00 00  58 db fc ff 46 ee 64 f7  |........X...F.d.|
00000030  03 00 00 00 84 db fc ff  94 db fc ff 00 c0 79 f7  |..............y.|
00000040

$ ./bin/example32 0123456789ABCDEF
You entered: 0123456789ABCDEFpassword123
You are user: 89ABCDEFpassword123

$ ./bin/example64 0123456789ABCDEFGHIJKLMNOPQRSTUV 64 | hexdump -C
WARNING: Debug mode is on and arguments were passed to dump memory.
00000000  30 31 32 33 34 35 36 37  38 39 41 42 43 44 45 46  |0123456789ABCDEF|
00000010  47 48 49 4a 4b 4c 4d 4e  4f 50 51 52 53 54 55 56  |GHIJKLMNOPQRSTUV|
00000020  70 61 73 73 77 6f 72 64  31 32 33 00 00 00 00 00  |password123.....|
00000030  00 00 00 00 00 00 00 00  ad ee 02 1f fc 7f 00 00  |................|
00000040

$ ./bin/example64 0123456789ABCDEFGHIJKLMNOPQRSTUV
You entered: 0123456789ABCDEFGHIJKLMNOPQRSTUVpassword123
You are user: GHIJKLMNOPQRSTUVpassword123