Direct Parameter Access not working, using printf in C

I am doing some computer security research, and I am trying to learn about string format vulnerabilities. I am running a program that contains this code:

char buf[1<<5];
strncpy(buf, data, sizeof(buf));
buf[sizeof(buf)-1]='\0';
fprintf(stderr, buf);

When I feed the program the argument "%08x.%08x.%08x.%08x.%08x" (which gets read into the "data" variable), I get the output: 00000020.b7fd7560.08048b09.00000019.78383025

I understand that each hex number is popped off the stack, and the "78383025" comes from the buffer itself. So there are 4 words--16 bytes--that I have to pop off before I get to the start of my buffer.

When I give the argument `perl -e 'pri开发者_JAVA技巧nt "\x2a\xf9\xff\xbf%08x.%08x.%08x.%08x_%s_";'`, the %s part prints the string located at memory address 0xbffff92a.

Now, I'd like to do this using direct parameter access. If I feed the program the argument `perl -e 'print "\x2a\xf9\xff\xbf%16$s";'`, I should expect the program to do the same thing as above. But all the program prints is the four characters at the start of the buffer. So, what gives??? Am I using the wrong syntax for DPA??? I am using Ubuntu 9.04, 32-bit by the way.

Here is some compilable code, not guaranteed to produce the same results though:

#include <stdio.h>

void run(const char* data) {
    char buf[1<<5];
    strncpy(buf, data, sizeof(buf));
    buf[sizeof(buf) - 1] = '\0';
    fprintf(stderr, buf);
}

int main(int argc, char* argv[]) {
    run(argv[1]);
    return 0;
}

---

%16$s refers to the 16-th argument after the format string, and tells printf to interpret it as a char* and display it as a string.

You seem to be using it as a means to skip 16 bytes before getting the string though, which is not exactly the same thing.

Since you want the 5-th argument, try something more like this format string :

"\x2a\xf9\xff\xbf%5$s"

Since you're using perl -e 'print "...";' to pass the data, you will have to escape the $ character. Ie. :

./a.out `perl -e 'print "\x2a\xf9\xff\xbf%5\\\$s";'`