Java JNA Read Process Memory Linux

Question

I'm trying to make a Java library to read/write memory values to processes on linux.

Every time I read a value, ptrace returns -1, and I read a different int everytime.

Here is my main class code:

public final class Main {

public static void main(String... args) {
    // 10702 ?        00:00:00 idea.sh
    Memory buffer = new Memory(4);
    System.out.println(ptrace.ptrace(1, 10702, new Pointer(0x7f1f80e81fb8L), buffer));
    System.out.println(buffer.getInt(0));
}

}

That outputs:

-1
2014720320

Here is my ptrace class:

public class ptrace {

static {
    Native.register(NativeLibrary.getInstance("c"));
}

public static native long ptrace(/*enum __ptrace_request*/ int request,
                           /*pid_t*/ int pid,
                                 Pointer addr,
                                 Pointer data);
}

Can anyone tell me why it fails to read the value at that Pointer?

Answer 1

You need to "attach" the running process to be able to read/write and be root as well when it's not your process (PTRACE_TRACEME).

E.g :

main:

//@see http://linux.die.net/include/sys/ptrace.h
private static final int PTRACE_PEEKDATA = 2;
private static final int PTRACE_ATTACH = 16;
private static final int PTRACE_DETACH = 17;

public static void main(String[] args) {
    int pid = 1015;

    try {
        ptrace.ptrace(PTRACE_ATTACH, pid, null, null);
        Memory buffer = new Memory(4);
        ptrace.ptrace(PTRACE_PEEKDATA, pid, new Pointer(0x7f27d6575000L), buffer);
        System.out.println(buffer.getChar(0));
    } catch (LastErrorException ex) {
        ex.printStackTrace();
    } finally {
        ptrace.ptrace(PTRACE_DETACH, pid, null, null);
    }

ptrace.java :

public class ptrace {

    static {
        Native.register(NativeLibrary.getInstance("c"));
    }

    public static native long ptrace(/*enum __ptrace_request*/int request,
            /*pid_t*/ int pid,
            Pointer addr,
            Pointer data) throws LastErrorException;
}

You can found more info on how to use ptrace here

LastErrorException is here just to get "errno". You can use ̀Native.getLastError() instead of Exception.

StackTace E.g:

com.sun.jna.LastErrorException: [1] (
at cg.ptrace.ptrace(Native Method)
at cg.Cg.main(Cg.java:123)

shell :

$ errno 1
EPERM 1 Operation not permitted