Question

I’m running some unsecure code which I have set its stdout and stderr streams to FileStreams wrapped in PrintStreams. (Standard output/error MUST be redirected.)

Is there any way to configure those redirected FileStreams/PrintStreams to set a maximum of say 10 MB written, so that, for example,

while (true) System.out.write("lots of bytes");

doesn’t write excessive amounts of data to the server’s disk.

The code does have a time limit of 15s, but I’d like a separate guard here.

Answer 1

One way to do it is to define a FilterOutputStream that you wrap the file stream in, which keeps an internal counter that it increments on every write, and after reaching a set threshold, starts throwing Exceptions or simply ignores the writes.

Something along the lines of:

import java.io.*;
class LimitOutputStream extends FilterOutputStream{

    private long limit;

    public LimitOutputStream(OutputStream out,long limit){
        super(out);
        this.limit = limit;
    }

    public void write(byte[]b) throws IOException{ 
        long left = Math.min(b.length,limit);
        if (left<=0)
            return;
        limit-=left;
        out.write(b, 0, (int)left);
    }

    public void write(int b) throws IOException{
        if (limit<=0)
            return;
        limit--;
        out.write(b);
    }

    public void write(byte[]b,int off, int len) throws IOException{
        long left = Math.min(len,limit);
        if (left<=0)
            return;
        limit-=left;
        out.write(b,off,(int)left);
    }  
}