How to I find out the size of a GZIP section embedded in firmware?

I am currently analyzing firmware images which contain many different sections, one of which is a GZIP section.

I am able to know the location of the start of the GZIP section using magic number and the GZIPInputStream in Java.

However, I need to know the compressed size o开发者_开发知识库f the gzip section. GZIPInputStream would return me the uncompressed file size.

Is there anybody who has an idea?

You can count the number of byte read using a custom InputStream. You would need to force the stream to read one byte at a time to ensure you don't read more than you need.

You can wrap your current InputStream in this

class CountingInputStream extends InputStream {
    final InputStream is;
    int counter = 0;

    public CountingInputStream(InputStream is) {
        this.is = is;
    }

    public int read() throws IOException {
        int read = is.read();
        if (read >= 0) counter++;
        return read;
    }
}

and then wrap it in a GZIPInputStream. The field counter will hold the number of bytes read.

To use this with BufferedInputStream you can do

 InputStream is = new BufferedInputStream(new FileInputStream(filename));
 // read some data or skip to where you want to start.

 CountingInputStream cis = new CountingInputStream(is);
 GZIPInputStream gzis = new GZIPInputStream(cis);
 // read some compressed data
 cis.read(...);

 int dataRead = cis.counter;

In general, there is no easy way to tell the size of the gzipped data, other than just going through all the blocks.

gzip is a stream compression format, meaning that all the compressed data is written in a single pass. There is no way to stash the compressed size anywhere---it can't be in the header, since that would require more than one pass, and it's useless to have it at the trailer, since if you can locate the trailer, then you already know the compressed size.