LinuxQuestions.org - Decrypt device driver

I got a task to make a small image, encrypt it with the simpliest method and write a decrypt driver.

Making an image:

Code:

dd if=/dev/zero of=myFS bs=1024 count=60

mkfs.ext2 myFS

Encrypting with XOR:

Code:

int main()

{

    std::fstream streamIn;

    std::fstream streamOut;



    const char key = 0b00001111;



    streamIn.open("myFS", std::ios_base::binary | std::ios_base::in);

    streamOut.open("newFS", std::ios_base::binary | std::ios_base::trunc | std::ios_base::out);



    char b;

    while (streamIn.read(&b, 1))

    {

        b ^= key;

        streamOut.write(&b, 1);

    }



    streamIn.close();

    streamOut.close();



    return 0;

}

Then writing driver. It is based on device-mapper target and the main working function has this prototype:

Code:

static int sddm_target_map(struct dm_target *ti, struct bio *bio);

You see, I have a bio structure, and I think, that I should decrypt every byte it contains. What I have now:

Code:

struct bio_vec vec;

struct bvec_iter it;

unsigned int len;

char *addr;

unsigned int i;



bio_for_each_segment(vec, bio, it)

{

    len = vec.bv_len;

    addr = (char*)(page_address(vec.bv_page) + vec.bv_offset);



    for (i = 0; i != len; ++i)

    {

        *(addr+i) ^= 0b00001111;

    }

}

Of course, it doesn't work. Attempts to mount it finished with error:

Code:

mount: wrong fs type, bad option, bad superblock on /dev/mapper/mydevice

Also saw another algorithm:

Code:

bio_for_each_segment(vec, bio, it)

{

    len = vec.bv_len;

    addr = kmap_atomic(vec.bv_page);

    pointer = (char *)(addr + vec.bv_offset);



    for (i = 0; i != len; ++i)

    {

        *(pointer+i) ^= 0b00001111;

    }



    kunmap_atomic(addr);

}

But it goes with the same error.

How can I implement my plan? Thanks in advance!