rand_ioctl.c

#include <fcntl.h>
#include <linux/types.h>
#include <linux/random.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>

#define MAX_BYTES   512
#define MAX_BITS    (8 * MAX_BYTES)

struct my_rand_pool_info {
    int entropy_count;
    int buf_size;
    unsigned char buf[MAX_BYTES];
} rpi;


static ssize_t
add_entropy(size_t nbits)
{
    int fd = open( "/dev/random", O_RDWR);
    if (fd < 0) {
        perror( "Cannot open /dev/random");
        return -1;
    }

    rpi.entropy_count = nbits;
    rpi.buf_size = 1;
    rpi.buf[0] = 0;

    if (ioctl(fd, RNDADDENTROPY, &rpi) == -1) {
        perror("ioctl");
        return -1;
    }
    return rpi.entropy_count;
}


int main( int argc, char *argv[]) {

    int nbits = 0;

    if (argc < 2) {
        fprintf( stderr, "Usage: rand_ioctl <num bits>\n");
        exit(1);
    }

    nbits = atoi( argv[1]);
    if (nbits <= 0)
        exit(0);
    else if (nbits > MAX_BITS)
        nbits = MAX_BITS;

    printf( "Adding %d bits of fake entropy to the pool.\n", nbits);

    ssize_t nbits_added = add_entropy( nbits);

    if (nbits_added >= 0) {
        printf( "%d bits added.\n", nbits_added);
        exit(0);
    }
    else
        exit(1);
}