isc/unix/entropy.c

	entropy.c revision 9c3531d72aeaad6c5f01efe6a1c82023e1379e4d
/*
 * Copyright (C) 2000  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM DISCLAIMS
 * ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL INTERNET SOFTWARE
 * CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL
 * DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR
 * PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS
 * ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS
 * SOFTWARE.
 */

/* $Id: entropy.c,v 1.42 2000/06/22 21:58:32 tale Exp $ */

#include <config.h>

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

#include <isc/buffer.h>
#include <isc/entropy.h>
#include <isc/list.h>
#include <isc/magic.h>
#include <isc/mem.h>
#include <isc/mutex.h>
#include <isc/region.h>
#include <isc/sha1.h>
#include <isc/string.h>
#include <isc/time.h>
#include <isc/util.h>

/*
 * Much of this code is modeled after the NetBSD /dev/random implementation,
 * written by Michael Graff <explorer@netbsd.org>.
 */

#define ENTROPY_MAGIC       ISC_MAGIC('E', 'n', 't', 'e')
#define SOURCE_MAGIC        ISC_MAGIC('E', 'n', 't', 's')

#define VALID_ENTROPY(e)    ISC_MAGIC_VALID(e, ENTROPY_MAGIC)
#define VALID_SOURCE(s)     ISC_MAGIC_VALID(s, SOURCE_MAGIC)

/***
 *** "constants."  Do not change these unless you _really_ know what
 *** you are doing.
 ***/

/*
 * size of entropy pool in 32-bit words.  This _MUST_ be a power of 2.
 */
#define RND_POOLWORDS   128
#define RND_POOLBITS    (RND_POOLWORDS * 32)

/*
 * Number of bytes returned per hash.  This must be true:
 *  threshold * 2 <= digest_size_in_bytes
 */
#define RND_ENTROPY_THRESHOLD   10
#define THRESHOLD_BITS      (RND_ENTROPY_THRESHOLD * 8)

/*
 * Size of the input event queue in samples.
 */
#define RND_EVENTQSIZE  32

/*
 * The number of times we'll "reseed" for pseudorandom seeds.  This is an
 * extremely weak pseudorandom seed.  If the caller is using lots of
 * pseudorandom data and they cannot provide a stronger random source,
 * there is little we can do other than hope they're smart enough to
 * call _adddata() with something better than we can come up with.
 */
#define RND_INITIALIZE  128

typedef struct {
    isc_uint32_t    magic;
    isc_uint32_t    cursor;     /* current add point in the pool */
    isc_uint32_t    entropy;    /* current entropy estimate in bits */
    isc_uint32_t    pseudo;     /* bits extracted in pseudorandom */
    isc_uint32_t    rotate;     /* how many bits to rotate by */
    isc_uint32_t    pool[RND_POOLWORDS];    /* random pool data */
} isc_entropypool_t;

struct isc_entropy {
    isc_uint32_t            magic;
    isc_mem_t              *mctx;
    isc_mutex_t         lock;
    unsigned int            refcnt;
    isc_uint32_t            initialized;
    isc_uint32_t            initcount;
    isc_entropypool_t       pool;
    unsigned int            nsources;
    isc_entropysource_t        *nextsource;
    ISC_LIST(isc_entropysource_t)   sources;
};

typedef struct {
    isc_uint32_t    last_time;  /* last time recorded */
    isc_uint32_t    last_delta; /* last delta value */
    isc_uint32_t    last_delta2;    /* last delta2 value */
    isc_uint32_t    nsamples;   /* number of samples filled in */
    isc_uint32_t   *samples;    /* the samples */
    isc_uint32_t   *extra;      /* extra samples added in */
} sample_queue_t;

typedef struct {
    sample_queue_t  samplequeue;
} isc_entropysamplesource_t;

typedef struct {
    isc_boolean_t       start_called;
    isc_entropystart_t  startfunc;
    isc_entropyget_t    getfunc;
    isc_entropystop_t   stopfunc;
    void               *arg;
    sample_queue_t      samplequeue;
} isc_cbsource_t;

typedef struct {
    int     fd;     /* fd for the file, or -1 if closed */
} isc_entropyfilesource_t;

struct isc_entropysource {
    isc_uint32_t    magic;
    unsigned int    type;
    isc_entropy_t  *ent;
    isc_uint32_t    total;      /* entropy from this source */
    ISC_LINK(isc_entropysource_t)   link;
    char        name[32];
    union {
        isc_entropysamplesource_t   sample;
        isc_entropyfilesource_t     file;
        isc_cbsource_t  callback;
    } sources;
};

#define ENTROPY_SOURCETYPE_SAMPLE   1   /* Type is a sample source */
#define ENTROPY_SOURCETYPE_FILE     2   /* Type is a file source */
#define ENTROPY_SOURCETYPE_CALLBACK 3   /* Type is a callback source */

/*
 * The random pool "taps"
 */
#define TAP1    99
#define TAP2    59
#define TAP3    31
#define TAP4     9
#define TAP5     7

static inline void
entropypool_add_word(isc_entropypool_t *, isc_uint32_t);

static void
fillpool(isc_entropy_t *, unsigned int, isc_boolean_t);

static int
wait_for_sources(isc_entropy_t *);

static unsigned int
crunchsamples(isc_entropy_t *, sample_queue_t *sq);

static inline void
reseed(isc_entropy_t *ent);

static void
samplequeue_release(isc_entropy_t *ent, sample_queue_t *sq) {
    REQUIRE(sq->samples != NULL);
    REQUIRE(sq->extra != NULL);

    isc_mem_put(ent->mctx, sq->samples, RND_EVENTQSIZE * 4);
    isc_mem_put(ent->mctx, sq->extra, RND_EVENTQSIZE * 4);
    sq->samples = NULL;
    sq->extra = NULL;
}

static isc_result_t
samplesource_allocate(isc_entropy_t *ent, sample_queue_t *sq) {
    sq->samples = isc_mem_get(ent->mctx, RND_EVENTQSIZE * 4);
    if (sq->samples == NULL)
        return (ISC_R_NOMEMORY);

    sq->extra = isc_mem_get(ent->mctx, RND_EVENTQSIZE * 4);
    if (sq->extra == NULL) {
        isc_mem_put(ent->mctx, sq->samples, RND_EVENTQSIZE * 4);
        sq->samples = NULL;
        return (ISC_R_NOMEMORY);
    }

    sq->nsamples = 0;

    return (ISC_R_SUCCESS);
}

/*
 * Add in entropy, even when the value we're adding in could be
 * very large.
 */
static inline void
add_entropy(isc_entropy_t *ent, isc_uint32_t entropy) {
    /* clamp input.  Yes, this must be done. */
    entropy = ISC_MIN(entropy, RND_POOLBITS);
    /* Add in the entropy we already have. */
    entropy += ent->pool.entropy;
    /* Clamp. */
    ent->pool.entropy = ISC_MIN(entropy, RND_POOLBITS);
}

/*
 * Decrement the amount of entropy the pool has.
 */
static inline void
subtract_entropy(isc_entropy_t *ent, isc_uint32_t entropy) {
    entropy = ISC_MIN(entropy, ent->pool.entropy);
    ent->pool.entropy -= entropy;
}

/*
 * Add in entropy, even when the value we're adding in could be
 * very large.
 */
static inline void
add_pseudo(isc_entropy_t *ent, isc_uint32_t pseudo) {
    /* clamp input.  Yes, this must be done. */
    pseudo = ISC_MIN(pseudo, RND_POOLBITS * 8);
    /* Add in the pseudo we already have. */
    pseudo += ent->pool.pseudo;
    /* Clamp. */
    ent->pool.pseudo = ISC_MIN(pseudo, RND_POOLBITS * 8);
}

/*
 * Decrement the amount of pseudo the pool has.
 */
static inline void
subtract_pseudo(isc_entropy_t *ent, isc_uint32_t pseudo) {
    pseudo = ISC_MIN(pseudo, ent->pool.pseudo);
    ent->pool.pseudo -= pseudo;
}

/*
 * Add one word to the pool, rotating the input as needed.
 */
static inline void
entropypool_add_word(isc_entropypool_t *rp, isc_uint32_t val) {
    /*
     * Steal some values out of the pool, and xor them into the
     * word we were given.
     *
     * Mix the new value into the pool using xor.  This will
     * prevent the actual values from being known to the caller
     * since the previous values are assumed to be unknown as well.
     */
    val ^= rp->pool[(rp->cursor + TAP1) & (RND_POOLWORDS - 1)];
    val ^= rp->pool[(rp->cursor + TAP2) & (RND_POOLWORDS - 1)];
    val ^= rp->pool[(rp->cursor + TAP3) & (RND_POOLWORDS - 1)];
    val ^= rp->pool[(rp->cursor + TAP4) & (RND_POOLWORDS - 1)];
    val ^= rp->pool[(rp->cursor + TAP5) & (RND_POOLWORDS - 1)];
    rp->pool[rp->cursor++] ^=
      ((val << rp->rotate) | (val >> (32 - rp->rotate)));

    /*
     * If we have looped around the pool, increment the rotate
     * variable so the next value will get xored in rotated to
     * a different position.
     * Increment by a value that is relativly prime to the word size
     * to try to spread the bits throughout the pool quickly when the
     * pool is empty.
     */
    if (rp->cursor == RND_POOLWORDS) {
        rp->cursor = 0;
        rp->rotate = (rp->rotate + 7) & 31;
    }
}

/*
 * Add a buffer's worth of data to the pool.
 *
 * Requires that the lock is held on the entropy pool.
 */
static void
entropypool_adddata(isc_entropy_t *ent, void *p, unsigned int len,
            isc_uint32_t entropy)
{
    isc_uint32_t val;
    isc_uint32_t addr;
    isc_uint8_t *buf;

    addr = (isc_uint32_t)p;
    buf = p;

    if ((addr & 0x03) != 0) {
        val = 0;
        switch (len) {
        case 3:
            val = *buf++;
            len--;
        case 2:
            val = val << 8 | *buf++;
            len--;
        case 1:
            val = val << 8 | *buf++;
            len--;
        }

        entropypool_add_word(&ent->pool, val);
    }

    for (; len > 3 ; len -= 4) {
        val = *((isc_uint32_t *)buf);

        entropypool_add_word(&ent->pool, val);
        buf += 4;
    }

    if (len != 0) {
        val = 0;
        switch (len) {
        case 3:
            val = *buf++;
        case 2:
            val = val << 8 | *buf++;
        case 1:
            val = val << 8 | *buf++;
        }

        entropypool_add_word(&ent->pool, val);
    }

    add_entropy(ent, entropy);
    subtract_pseudo(ent, entropy);
}

static inline void
reseed(isc_entropy_t *ent) {
    isc_result_t result;
    isc_time_t t;
    pid_t pid;

    if (ent->initcount == 0) {
        pid = getpid();
        entropypool_adddata(ent, &pid, sizeof pid, 0);
        pid = getppid();
        entropypool_adddata(ent, &pid, sizeof pid, 0);
    }

    /*
     * After we've reseeded 100 times, only add new timing info every
     * 50 requests.  This will keep us from using lots and lots of
     * CPU just to return bad pseudorandom data anyway.
     */
    if (ent->initcount > 100)
        if ((ent->initcount % 50) != 0)
            return;

    result = isc_time_now(&t);
    if (result == ISC_R_SUCCESS) {
        entropypool_adddata(ent, &t, sizeof t, 0);
        ent->initcount++;
    }
}

static unsigned int
get_from_filesource(isc_entropysource_t *source, isc_uint32_t desired) {
    isc_entropy_t *ent = source->ent;
    unsigned char buf[128];
    int fd = source->sources.file.fd;
    ssize_t n, ndesired;
    unsigned int added;

    if (fd == -1)
        return (0);

    desired = desired / 8 + (((desired & 0x07) > 0) ? 1 : 0);

    added = 0;
    while (desired > 0) {
        ndesired = ISC_MIN(desired, sizeof(buf));
        n = read(fd, buf, ndesired);
        if (n < 0) {
            if (errno == EAGAIN)
                goto out;
            close(fd);
            source->sources.file.fd = -1;
            goto out;
        }
        if (n == 0) {
            close(fd);
            source->sources.file.fd = -1;
            goto out;
        }

        entropypool_adddata(ent, buf, n, n * 8);
        added += n * 8;
        desired -= n;
    }

 out:
    return (added);
}

static unsigned int
get_from_callback(isc_entropysource_t *source, unsigned int desired,
          isc_boolean_t blocking)
{
    isc_entropy_t *ent = source->ent;
    isc_cbsource_t *cbs = &source->sources.callback;
    unsigned int added;
    unsigned int got;
    isc_result_t result;

    if (desired == 0)
        return (0);

    if (!cbs->start_called && cbs->startfunc != NULL) {
        result = cbs->startfunc(source, cbs->arg, blocking);
        if (result != ISC_R_SUCCESS)
            return (0);
        cbs->start_called = ISC_TRUE;
    }

    added = 0;
    result = ISC_R_SUCCESS;
    while (desired > 0 && result == ISC_R_SUCCESS) {
        result = cbs->getfunc(source, cbs->arg, blocking);
        if (result == ISC_R_QUEUEFULL) {
            got = crunchsamples(ent, &cbs->samplequeue);
            added += got;
            desired -= ISC_MIN(got, desired);
            result = ISC_R_SUCCESS;
        }
    }

    return (added);
}

/*
 * Poll each source, trying to get data from it to stuff into the entropy
 * pool.
 */
static void
fillpool(isc_entropy_t *ent, unsigned int desired, isc_boolean_t blocking) {
    unsigned int added;
    unsigned int remaining;
    unsigned int needed;
    unsigned int nsource;
    isc_entropysource_t *source;

    REQUIRE(VALID_ENTROPY(ent));

    needed = desired;

    /*
     * This logic is a little strange, so an explanation is in order.
     *
     * If needed is 0, it means we are being asked to "fill to whatever
     * we think is best."  This means that if we have at least a
     * partially full pool (say, > 1/4th of the pool) we probably don't
     * need to add anything.
     *
     * Also, we will check to see if the "pseudo" count is too high.
     * If it is, try to mix in better data.  Too high is currently
     * defined as 1/4th of the pool.
     *
     * Next, if we are asked to add a specific bit of entropy, make
     * certain that we will do so.  Clamp how much we try to add to
     * (DIGEST_SIZE * 8 < needed < POOLBITS - entropy).
     *
     * Note that if we are in a blocking mode, we will only try to
     * get as much data as we need, not as much as we might want
     * to build up.
     */
    if (needed == 0) {
        REQUIRE(!blocking);

        if ((ent->pool.entropy >= RND_POOLBITS / 4)
            && (ent->pool.pseudo <= RND_POOLBITS / 4))
            return;

        needed = THRESHOLD_BITS * 4;
    } else {
        needed = ISC_MAX(needed, THRESHOLD_BITS);
        needed = ISC_MIN(needed, RND_POOLBITS);
    }

    /*
     * In any case, clamp how much we need to how much we can add.
     */
    needed = ISC_MIN(needed, RND_POOLBITS - ent->pool.entropy);

    /*
     * But wait!  If we're not yet initialized, we need at least
     *  THRESHOLD_BITS
     * of randomness.
     */
    if (ent->initialized < THRESHOLD_BITS)
        needed = ISC_MAX(needed, THRESHOLD_BITS - ent->initialized);

    /*
     * Poll each file source to see if we can read anything useful from
     * it.  XXXMLG When where are multiple sources, we should keep a
     * record of which one we last used so we can start from it (or the
     * next one) to avoid letting some sources build up entropy while
     * others are always drained.
     */

    added = 0;
    remaining = needed;
    if (ent->nextsource == NULL) {
        ent->nextsource = ISC_LIST_HEAD(ent->sources);
        if (ent->nextsource == NULL)
            return;
    }
    source = ent->nextsource;
 again_file:
    for (nsource = 0 ; nsource < ent->nsources ; nsource++) {
        unsigned int got;

        if (remaining == 0)
            break;

        got = 0;

        if (source->type == ENTROPY_SOURCETYPE_FILE)
            got = get_from_filesource(source, remaining);

        added += got;

        remaining -= ISC_MIN(remaining, got);

        source = ISC_LIST_NEXT(source, link);
        if (source == NULL)
            source = ISC_LIST_HEAD(ent->sources);

#if 0
        if (added >= needed)
            break;
#endif
    }
    ent->nextsource = source;

    if (blocking && remaining != 0) {
        int fds;
        fds = wait_for_sources(ent);
        if (fds > 0)
            goto again_file;
    }

    /*
     * Here, if there are bits remaining to be had and we can block,
     * check to see if we have a callback source.  If so, call them.
     */
    source = ISC_LIST_HEAD(ent->sources);
    while ((remaining != 0) && (source != NULL)) {
        unsigned int got;

        got = 0;

        if (source->type == ENTROPY_SOURCETYPE_CALLBACK)
            got = get_from_callback(source, remaining, blocking);

        added += got;
        remaining -= ISC_MIN(remaining, got);

        if (added >= needed)
            break;

        source = ISC_LIST_NEXT(source, link);
    }

    /*
     * Mark as initialized if we've added enough data.
     */
    if (ent->initialized < THRESHOLD_BITS)
        ent->initialized += added;
}

static int
wait_for_sources(isc_entropy_t *ent) {
    isc_entropysource_t *source;
    int maxfd, fd;
    int cc;
    fd_set reads;

    maxfd = -1;
    FD_ZERO(&reads);

    source = ISC_LIST_HEAD(ent->sources);
    while (source != NULL) {
        if (source->type == ENTROPY_SOURCETYPE_FILE) {
            fd = source->sources.file.fd;
            if (fd >= 0) {
                maxfd = ISC_MAX(maxfd, fd);
                FD_SET(fd, &reads);
            }
        }
        source = ISC_LIST_NEXT(source, link);
    }

    if (maxfd < 0)
        return (-1);

    cc = select(maxfd + 1, &reads, NULL, NULL, NULL);
    if (cc < 0)
        return (-1);

    return (cc);
}

/*
 * Extract some number of bytes from the random pool, decreasing the
 * estimate of randomness as each byte is extracted.
 *
 * Do this by stiring the pool and returning a part of hash as randomness.
 * Note that no secrets are given away here since parts of the hash are
 * xored together before returned.
 *
 * Honor the request from the caller to only return good data, any data,
 * etc.
 */
isc_result_t
isc_entropy_getdata(isc_entropy_t *ent, void *data, unsigned int length,
            unsigned int *returned, unsigned int flags)
{
    unsigned int i;
    isc_sha1_t hash;
    unsigned char digest[ISC_SHA1_DIGESTLENGTH];
    isc_uint32_t remain, deltae, count, total;
    isc_uint8_t *buf;
    isc_boolean_t goodonly, partial, blocking;

    REQUIRE(VALID_ENTROPY(ent));
    REQUIRE(data != NULL);
    REQUIRE(length > 0);

    goodonly = ISC_TF((flags & ISC_ENTROPY_GOODONLY) != 0);
    partial = ISC_TF((flags & ISC_ENTROPY_PARTIAL) != 0);
    blocking = ISC_TF((flags & ISC_ENTROPY_BLOCKING) != 0);

    LOCK(&ent->lock);

    remain = length;
    buf = data;
    total = 0;
    while (remain != 0) {
        count = ISC_MIN(remain, RND_ENTROPY_THRESHOLD);

        /*
         * If we are extracting good data only, make certain we
         * have enough data in our pool for this pass.  If we don't,
         * get some, and fail if we can't, and partial returns
         * are not ok.
         */
        if (goodonly) {
            unsigned int fillcount;

            fillcount = ISC_MAX(remain * 8, count * 8);

            /*
             * If, however, we have at least THRESHOLD_BITS
             * of entropy in the pool, don't block here.  It is
             * better to drain the pool once in a while and
             * then refill it than it is to constantly keep the
             * pool full.
             */
            if (ent->pool.entropy >= THRESHOLD_BITS)
                fillpool(ent, fillcount, ISC_FALSE);
            else
                fillpool(ent, fillcount, blocking);

            if (ent->pool.entropy < THRESHOLD_BITS) {
                if (!blocking && !partial)
                    goto zeroize;
                else if (partial)
                    goto partial_output;
            }
        } else {
            /*
             * If we've extracted half our pool size in bits
             * since the last refresh, try to refresh here.
             */
            if (ent->initialized < THRESHOLD_BITS)
                fillpool(ent, THRESHOLD_BITS, blocking);
            else
                fillpool(ent, 0, ISC_FALSE);

            /*
             * If we've not initialized with enough good random
             * data, seed with our crappy code.
             */
            if (ent->initialized < THRESHOLD_BITS)
                reseed(ent);
        }

        isc_sha1_init(&hash);
        isc_sha1_update(&hash, (void *)(ent->pool.pool),
                RND_POOLWORDS * 4);
        isc_sha1_final(&hash, digest);

        /*
         * Stir the extracted data (all of it) back into the pool.
         */
        entropypool_adddata(ent, digest, ISC_SHA1_DIGESTLENGTH, 0);

        for (i = 0; i < count; i++)
            buf[i] = digest[i] ^ digest[i + RND_ENTROPY_THRESHOLD];

        buf += count;
        remain -= count;

        deltae = count * 8;
        deltae = ISC_MIN(deltae, ent->pool.entropy);
        total += deltae;
        subtract_entropy(ent, deltae);
        add_pseudo(ent, count * 8);
    }

 partial_output:

    memset(digest, 0, sizeof(digest));

    if (returned != NULL)
        *returned = (length - remain);

    UNLOCK(&ent->lock);

    return (ISC_R_SUCCESS);

 zeroize:
    /* put the entropy we almost extracted back */
    add_entropy(ent, total);
    memset(data, 0, length);
    memset(digest, 0, sizeof(digest));
    if (returned != NULL)
        *returned = 0;

    UNLOCK(&ent->lock);

    return (ISC_R_NOENTROPY);
}

static void
isc_entropypool_init(isc_entropypool_t *pool) {
    pool->cursor = RND_POOLWORDS - 1;
    pool->entropy = 0;
    pool->pseudo = 0;
    pool->rotate = 0;
    memset(pool->pool, 0, RND_POOLWORDS);
}

static void
isc_entropypool_invalidate(isc_entropypool_t *pool) {
    pool->cursor = 0;
    pool->entropy = 0;
    pool->pseudo = 0;
    pool->rotate = 0;
    memset(pool->pool, 0, RND_POOLWORDS);
}

isc_result_t
isc_entropy_create(isc_mem_t *mctx, isc_entropy_t **entp) {
    isc_result_t ret;
    isc_entropy_t *ent;

    REQUIRE(mctx != NULL);
    REQUIRE(entp != NULL && *entp == NULL);

    ent = isc_mem_get(mctx, sizeof(isc_entropy_t));
    if (ent == NULL)
        return (ISC_R_NOMEMORY);

    /*
     * We need a lock.
     */
    if (isc_mutex_init(&ent->lock) != ISC_R_SUCCESS) {
        ret = ISC_R_UNEXPECTED;
        goto errout;
    }

    /*
     * From here down, no failures will/can occur.
     */
    ISC_LIST_INIT(ent->sources);
    ent->nextsource = NULL;
    ent->nsources = 0;
    ent->mctx = NULL;
    isc_mem_attach(mctx, &ent->mctx);
    ent->refcnt = 1;
    ent->initialized = 0;
    ent->initcount = 0;
    ent->magic = ENTROPY_MAGIC;

    isc_entropypool_init(&ent->pool);

    *entp = ent;
    return (ISC_R_SUCCESS);

 errout:
    isc_mem_put(mctx, ent, sizeof(isc_entropy_t));

    return (ret);
}

/*
 * Requires "ent" be locked.
 */
static void
destroysource(isc_entropysource_t **sourcep) {
    isc_entropysource_t *source;
    isc_entropy_t *ent;
    isc_cbsource_t *cbs;
    int fd;

    source = *sourcep;
    *sourcep = NULL;
    ent = source->ent;

    ISC_LIST_UNLINK(ent->sources, source, link);
    ent->nextsource = NULL;
    REQUIRE(ent->nsources > 0);
    ent->nsources--;

    switch (source->type) {
    case ENTROPY_SOURCETYPE_FILE:
        fd = source->sources.file.fd;
        if (fd >= 0)
            close(fd);
        break;
    case ENTROPY_SOURCETYPE_SAMPLE:
        samplequeue_release(ent, &source->sources.sample.samplequeue);
        break;
    case ENTROPY_SOURCETYPE_CALLBACK:
        cbs = &source->sources.callback;
        if (cbs->start_called && cbs->stopfunc != NULL) {
            cbs->stopfunc(source, cbs->arg);
            cbs->start_called = ISC_FALSE;
        }
        samplequeue_release(ent, &cbs->samplequeue);
        break;
    }

    memset(source, 0, sizeof(isc_entropysource_t));

    isc_mem_put(ent->mctx, source, sizeof(isc_entropysource_t));
}

static inline isc_boolean_t
destroy_check(isc_entropy_t *ent) {
    isc_entropysource_t *source;

    if (ent->refcnt > 0)
        return (ISC_FALSE);

    source = ISC_LIST_HEAD(ent->sources);
    while (source != NULL) {
        switch (source->type) {
        case ENTROPY_SOURCETYPE_FILE:
            break;
        default:
            return (ISC_FALSE);
        }
        source = ISC_LIST_NEXT(source, link);
    }

    return (ISC_TRUE);
}

static void
destroy(isc_entropy_t **entp) {
    isc_entropy_t *ent;
    isc_entropysource_t *source;
    isc_mem_t *mctx;

    REQUIRE(entp != NULL && *entp != NULL);
    ent = *entp;
    *entp = NULL;

    LOCK(&ent->lock);

    REQUIRE(ent->refcnt == 0);

    /*
     * Here, detach non-sample sources.
     */
    source = ISC_LIST_HEAD(ent->sources);
    while (source != NULL) {
        switch(source->type) {
        case ENTROPY_SOURCETYPE_FILE:
            destroysource(&source);
            break;
        }
        source = ISC_LIST_HEAD(ent->sources);
    }

    /*
     * If there are other types of sources, we've found a bug.
     */
    REQUIRE(ISC_LIST_EMPTY(ent->sources));

    mctx = ent->mctx;

    isc_entropypool_invalidate(&ent->pool);

    UNLOCK(&ent->lock);

    isc_mutex_destroy(&ent->lock);

    memset(ent, 0, sizeof(isc_entropy_t));
    isc_mem_put(mctx, ent, sizeof(isc_entropy_t));
    isc_mem_detach(&mctx);
}

/*
 * Make a fd non-blocking
 */
static isc_result_t
make_nonblock(int fd) {
    int ret;
    int flags;

    flags = fcntl(fd, F_GETFL, 0);
    flags |= O_NONBLOCK;
    ret = fcntl(fd, F_SETFL, flags);

    if (ret == -1) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "fcntl(%d, F_SETFL, %d): %s",
                 fd, flags, strerror(errno));

        return (ISC_R_UNEXPECTED);
    }

    return (ISC_R_SUCCESS);
}

isc_result_t
isc_entropy_createfilesource(isc_entropy_t *ent, const char *fname) {
    int fd;
    isc_result_t ret;
    isc_entropysource_t *source;

    REQUIRE(VALID_ENTROPY(ent));
    REQUIRE(fname != NULL);

    LOCK(&ent->lock);

    source = NULL;
    fd = -1;

    fd = open(fname, O_RDONLY | O_NONBLOCK, 0);
    if (fd < 0) {
        ret = ISC_R_IOERROR;
        goto errout;
    }

    ret = make_nonblock(fd);
    if (ret != ISC_R_SUCCESS)
        goto errout;

    source = isc_mem_get(ent->mctx, sizeof(isc_entropysource_t));
    if (source == NULL) {
        ret = ISC_R_NOMEMORY;
        goto errout;
    }

    /*
     * From here down, no failures can occur.
     */
    source->magic = SOURCE_MAGIC;
    source->type = ENTROPY_SOURCETYPE_FILE;
    source->ent = ent;
    source->total = 0;
    memset(source->name, 0, sizeof(source->name));
    ISC_LINK_INIT(source, link);
    source->sources.file.fd = fd;

    /*
     * Hook it into the entropy system.
     */
    ISC_LIST_APPEND(ent->sources, source, link);
    ent->nsources++;

    UNLOCK(&ent->lock);
    return (ISC_R_SUCCESS);

 errout:
    if (source != NULL)
        isc_mem_put(ent->mctx, source, sizeof(isc_entropysource_t));
    if (fd >= 0)
        close(fd);

    UNLOCK(&ent->lock);

    return (ret);
}

void
isc_entropy_destroysource(isc_entropysource_t **sourcep) {
    isc_entropysource_t *source;
    isc_entropy_t *ent;
    isc_boolean_t killit;

    REQUIRE(sourcep != NULL);
    REQUIRE(VALID_SOURCE(*sourcep));

    source = *sourcep;
    *sourcep = NULL;

    ent = source->ent;
    REQUIRE(VALID_ENTROPY(ent));

    LOCK(&ent->lock);

    destroysource(&source);

    killit = destroy_check(ent);

    UNLOCK(&ent->lock);

    if (killit)
        destroy(&ent);
}

isc_result_t
isc_entropy_createcallbacksource(isc_entropy_t *ent,
                 isc_entropystart_t start,
                 isc_entropyget_t get,
                 isc_entropystop_t stop,
                 void *arg,
                 isc_entropysource_t **sourcep)
{
    isc_result_t ret;
    isc_entropysource_t *source;
    isc_cbsource_t *cbs;

    REQUIRE(VALID_ENTROPY(ent));
    REQUIRE(get != NULL);
    REQUIRE(sourcep != NULL && *sourcep == NULL);

    LOCK(&ent->lock);

    source = isc_mem_get(ent->mctx, sizeof(isc_entropysource_t));
    if (source == NULL) {
        ret = ISC_R_NOMEMORY;
        goto errout;
    }

    cbs = &source->sources.callback;

    ret = samplesource_allocate(ent, &cbs->samplequeue);
    if (ret != ISC_R_SUCCESS)
        goto errout;

    cbs->start_called = ISC_FALSE;
    cbs->startfunc = start;
    cbs->getfunc = get;
    cbs->stopfunc = stop;
    cbs->arg = arg;

    /*
     * From here down, no failures can occur.
     */
    source->magic = SOURCE_MAGIC;
    source->type = ENTROPY_SOURCETYPE_CALLBACK;
    source->ent = ent;
    source->total = 0;
    memset(source->name, 0, sizeof(source->name));
    ISC_LINK_INIT(source, link);

    /*
     * Hook it into the entropy system.
     */
    ISC_LIST_APPEND(ent->sources, source, link);
    ent->nsources++;

    *sourcep = source;

    UNLOCK(&ent->lock);
    return (ISC_R_SUCCESS);

 errout:
    if (source != NULL)
        isc_mem_put(ent->mctx, source, sizeof(isc_entropysource_t));

    UNLOCK(&ent->lock);

    return (ret);
}

void
isc_entropy_stopcallbacksources(isc_entropy_t *ent) {
    isc_entropysource_t *source;
    isc_cbsource_t *cbs;

    REQUIRE(VALID_ENTROPY(ent));

    LOCK(&ent->lock);

    source = ISC_LIST_HEAD(ent->sources);
    while (source != NULL) {
        if (source->type == ENTROPY_SOURCETYPE_CALLBACK) {
            cbs = &source->sources.callback;
            if (cbs->start_called && cbs->stopfunc != NULL) {
                cbs->stopfunc(source, cbs->arg);
                cbs->start_called = ISC_FALSE;
            }
        }

        source = ISC_LIST_NEXT(source, link);
    }

    UNLOCK(&ent->lock);
}

isc_result_t
isc_entropy_createsamplesource(isc_entropy_t *ent,
                   isc_entropysource_t **sourcep)
{
    isc_result_t ret;
    isc_entropysource_t *source;
    sample_queue_t *sq;

    REQUIRE(VALID_ENTROPY(ent));
    REQUIRE(sourcep != NULL && *sourcep == NULL);

    LOCK(&ent->lock);

    source = isc_mem_get(ent->mctx, sizeof(isc_entropysource_t));
    if (source == NULL) {
        ret = ISC_R_NOMEMORY;
        goto errout;
    }

    sq = &source->sources.sample.samplequeue;
    ret = samplesource_allocate(ent, sq);
    if (ret != ISC_R_SUCCESS)
        goto errout;

    /*
     * From here down, no failures can occur.
     */
    source->magic = SOURCE_MAGIC;
    source->type = ENTROPY_SOURCETYPE_SAMPLE;
    source->ent = ent;
    source->total = 0;
    memset(source->name, 0, sizeof(source->name));
    ISC_LINK_INIT(source, link);

    /*
     * Hook it into the entropy system.
     */
    ISC_LIST_APPEND(ent->sources, source, link);
    ent->nsources++;

    *sourcep = source;

    UNLOCK(&ent->lock);
    return (ISC_R_SUCCESS);

 errout:
    if (source != NULL)
        isc_mem_put(ent->mctx, source, sizeof(isc_entropysource_t));

    UNLOCK(&ent->lock);

    return (ret);
}

static inline unsigned int
estimate_entropy(sample_queue_t *sq, isc_uint32_t t)
{
    isc_int32_t     delta;
    isc_int32_t     delta2;
    isc_int32_t     delta3;

    /*
     * If the time counter has overflowed, calculate the real difference.
     * If it has not, it is simplier.
     */
    if (t < sq->last_time)
        delta = UINT_MAX - sq->last_time + t;
    else
        delta = sq->last_time - t;

    if (delta < 0)
        delta = -delta;

    /*
     * Calculate the second and third order differentials
     */
    delta2 = sq->last_delta - delta;
    if (delta2 < 0)
        delta2 = -delta2;

    delta3 = sq->last_delta2 - delta2;
    if (delta3 < 0)
        delta3 = -delta3;

    sq->last_time = t;
    sq->last_delta = delta;
    sq->last_delta2 = delta2;

    /*
     * If any delta is 0, we got no entropy.  If all are non-zero, we
     * might have something.
     */
    if (delta == 0 || delta2 == 0 || delta3 == 0)
        return 0;

    /*
     * We could find the smallest delta and claim we got log2(delta)
     * bits, but for now return that we found 1 bit.
     */
    return 1;
}

static unsigned int
crunchsamples(isc_entropy_t *ent, sample_queue_t *sq) {
    unsigned int ns;
    unsigned int added;

    if (sq->nsamples < 6)
        return (0);

    added = 0;
    sq->last_time = sq->samples[0];
    sq->last_delta = 0;
    sq->last_delta2 = 0;

    /*
     * Prime the values by adding in the first 4 samples in.  This
     * should completely initialize the delta calculations.
     */
    for (ns = 0 ; ns < 4 ; ns++)
        (void)estimate_entropy(sq, sq->samples[ns]);

    for (ns = 4 ; ns < sq->nsamples ; ns++)
        added += estimate_entropy(sq, sq->samples[ns]);

    entropypool_adddata(ent, sq->samples, sq->nsamples * 4, added);
    entropypool_adddata(ent, sq->extra, sq->nsamples * 4, 0);

    /*
     * Move the last 4 samples into the first 4 positions, and start
     * adding new samples from that point.
     */
    for (ns = 0 ; ns < 4 ; ns++) {
        sq->samples[ns] = sq->samples[sq->nsamples - 4 + ns];
        sq->extra[ns] = sq->extra[sq->nsamples - 4 + ns];
    }

    sq->nsamples = 0;

    return (added);
}

/*
 * Add a sample, and return ISC_R_SUCCESS if the queue has become full,
 * ISC_R_NOENTROPY if it has space remaining, and ISC_R_NOMORE if the
 * queue was full when this function was called.
 */
static isc_result_t
addsample(sample_queue_t *sq, isc_uint32_t sample, isc_uint32_t extra) {
    if (sq->nsamples >= RND_EVENTQSIZE)
        return (ISC_R_NOMORE);

    sq->samples[sq->nsamples] = sample;
    sq->extra[sq->nsamples] = extra;
    sq->nsamples++;

    if (sq->nsamples >= RND_EVENTQSIZE)
        return (ISC_R_QUEUEFULL);

    return (ISC_R_SUCCESS);
}

isc_result_t
isc_entropy_addsample(isc_entropysource_t *source, isc_uint32_t sample,
              isc_uint32_t extra)
{
    isc_entropy_t *ent;
    sample_queue_t *sq;
    unsigned int entropy;
    isc_result_t result;

    REQUIRE(VALID_SOURCE(source));

    ent = source->ent;

    LOCK(&ent->lock);

    sq = &source->sources.sample.samplequeue;
    result = addsample(sq, sample, extra);
    if (result == ISC_R_QUEUEFULL) {
        entropy = crunchsamples(ent, sq);
        add_entropy(ent, entropy);
    }

    UNLOCK(&ent->lock);

    return (result);
}

isc_result_t
isc_entropy_addcallbacksample(isc_entropysource_t *source, isc_uint32_t sample,
                  isc_uint32_t extra)
{
    isc_entropy_t *ent;
    sample_queue_t *sq;
    isc_result_t result;

    REQUIRE(VALID_SOURCE(source));
    REQUIRE(source->type == ENTROPY_SOURCETYPE_CALLBACK);

    ent = source->ent;

    sq = &source->sources.callback.samplequeue;
    result = addsample(sq, sample, extra);

    return (result);
}

void
isc_entropy_putdata(isc_entropy_t *ent, void *data, unsigned int length,
            isc_uint32_t entropy)
{
    REQUIRE(VALID_ENTROPY(ent));

    LOCK(&ent->lock);

    entropypool_adddata(ent, data, length, entropy);

    if (ent->initialized < THRESHOLD_BITS)
        ent->initialized = THRESHOLD_BITS;

    UNLOCK(&ent->lock);
}

static void
dumpstats(isc_entropy_t *ent, FILE *out) {
    fprintf(out,
        "Entropy pool %p:  refcnt %u"
        " cursor %u, rotate %u entropy %u pseudo %u nsources %u"
        " nextsource %p initialized %u initcount %u\n",
        ent, ent->refcnt,
        ent->pool.cursor, ent->pool.rotate,
        ent->pool.entropy, ent->pool.pseudo,
        ent->nsources, ent->nextsource, ent->initialized,
        ent->initcount);
}

/*
 * This function ignores locking.  Use at your own risk.
 */
void
isc_entropy_stats(isc_entropy_t *ent, FILE *out) {
    REQUIRE(VALID_ENTROPY(ent));

    LOCK(&ent->lock);
    dumpstats(ent, out);
    UNLOCK(&ent->lock);
}

void
isc_entropy_attach(isc_entropy_t *ent, isc_entropy_t **entp) {
    REQUIRE(VALID_ENTROPY(ent));
    REQUIRE(entp != NULL && *entp == NULL);

    LOCK(&ent->lock);

    ent->refcnt++;
    *entp = ent;

    UNLOCK(&ent->lock);
}

void
isc_entropy_detach(isc_entropy_t **entp) {
    isc_entropy_t *ent;
    isc_boolean_t killit;

    REQUIRE(entp != NULL && VALID_ENTROPY(*entp));
    ent = *entp;
    *entp = NULL;

    LOCK(&ent->lock);

    REQUIRE(ent->refcnt > 0);
    ent->refcnt--;

    killit = destroy_check(ent);

    UNLOCK(&ent->lock);

    if (killit)
        destroy(&ent);
}