Re: [Qemu-devel] [RFC V8 06/13] quorum: Add quorum mechanism.

[Kevin Wolf]

Am 28.01.2013 18:07, schrieb Benoît Canet:
> Use gnutls's SHA-256 to compare versions.
> 
> Signed-off-by: Benoit Canet <address@hidden>
> ---
>  block/quorum.c |  303 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  configure      |   22 ++++
>  2 files changed, 324 insertions(+), 1 deletion(-)
> 
> diff --git a/block/quorum.c b/block/quorum.c
> index e3c6aad..4c552e4 100644
> --- a/block/quorum.c
> +++ b/block/quorum.c
> @@ -13,8 +13,30 @@
>   * See the COPYING file in the top-level directory.
>   */
>  
> +#include <gnutls/gnutls.h>
> +#include <gnutls/crypto.h>
>  #include "block/block_int.h"
>  
> +#define HASH_LENGTH 32
> +
> +typedef union QuorumVoteValue {
> +    char h[HASH_LENGTH];       /* SHA-256 hash */
> +    unsigned long l;  /* simpler hash */
> +} QuorumVoteValue;
> +
> +typedef struct QuorumVoteItem {
> +    int index;
> +    QLIST_ENTRY(QuorumVoteItem) next;
> +} QuorumVoteItem;
> +
> +typedef struct QuorumVoteVersion {
> +    QuorumVoteValue value;
> +    int index;
> +    int vote_count;
> +    QLIST_HEAD(, QuorumVoteItem) items;
> +    QLIST_ENTRY(QuorumVoteVersion) next;
> +} QuorumVoteVersion;

I wonder if it wouldn't become simpler if you used arrays instead of
lists. We know that s->total is the upper limit for entries.

> +
>  typedef struct {
>      BlockDriverState **bs;
>      unsigned long long threshold;
> @@ -32,6 +54,11 @@ typedef struct QuorumSingleAIOCB {
>      QuorumAIOCB *parent;
>  } QuorumSingleAIOCB;
>  
> +typedef struct QuorumVotes {
> +    QLIST_HEAD(, QuorumVoteVersion) vote_list;
> +    int (*compare)(QuorumVoteValue *a, QuorumVoteValue *b);
> +} QuorumVotes;

Can this be directly embedded into QuorumAIOCB?

compare is always quorum_sha256_compare, so why even have a field? We
can still introduce it once we add different options.

> +
>  struct QuorumAIOCB {
>      BlockDriverAIOCB common;
>      BDRVQuorumState *bqs;
> @@ -48,6 +75,8 @@ struct QuorumAIOCB {
>      int success_count;          /* number of successfully completed AIOCB */
>      bool *finished;             /* completion signal for cancel */
>  
> +    QuorumVotes votes;
> +
>      void (*vote)(QuorumAIOCB *acb);
>      int vote_ret;
>  };
> @@ -84,6 +113,11 @@ static void quorum_aio_bh(void *opaque)
>      }
>  
>      qemu_bh_delete(acb->bh);
> +
> +    if (acb->vote_ret) {
> +        ret = acb->vote_ret;
> +    }
> +
>      acb->common.cb(acb->common.opaque, ret);
>      if (acb->finished) {
>          *acb->finished = true;
> @@ -95,6 +129,11 @@ static void quorum_aio_bh(void *opaque)
>      qemu_aio_release(acb);
>  }
>  
> +static int quorum_sha256_compare(QuorumVoteValue *a, QuorumVoteValue *b)
> +{
> +    return memcmp(a, b, HASH_LENGTH);
> +}

Comparing a.h and b.h would be cleaner.

> +
>  static QuorumAIOCB *quorum_aio_get(BDRVQuorumState *s,
>                                     BlockDriverState *bs,
>                                     QEMUIOVector *qiov,
> @@ -118,6 +157,8 @@ static QuorumAIOCB *quorum_aio_get(BDRVQuorumState *s,
>      acb->vote = NULL;
>      acb->vote_ret = 0;
>      acb->finished = NULL;
> +    acb->votes.compare = quorum_sha256_compare;
> +    QLIST_INIT(&acb->votes.vote_list);
>  
>      for (i = 0; i < s->total; i++) {
>          acb->aios[i].buf = NULL;
> @@ -145,10 +186,268 @@ static void quorum_aio_cb(void *opaque, int ret)
>          return;
>      }
>  
> +    /* Do the vote */
> +    if (acb->vote) {
> +        acb->vote(acb);
> +    }

This is NULL for all writes and quorum_vote for all reads. Is there any
chance that more options will be introduced? If not, why not have a bool
is_read and directly call the function here?

> +
>      acb->bh = qemu_bh_new(quorum_aio_bh, acb);
>      qemu_bh_schedule(acb->bh);
>  }
>  
> +static void quorum_print_bad(QuorumAIOCB *acb, const char *filename)
> +{
> +    fprintf(stderr, "quorum: corrected error in quorum file %s: sector_num=%"
> +            PRId64 " nb_sectors=%i\n", filename, acb->sector_num,
> +            acb->nb_sectors);
> +}
> +
> +static void quorum_print_failure(QuorumAIOCB *acb)
> +{
> +    fprintf(stderr, "quorum: failure sector_num=%" PRId64 " nb_sectors=%i\n",
> +            acb->sector_num, acb->nb_sectors);
> +}
> +
> +static void quorum_print_bad_versions(QuorumAIOCB *acb,
> +                                      QuorumVoteValue *value)
> +{
> +    QuorumVoteVersion *version;
> +    QuorumVoteItem *item;
> +    BDRVQuorumState *s = acb->bqs;
> +
> +    QLIST_FOREACH(version, &acb->votes.vote_list, next) {
> +        if (!acb->votes.compare(&version->value, value)) {
> +            continue;
> +        }
> +        QLIST_FOREACH(item, &version->items, next) {
> +            quorum_print_bad(acb, s->filenames[item->index]);
> +        }
> +    }
> +}
> +
> +static void quorum_copy_qiov(QEMUIOVector *dest, QEMUIOVector *source)
> +{
> +    int i;
> +    assert(dest->niov == source->niov);
> +    assert(dest->size == source->size);
> +    for (i = 0; i < source->niov; i++) {
> +        assert(dest->iov[i].iov_len == source->iov[i].iov_len);
> +        memcpy(dest->iov[i].iov_base,
> +               source->iov[i].iov_base,
> +               source->iov[i].iov_len);
> +    }
> +}
> +
> +static void quorum_count_vote(QuorumVotes *votes,
> +                              QuorumVoteValue *value,
> +                              int index)
> +{
> +    QuorumVoteVersion *v = NULL, *version = NULL;
> +    QuorumVoteItem *item;
> +
> +    /* look if we have something with this hash */
> +    QLIST_FOREACH(v, &votes->vote_list, next) {
> +        if (!votes->compare(&v->value, value)) {
> +            version = v;
> +            break;
> +        }
> +    }
> +
> +    /* It's a version not yet in the list add it */
> +    if (!version) {
> +        version = g_new0(QuorumVoteVersion, 1);
> +        QLIST_INIT(&version->items);
> +        memcpy(&version->value, value, sizeof(version->value));
> +        version->index = index;
> +        version->vote_count = 0;
> +        QLIST_INSERT_HEAD(&votes->vote_list, version, next);
> +    }
> +
> +    version->vote_count++;
> +
> +    item = g_new0(QuorumVoteItem, 1);
> +    item->index = index;
> +    QLIST_INSERT_HEAD(&version->items, item, next);
> +}
> +
> +static void quorum_free_vote_list(QuorumVotes *votes)
> +{
> +    QuorumVoteVersion *version, *next_version;
> +    QuorumVoteItem *item, *next_item;
> +
> +    QLIST_FOREACH_SAFE(version, &votes->vote_list, next, next_version) {
> +        QLIST_REMOVE(version, next);
> +        QLIST_FOREACH_SAFE(item, &version->items, next, next_item) {
> +            QLIST_REMOVE(item, next);
> +            g_free(item);
> +        }
> +        g_free(version);
> +    }
> +}
> +
> +static int quorum_compute_hash(QuorumAIOCB *acb, int i, QuorumVoteValue 
> *hash)
> +{
> +    int j, ret;
> +    gnutls_hash_hd_t dig;
> +    QEMUIOVector *qiov = &acb->aios[i].qiov;
> +
> +    ret = gnutls_hash_init(&dig, GNUTLS_DIG_SHA256);
> +
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    for (j = 0; j < qiov->niov; j++) {
> +        ret = gnutls_hash(dig, qiov->iov[j].iov_base, qiov->iov[j].iov_len);
> +        if (ret < 0) {
> +            return ret;

I have no clue about this lib, but sure that you don't need to call deinit?

> +        }
> +    }
> +
> +    gnutls_hash_deinit(dig, (void *) hash);
> +
> +    return 0;
> +}
> +
> +static QuorumVoteVersion *quorum_get_vote_winner(QuorumVotes *votes)
> +{
> +    int i = 0;
> +    QuorumVoteVersion *candidate, *winner = NULL;
> +
> +    QLIST_FOREACH(candidate, &votes->vote_list, next) {
> +        if (candidate->vote_count > i) {
> +            i = candidate->vote_count;
> +            winner = candidate;
> +        }
> +    }
> +
> +    return winner;
> +}
> +
> +static bool quorum_iovec_compare(QEMUIOVector *a, QEMUIOVector *b)
> +{
> +    int i;
> +    int result;
> +
> +    assert(a->niov == b->niov);
> +    for (i = 0; i < a->niov; i++) {
> +        assert(a->iov[i].iov_len == b->iov[i].iov_len);
> +        result = memcmp(a->iov[i].iov_base,
> +                        b->iov[i].iov_base,
> +                        a->iov[i].iov_len);
> +        if (result) {
> +            return false;
> +        }
> +    }
> +
> +    return true;
> +}

qemu_iovec_compare() seems to do exactly the same, except that this
doesn't return the offset of the first difference.

Why is it a good idea to duplicate the code?

> +
> +static void GCC_FMT_ATTR(2, 3) quorum_err(QuorumAIOCB *acb,
> +                                          const char *fmt, ...)
> +{
> +    va_list ap;
> +
> +    va_start(ap, fmt);
> +    fprintf(stderr, "quorum: sector_num=%" PRId64 " nb_sectors=%d ",
> +            acb->sector_num, acb->nb_sectors);
> +    vfprintf(stderr, fmt, ap);
> +    fprintf(stderr, "\n");
> +    va_end(ap);
> +    exit(1);
> +}
> +
> +static bool quorum_compare(QuorumAIOCB *acb,
> +                           QEMUIOVector *a,
> +                           QEMUIOVector *b)
> +{
> +    BDRVQuorumState *s = acb->bqs;
> +    bool blkverify = false;
> +    ssize_t offset;
> +
> +    if (s->total == 2 && s->threshold == 2) {
> +        blkverify = true;
> +    }
> +
> +    if (blkverify) {
> +        offset = qemu_iovec_compare(a, b);
> +        if (offset != -1) {
> +            quorum_err(acb, "contents mismatch in sector %" PRId64,
> +                       acb->sector_num +
> +                       (uint64_t)(offset / BDRV_SECTOR_SIZE));
> +        }
> +        return true;
> +    }

wtf is happening here? Why an uncommented special case for two images
that results in an entirely different behaviour? Why split the if block
and introduce a useless blkverify variable?

> +
> +    return quorum_iovec_compare(a, b);
> +}
> +
> +
> +static void quorum_vote(QuorumAIOCB *acb)
> +{
> +    bool quorum = true;
> +    int i, j, ret;
> +    QuorumVoteValue hash;
> +    BDRVQuorumState *s = acb->bqs;
> +    QuorumVoteVersion *winner;
> +
> +    /* get the index of the first successful read */
> +    for (i = 0; i < s->total; i++) {
> +        if (!acb->aios[i].ret) {
> +            break;
> +        }
> +    }
> +
> +    /* compare this read with all other successful read looking for quorum */
> +    for (j = i + 1; j < s->total; j++) {
> +        if (acb->aios[j].ret) {
> +            continue;
> +        }
> +        quorum = quorum_compare(acb, &acb->aios[i].qiov, &acb->aios[j].qiov);
> +        if (!quorum) {
> +            break;
> +       }
> +    }
> +
> +    /* Every successful read agrees -> Quorum */
> +    if (quorum) {
> +        quorum_copy_qiov(acb->qiov, &acb->aios[i].qiov);
> +        return;
> +    }
> +
> +    /* compute hashs for each successful read, also store indexes */
> +    for (i = 0; i < s->total; i++) {
> +        if (acb->aios[i].ret) {
> +            continue;
> +        }
> +        ret = quorum_compute_hash(acb, i, &hash);
> +        assert(ret == 0);

So you're claiming that this function cannot possibly fail? Why does it
even have a return code then?

> +        quorum_count_vote(&acb->votes, &hash, i);
> +    }
> +
> +    /* vote to select the most represented version */
> +    winner = quorum_get_vote_winner(&acb->votes);
> +    assert(winner != NULL);

Won't this assertion fail if all requests returned an error?

> +
> +    /* if the winner count is smaller than threshold read fail */

s/than/then/

> +    if (winner->vote_count < s->threshold) {
> +        quorum_print_failure(acb);
> +        acb->vote_ret = -EIO;
> +        fprintf(stderr, "quorum: vote result inferior to threshold\n");

Block drivers usually shouldn't print anything to stderr or stdout.

> +        goto free_exit;
> +    }
> +
> +    /* we have a winner: copy it */
> +    quorum_copy_qiov(acb->qiov, &acb->aios[winner->index].qiov);
> +
> +    /* some versions are bad print them */
> +    quorum_print_bad_versions(acb, &winner->value);

Same here. Is this driver meant to be used in production or only for
debugging? Maybe it should have a debug mode that must explicitly be
enabled and messages are only printed in this mode?

> +
> +free_exit:
> +    /* free lists */
> +    quorum_free_vote_list(&acb->votes);
> +}
> +
>  static BlockDriverAIOCB *quorum_aio_readv(BlockDriverState *bs,
>                                           int64_t sector_num,
>                                           QEMUIOVector *qiov,
> @@ -161,6 +460,8 @@ static BlockDriverAIOCB 
> *quorum_aio_readv(BlockDriverState *bs,
>                                        nb_sectors, cb, opaque);
>      int i;
>  
> +    acb->vote = quorum_vote;
> +
>      for (i = 0; i < s->total; i++) {
>          acb->aios[i].buf = qemu_blockalign(bs->file, qiov->size);
>          qemu_iovec_init(&acb->aios[i].qiov, qiov->niov);
> @@ -168,7 +469,7 @@ static BlockDriverAIOCB 
> *quorum_aio_readv(BlockDriverState *bs,
>      }
>  
>      for (i = 0; i < s->total; i++) {
> -        bdrv_aio_readv(s->bs[i], sector_num, qiov, nb_sectors,
> +        bdrv_aio_readv(s->bs[i], sector_num, &acb->aios[i].qiov, nb_sectors,
>                         quorum_aio_cb, &acb->aios[i]);
>      }
>  
> diff --git a/configure b/configure
> index 4ebb60d..0832d26 100755
> --- a/configure
> +++ b/configure
> @@ -1733,6 +1733,28 @@ EOF
>  fi
>  
>  ##########################################
> +# Quorum gnutls detection
> +cat > $TMPC <<EOF
> +#include <gnutls/gnutls.h>
> +#include <gnutls/crypto.h>
> +int main(void) {char data[4096], digest[32];
> +gnutls_hash_fast(GNUTLS_DIG_SHA256, data, 4096, digest);
> +return 0;
> +}
> +EOF
> +qcow_tls_cflags=`$pkg_config --cflags gnutls 2> /dev/null`

Wait, what has qcow to do with it?

> +qcow_tls_libs=`$pkg_config --libs gnutls 2> /dev/null`
> +if compile_prog "$qcow_tls_cflags" "$qcow_tls_libs" ; then
> +  qcow_tls=yes
> +  libs_softmmu="$qcow_tls_libs $libs_softmmu"
> +  libs_tools="$qcow_tls_libs $libs_softmmu"
> +  QEMU_CFLAGS="$QEMU_CFLAGS $qcow_tls_cflags"
> +else
> +  echo "gnutls > 2.10.0 required to compile QEMU"
> +  exit 1
> +fi

Should this really be a hard dependency? Why not just disable quorum if
the lib isn't there?

Kevin