Re: [Qemu-devel] [PATCH v2 2/2] qcow2: Fix fail path in realloc_refcount_block()

[Laszlo Ersek <lersek@redhat.com>]

On 03/17/14 23:04, Max Reitz wrote:
> If qcow2_alloc_clusters() fails, new_offset and ret will both be
> negative after the fail label, thus passing the first if condition and
> subsequently resulting in a call of qcow2_free_clusters() with an
> invalid (negative) offset parameter. Fix this by introducing a new label
> "fail_free_cluster" which is only invoked if new_offset is indeed
> pointing to a newly allocated cluster that should be cleaned up by
> freeing it.
> 
> While we're at it, clean up the whole fail path. qcow2_cache_put()
> should (and actually can) never fail, hence the return value can safely
> be ignored (aside from asserting that it indeed did not fail).
> 
> Furthermore, there is no reason to give QCOW2_DISCARD_ALWAYS to
> qcow2_free_clusters(), a mere QCOW2_DISCARD_OTHER will suffice.
> 
> Ultimately, rename the "fail" label to "done", as it is invoked both on
> failure and success.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> Suggested-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  block/qcow2-refcount.c | 34 ++++++++++++++++++----------------
>  1 file changed, 18 insertions(+), 16 deletions(-)
> 
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 3f2ed08..4a2df5f 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -1399,14 +1399,14 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
>          fprintf(stderr, "Could not allocate new cluster: %s\n",
>                  strerror(-new_offset));
>          ret = new_offset;
> -        goto fail;
> +        goto done;
>      }
>  
>      /* fetch current refcount block content */
>      ret = qcow2_cache_get(bs, s->refcount_block_cache, offset, &refcount_block);
>      if (ret < 0) {
>          fprintf(stderr, "Could not fetch refcount block: %s\n", strerror(-ret));
> -        goto fail;
> +        goto fail_free_cluster;
>      }
>  
>      /* new block has not yet been entered into refcount table, therefore it is
> @@ -1417,8 +1417,7 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
>                  "check failed: %s\n", strerror(-ret));
>          /* the image will be marked corrupt, so don't even attempt on freeing
>           * the cluster */
> -        new_offset = 0;
> -        goto fail;
> +        goto done;
>      }
>  
>      /* write to new block */
> @@ -1426,7 +1425,7 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
>              s->cluster_sectors);
>      if (ret < 0) {
>          fprintf(stderr, "Could not write refcount block: %s\n", strerror(-ret));
> -        goto fail;
> +        goto fail_free_cluster;
>      }
>  
>      /* update refcount table */
> @@ -1436,24 +1435,27 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
>      if (ret < 0) {
>          fprintf(stderr, "Could not update refcount table: %s\n",
>                  strerror(-ret));
> -        goto fail;
> +        goto fail_free_cluster;
>      }
>  
> -fail:
> -    if (new_offset && (ret < 0)) {
> -        qcow2_free_clusters(bs, new_offset, s->cluster_size,
> -                QCOW2_DISCARD_ALWAYS);
> -    }
> +    goto done;
> +
> +fail_free_cluster:
> +    qcow2_free_clusters(bs, new_offset, s->cluster_size, QCOW2_DISCARD_OTHER);
> +
> +done:
>      if (refcount_block) {
> -        if (ret < 0) {
> -            qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
> -        } else {
> -            ret = qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
> -        }
> +        /* This should never fail, as it would only do so if the given refcount
> +         * block cannot be found in the cache. As this is impossible as long as
> +         * there are no bugs, assert the success. */
> +        int tmp = qcow2_cache_put(bs, s->refcount_block_cache, &refcount_block);
> +        assert(tmp == 0);
>      }
> +
>      if (ret < 0) {
>          return ret;
>      }
> +
>      return new_offset;
>  }
>  
> 

I liked the other version better. Especially crossing the trajectories
of the gotos (where "new_offset" used to be zeroed) makes me want to
avert my eyes. However, I've obsessed enough; the code seems correct.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks!
Laszlo