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Re: [Qemu-devel] [PATCH v2 2/2] qcow2: Fix fail path in realloc_refcount
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH v2 2/2] qcow2: Fix fail path in realloc_refcount_block() |
Date: |
Mon, 17 Mar 2014 23:18:49 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0 |
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 <address@hidden>
> Suggested-by: Laszlo Ersek <address@hidden>
> ---
> 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 <address@hidden>
Thanks!
Laszlo