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

["Benoît Canet" <benoit.canet@irqsave.net>]

The Saturday 15 Mar 2014 à 21:55:54 (+0100), 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 checking for new_offset
> being positive instead.
>
> While we're at it, clean up the whole fail path. qcow2_cache_put()
> actually can never fail, hence the return value can safely be ignored

I would say "actually should never fail".

> (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.
>
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> Suggested-by: Laszlo Ersek <lersek@redhat.com>
> ---
>  block/qcow2-refcount.c | 20 +++++++++++---------
>  1 file changed, 11 insertions(+), 9 deletions(-)
>
> diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
> index 6151148..b111319 100644
> --- a/block/qcow2-refcount.c
> +++ b/block/qcow2-refcount.c
> @@ -1440,20 +1440,22 @@ static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
>      }
>
>  fail:

I don't like the label name because its real meaning is exit: it handle both
fail and regular exits.

> -    if (new_offset && (ret < 0)) {
> -        qcow2_free_clusters(bs, new_offset, s->cluster_size,
> -                QCOW2_DISCARD_ALWAYS);
> -    }
>      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) {
> +        if (new_offset > 0) {
> +            qcow2_free_clusters(bs, new_offset, s->cluster_size,
> +                                QCOW2_DISCARD_OTHER);
> +        }
>          return ret;
>      }
> +
>      return new_offset;

The function documentation says:
/*
 * Allocates a new cluster for the given refcount block (represented by its
 * offset in the image file) and copies the current content there. This function
 * does _not_ decrement the reference count for the currently occupied cluster.
 *
 * This function prints an informative message to stderr on error (and returns
 * -errno); on success, 0 is returned.
 */
static int64_t realloc_refcount_block(BlockDriverState *bs, int reftable_index,
                                      uint64_t offset)


So why are we returning new_offset on success ? 

>  }
>
> --
> 1.9.0
>
>