[PATCH] rbd: have rbd_obj_method_sync() return transfer count

[PATCH] rbd: have rbd_obj_method_sync() return transfer count

[Alex Elder]

Callers of rbd_obj_method_sync() don't know how many bytes of data
got returned by the class method call.  As a result, they have been
assuming enough got returned to decode whatever was expected.

This isn't safe.  We know how many bytes got transferred, so have
rbd_obj_method_sync() return that amount (rather than just 0) if
the call is successful.

Change all callers to use this return value to ensure decoding of
the results is done safely.

On the other hand, most callers of rbd_obj_method_sync() only
indicate success or failure, so all of *their* callers can simply
test for non-zero result.

This resolves:
    http://tracker.ceph.com/issues/4773

Signed-off-by: Alex Elder <elder@inktank.com>
---
 drivers/block/rbd.c |   60
++++++++++++++++++++++++++++-----------------------
 1 file changed, 33 insertions(+), 27 deletions(-)

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index 46d9bf7..3013cdb0 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -2642,7 +2642,7 @@ static int rbd_obj_method_sync(struct rbd_device
*rbd_dev,
 	 * method.  Currently if this is present it will be a
 	 * snapshot id.
 	 */
-	page_count = (u32) calc_pages_for(0, inbound_size);
+	page_count = (u32)calc_pages_for(0, inbound_size);
 	pages = ceph_alloc_page_vector(page_count, GFP_KERNEL);
 	if (IS_ERR(pages))
 		return PTR_ERR(pages);
@@ -2689,7 +2689,9 @@ static int rbd_obj_method_sync(struct rbd_device
*rbd_dev,
 	ret = obj_request->result;
 	if (ret < 0)
 		goto out;
-	ret = 0;
+
+	rbd_assert(obj_request->xferred < (u64)INT_MAX);
+	ret = (int)obj_request->xferred;
 	ceph_copy_from_page_vector(pages, inbound, 0, obj_request->xferred);
 	if (version)
 		*version = obj_request->version;
@@ -3582,13 +3584,15 @@ static int _rbd_dev_v2_snap_size(struct
rbd_device *rbd_dev, u64 snap_id,
 	dout("%s: rbd_obj_method_sync returned %d\n", __func__, ret);
 	if (ret < 0)
 		return ret;
+	if (ret < sizeof (size_buf))
+		return -ERANGE;

 	*order = size_buf.order;
 	*snap_size = le64_to_cpu(size_buf.size);

 	dout("  snap_id 0x%016llx order = %u, snap_size = %llu\n",
-		(unsigned long long) snap_id, (unsigned int) *order,
-		(unsigned long long) *snap_size);
+		(unsigned long long)snap_id, (unsigned int)*order,
+		(unsigned long long)*snap_size);

 	return 0;
 }
@@ -3619,8 +3623,8 @@ static int rbd_dev_v2_object_prefix(struct
rbd_device *rbd_dev)

 	p = reply_buf;
 	rbd_dev->header.object_prefix = ceph_extract_encoded_string(&p,
-						p + RBD_OBJ_PREFIX_LEN_MAX,
-						NULL, GFP_NOIO);
+						p + ret, NULL, GFP_NOIO);
+	ret = 0;

 	if (IS_ERR(rbd_dev->header.object_prefix)) {
 		ret = PTR_ERR(rbd_dev->header.object_prefix);
@@ -3628,7 +3632,6 @@ static int rbd_dev_v2_object_prefix(struct
rbd_device *rbd_dev)
 	} else {
 		dout("  object_prefix = %s\n", rbd_dev->header.object_prefix);
 	}
-
 out:
 	kfree(reply_buf);

@@ -3653,6 +3656,8 @@ static int _rbd_dev_v2_snap_features(struct
rbd_device *rbd_dev, u64 snap_id,
 	dout("%s: rbd_obj_method_sync returned %d\n", __func__, ret);
 	if (ret < 0)
 		return ret;
+	if (ret < sizeof (features_buf))
+		return -ERANGE;

 	incompat = le64_to_cpu(features_buf.incompat);
 	if (incompat & ~RBD_FEATURES_SUPPORTED)
@@ -3661,9 +3666,9 @@ static int _rbd_dev_v2_snap_features(struct
rbd_device *rbd_dev, u64 snap_id,
 	*snap_features = le64_to_cpu(features_buf.features);

 	dout("  snap_id 0x%016llx features = 0x%016llx incompat = 0x%016llx\n",
-		(unsigned long long) snap_id,
-		(unsigned long long) *snap_features,
-		(unsigned long long) le64_to_cpu(features_buf.incompat));
+		(unsigned long long)snap_id,
+		(unsigned long long)*snap_features,
+		(unsigned long long)le64_to_cpu(features_buf.incompat));

 	return 0;
 }
@@ -3709,9 +3714,9 @@ static int rbd_dev_v2_parent_info(struct
rbd_device *rbd_dev)
 	if (ret < 0)
 		goto out_err;

-	ret = -ERANGE;
 	p = reply_buf;
 	end = reply_buf + size;
+	ret = -ERANGE;
 	ceph_decode_64_safe(&p, end, parent_spec->pool_id, out_err);
 	if (parent_spec->pool_id == CEPH_NOPOOL)
 		goto out;	/* No parent?  No problem. */
@@ -3719,8 +3724,8 @@ static int rbd_dev_v2_parent_info(struct
rbd_device *rbd_dev)
 	/* The ceph file layout needs to fit pool id in 32 bits */

 	ret = -EIO;
-	if (WARN_ON(parent_spec->pool_id > (u64) U32_MAX))
-		goto out;
+	if (WARN_ON(parent_spec->pool_id > (u64)U32_MAX))
+		goto out_err;

 	image_id = ceph_extract_encoded_string(&p, end, NULL, GFP_KERNEL);
 	if (IS_ERR(image_id)) {
@@ -3765,7 +3770,7 @@ static char *rbd_dev_image_name(struct rbd_device
*rbd_dev)

 	p = image_id;
 	end = image_id + image_id_size;
-	ceph_encode_string(&p, end, rbd_dev->spec->image_id, (u32) len);
+	ceph_encode_string(&p, end, rbd_dev->spec->image_id, (u32)len);

 	size = sizeof (__le32) + RBD_IMAGE_NAME_LEN_MAX;
 	reply_buf = kmalloc(size, GFP_KERNEL);
@@ -3885,9 +3890,9 @@ static int rbd_dev_v2_snap_context(struct
rbd_device *rbd_dev, u64 *ver)
 	if (ret < 0)
 		goto out;

-	ret = -ERANGE;
 	p = reply_buf;
-	end = reply_buf + size;
+	end = reply_buf + ret;
+	ret = -ERANGE;
 	ceph_decode_64_safe(&p, end, seq, out);
 	ceph_decode_32_safe(&p, end, snap_count, out);

@@ -3912,6 +3917,7 @@ static int rbd_dev_v2_snap_context(struct
rbd_device *rbd_dev, u64 *ver)
 		ret = -ENOMEM;
 		goto out;
 	}
+	ret = 0;

 	atomic_set(&snapc->nref, 1);
 	snapc->seq = seq;
@@ -3922,12 +3928,11 @@ static int rbd_dev_v2_snap_context(struct
rbd_device *rbd_dev, u64 *ver)
 	rbd_dev->header.snapc = snapc;

 	dout("  snap context seq = %llu, snap_count = %u\n",
-		(unsigned long long) seq, (unsigned int) snap_count);
-
+		(unsigned long long)seq, (unsigned int)snap_count);
 out:
 	kfree(reply_buf);

-	return 0;
+	return ret;
 }

 static char *rbd_dev_v2_snap_name(struct rbd_device *rbd_dev, u32 which)
@@ -3962,7 +3967,7 @@ static char *rbd_dev_v2_snap_name(struct
rbd_device *rbd_dev, u32 which)
 		goto out;
 	} else {
 		dout("  snap_id 0x%016llx snap_name = %s\n",
-			(unsigned long long) le64_to_cpu(snap_id), snap_name);
+			(unsigned long long)le64_to_cpu(snap_id), snap_name);
 	}
 	kfree(reply_buf);

@@ -4559,8 +4564,10 @@ static int rbd_dev_image_id(struct rbd_device
*rbd_dev)

 	p = response;
 	rbd_dev->spec->image_id = ceph_extract_encoded_string(&p,
-						p + RBD_IMAGE_ID_LEN_MAX,
+						p + ret,
 						NULL, GFP_NOIO);
+	ret = 0;
+
 	if (IS_ERR(rbd_dev->spec->image_id)) {
 		ret = PTR_ERR(rbd_dev->spec->image_id);
 		rbd_dev->spec->image_id = NULL;
@@ -4605,7 +4612,7 @@ static int rbd_dev_v1_probe(struct rbd_device
*rbd_dev)
 	/* Version 1 images have no parent (no layering) */

 	rbd_dev->parent_spec = NULL;
-	rbd_dev->parent_overlap = 0;
+	rbd_dev->parent_overlap =40;

 	rbd_dev->image_format = 1;

@@ -4641,28 +4648,27 @@ static int rbd_dev_v2_probe(struct rbd_device
*rbd_dev)
 			RBD_HEADER_PREFIX, rbd_dev->spec->image_id);

 	/* Get the size and object order for the image */
-
 	ret = rbd_dev_v2_image_size(rbd_dev);
-	if (ret < 0)
+	if (ret)
 		goto out_err;

 	/* Get the object prefix (a.k.a. block_name) for the image */

 	ret = rbd_dev_v2_object_prefix(rbd_dev);
-	if (ret < 0)
+	if (ret)
 		goto out_err;

 	/* Get the and check features for the image */

 	ret = rbd_dev_v2_features(rbd_dev);
-	if (ret < 0)
+	if (ret)
 		goto out_err;

 	/* If the image supports layering, get the parent info */

 	if (rbd_dev->header.features & RBD_FEATURE_LAYERING) {
 		ret = rbd_dev_v2_parent_info(rbd_dev);
-		if (ret < 0)
+		if (ret)
 			goto out_err;
 	}

-- 
1.7.9.5

Re: [PATCH] rbd: have rbd_obj_method_sync() return transfer count

[Josh Durgin]

A couple small things below. With those fixed:

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

On 04/21/2013 02:17 PM, Alex Elder wrote:
> Callers of rbd_obj_method_sync() don't know how many bytes of data
> got returned by the class method call.  As a result, they have been
> assuming enough got returned to decode whatever was expected.
>
> This isn't safe.  We know how many bytes got transferred, so have
> rbd_obj_method_sync() return that amount (rather than just 0) if
> the call is successful.
>
> Change all callers to use this return value to ensure decoding of
> the results is done safely.
>
> On the other hand, most callers of rbd_obj_method_sync() only
> indicate success or failure, so all of *their* callers can simply
> test for non-zero result.
>
> This resolves:
>      http://tracker.ceph.com/issues/4773
>
> Signed-off-by: Alex Elder <elder@inktank.com>
> ---
>   drivers/block/rbd.c |   60
> ++++++++++++++++++++++++++++-----------------------
>   1 file changed, 33 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
> index 46d9bf7..3013cdb0 100644
> --- a/drivers/block/rbd.c
> +++ b/drivers/block/rbd.c
> @@ -2642,7 +2642,7 @@ static int rbd_obj_method_sync(struct rbd_device
> *rbd_dev,
>   	 * method.  Currently if this is present it will be a
>   	 * snapshot id.
>   	 */
> -	page_count = (u32) calc_pages_for(0, inbound_size);
> +	page_count = (u32)calc_pages_for(0, inbound_size);
>   	pages = ceph_alloc_page_vector(page_count, GFP_KERNEL);
>   	if (IS_ERR(pages))
>   		return PTR_ERR(pages);
> @@ -2689,7 +2689,9 @@ static int rbd_obj_method_sync(struct rbd_device
> *rbd_dev,
>   	ret = obj_request->result;
>   	if (ret < 0)
>   		goto out;
> -	ret = 0;
> +
> +	rbd_assert(obj_request->xferred < (u64)INT_MAX);
> +	ret = (int)obj_request->xferred;
>   	ceph_copy_from_page_vector(pages, inbound, 0, obj_request->xferred);
>   	if (version)
>   		*version = obj_request->version;
> @@ -3582,13 +3584,15 @@ static int _rbd_dev_v2_snap_size(struct
> rbd_device *rbd_dev, u64 snap_id,
>   	dout("%s: rbd_obj_method_sync returned %d\n", __func__, ret);
>   	if (ret < 0)
>   		return ret;
> +	if (ret < sizeof (size_buf))
> +		return -ERANGE;
>
>   	*order = size_buf.order;
>   	*snap_size = le64_to_cpu(size_buf.size);
>
>   	dout("  snap_id 0x%016llx order = %u, snap_size = %llu\n",
> -		(unsigned long long) snap_id, (unsigned int) *order,
> -		(unsigned long long) *snap_size);
> +		(unsigned long long)snap_id, (unsigned int)*order,
> +		(unsigned long long)*snap_size);
>
>   	return 0;
>   }
> @@ -3619,8 +3623,8 @@ static int rbd_dev_v2_object_prefix(struct
> rbd_device *rbd_dev)
>
>   	p = reply_buf;
>   	rbd_dev->header.object_prefix = ceph_extract_encoded_string(&p,
> -						p + RBD_OBJ_PREFIX_LEN_MAX,
> -						NULL, GFP_NOIO);
> +						p + ret, NULL, GFP_NOIO);
> +	ret = 0;
>
>   	if (IS_ERR(rbd_dev->header.object_prefix)) {
>   		ret = PTR_ERR(rbd_dev->header.object_prefix);
> @@ -3628,7 +3632,6 @@ static int rbd_dev_v2_object_prefix(struct
> rbd_device *rbd_dev)
>   	} else {
>   		dout("  object_prefix = %s\n", rbd_dev->header.object_prefix);
>   	}
> -
>   out:
>   	kfree(reply_buf);
>
> @@ -3653,6 +3656,8 @@ static int _rbd_dev_v2_snap_features(struct
> rbd_device *rbd_dev, u64 snap_id,
>   	dout("%s: rbd_obj_method_sync returned %d\n", __func__, ret);
>   	if (ret < 0)
>   		return ret;
> +	if (ret < sizeof (features_buf))
> +		return -ERANGE;
>
>   	incompat = le64_to_cpu(features_buf.incompat);
>   	if (incompat & ~RBD_FEATURES_SUPPORTED)
> @@ -3661,9 +3666,9 @@ static int _rbd_dev_v2_snap_features(struct
> rbd_device *rbd_dev, u64 snap_id,
>   	*snap_features = le64_to_cpu(features_buf.features);
>
>   	dout("  snap_id 0x%016llx features = 0x%016llx incompat = 0x%016llx\n",
> -		(unsigned long long) snap_id,
> -		(unsigned long long) *snap_features,
> -		(unsigned long long) le64_to_cpu(features_buf.incompat));
> +		(unsigned long long)snap_id,
> +		(unsigned long long)*snap_features,
> +		(unsigned long long)le64_to_cpu(features_buf.incompat));
>
>   	return 0;
>   }
> @@ -3709,9 +3714,9 @@ static int rbd_dev_v2_parent_info(struct
> rbd_device *rbd_dev)
>   	if (ret < 0)
>   		goto out_err;
>
> -	ret = -ERANGE;
>   	p = reply_buf;
>   	end = reply_buf + size;
> +	ret = -ERANGE;

maybe check for the full parent_spec here? even if there's no parent,
a complete parent_spec should be returned.

>   	ceph_decode_64_safe(&p, end, parent_spec->pool_id, out_err);
>   	if (parent_spec->pool_id == CEPH_NOPOOL)
>   		goto out;	/* No parent?  No problem. */
> @@ -3719,8 +3724,8 @@ static int rbd_dev_v2_parent_info(struct
> rbd_device *rbd_dev)
>   	/* The ceph file layout needs to fit pool id in 32 bits */
>
>   	ret = -EIO;
> -	if (WARN_ON(parent_spec->pool_id > (u64) U32_MAX))
> -		goto out;
> +	if (WARN_ON(parent_spec->pool_id > (u64)U32_MAX))
> +		goto out_err;
>
>   	image_id = ceph_extract_encoded_string(&p, end, NULL, GFP_KERNEL);
>   	if (IS_ERR(image_id)) {
> @@ -3765,7 +3770,7 @@ static char *rbd_dev_image_name(struct rbd_device
> *rbd_dev)
>
>   	p = image_id;
>   	end = image_id + image_id_size;
> -	ceph_encode_string(&p, end, rbd_dev->spec->image_id, (u32) len);
> +	ceph_encode_string(&p, end, rbd_dev->spec->image_id, (u32)len);
>
>   	size = sizeof (__le32) + RBD_IMAGE_NAME_LEN_MAX;
>   	reply_buf = kmalloc(size, GFP_KERNEL);
> @@ -3885,9 +3890,9 @@ static int rbd_dev_v2_snap_context(struct
> rbd_device *rbd_dev, u64 *ver)
>   	if (ret < 0)
>   		goto out;
>
> -	ret = -ERANGE;
>   	p = reply_buf;
> -	end = reply_buf + size;
> +	end = reply_buf + ret;
> +	ret = -ERANGE;
>   	ceph_decode_64_safe(&p, end, seq, out);
>   	ceph_decode_32_safe(&p, end, snap_count, out);
>
> @@ -3912,6 +3917,7 @@ static int rbd_dev_v2_snap_context(struct
> rbd_device *rbd_dev, u64 *ver)
>   		ret = -ENOMEM;
>   		goto out;
>   	}
> +	ret = 0;
>
>   	atomic_set(&snapc->nref, 1);
>   	snapc->seq = seq;
> @@ -3922,12 +3928,11 @@ static int rbd_dev_v2_snap_context(struct
> rbd_device *rbd_dev, u64 *ver)
>   	rbd_dev->header.snapc = snapc;
>
>   	dout("  snap context seq = %llu, snap_count = %u\n",
> -		(unsigned long long) seq, (unsigned int) snap_count);
> -
> +		(unsigned long long)seq, (unsigned int)snap_count);
>   out:
>   	kfree(reply_buf);
>
> -	return 0;
> +	return ret;
>   }
>
>   static char *rbd_dev_v2_snap_name(struct rbd_device *rbd_dev, u32 which)
> @@ -3962,7 +3967,7 @@ static char *rbd_dev_v2_snap_name(struct
> rbd_device *rbd_dev, u32 which)
>   		goto out;
>   	} else {
>   		dout("  snap_id 0x%016llx snap_name = %s\n",
> -			(unsigned long long) le64_to_cpu(snap_id), snap_name);
> +			(unsigned long long)le64_to_cpu(snap_id), snap_name);
>   	}
>   	kfree(reply_buf);
>
> @@ -4559,8 +4564,10 @@ static int rbd_dev_image_id(struct rbd_device
> *rbd_dev)
>
>   	p = response;
>   	rbd_dev->spec->image_id = ceph_extract_encoded_string(&p,
> -						p + RBD_IMAGE_ID_LEN_MAX,
> +						p + ret,
>   						NULL, GFP_NOIO);
> +	ret = 0;
> +
>   	if (IS_ERR(rbd_dev->spec->image_id)) {
>   		ret = PTR_ERR(rbd_dev->spec->image_id);
>   		rbd_dev->spec->image_id = NULL;
> @@ -4605,7 +4612,7 @@ static int rbd_dev_v1_probe(struct rbd_device
> *rbd_dev)
>   	/* Version 1 images have no parent (no layering) */
>
>   	rbd_dev->parent_spec = NULL;
> -	rbd_dev->parent_overlap = 0;
> +	rbd_dev->parent_overlap =40;

extraneous 4

>
>   	rbd_dev->image_format = 1;
>
> @@ -4641,28 +4648,27 @@ static int rbd_dev_v2_probe(struct rbd_device
> *rbd_dev)
>   			RBD_HEADER_PREFIX, rbd_dev->spec->image_id);
>
>   	/* Get the size and object order for the image */
> -
>   	ret = rbd_dev_v2_image_size(rbd_dev);
> -	if (ret < 0)
> +	if (ret)
>   		goto out_err;
>
>   	/* Get the object prefix (a.k.a. block_name) for the image */
>
>   	ret = rbd_dev_v2_object_prefix(rbd_dev);
> -	if (ret < 0)
> +	if (ret)
>   		goto out_err;
>
>   	/* Get the and check features for the image */
>
>   	ret = rbd_dev_v2_features(rbd_dev);
> -	if (ret < 0)
> +	if (ret)
>   		goto out_err;
>
>   	/* If the image supports layering, get the parent info */
>
>   	if (rbd_dev->header.features & RBD_FEATURE_LAYERING) {
>   		ret = rbd_dev_v2_parent_info(rbd_dev);
> -		if (ret < 0)
> +		if (ret)
>   			goto out_err;
>   	}
>

Re: [PATCH] rbd: have rbd_obj_method_sync() return transfer count

[Alex Elder]

On 04/22/2013 04:57 PM, Josh Durgin wrote:
> A couple small things below. With those fixed:
> 
> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
> 
> On 04/21/2013 02:17 PM, Alex Elder wrote:
>> Callers of rbd_obj_method_sync() don't know how many bytes of data
>> got returned by the class method call.  As a result, they have been
>> assuming enough got returned to decode whatever was expected.
>>
>> This isn't safe.  We know how many bytes got transferred, so have
>> rbd_obj_method_sync() return that amount (rather than just 0) if
>> the call is successful.

. . .

>> @@ -3709,9 +3714,9 @@ static int rbd_dev_v2_parent_info(struct
>> rbd_device *rbd_dev)
>>       if (ret < 0)
>>           goto out_err;
>>
>> -    ret = -ERANGE;
>>       p = reply_buf;
>>       end = reply_buf + size;

You didn't mention this, but now that I look at this I
managed to screw up the point of this patch in this spot.
The above line should (and will) be:
         end = reply_buf + ret;
(That was why the "ret = -ERANGE" had to be moved to begin
with.)  I did it right in one similar spot elsewhere in
the patch.

>> +    ret = -ERANGE;
> 
> maybe check for the full parent_spec here? even if there's no parent,
> a complete parent_spec should be returned.

I'm not sure I understand this comment.

As it stands, we allocate a local parent_spec structure, and
that gets filled based on what comes back from the "get_parent"
method call.  If anything goes wrong, we discard the parent
spec and return an error.  Only when all goes well do we assign
    rbd_dev->parent_spec = parent_spec;
(and the same goes for the overlap).  ceph_extract_encoded_string()
will return an error if it ends up short, and the other things
that extract values also check for length.

Is that what you were referring to--return either a complete
one or none?  Or was there something else?

>>       ceph_decode_64_safe(&p, end, parent_spec->pool_id, out_err);
>>       if (parent_spec->pool_id == CEPH_NOPOOL)
>>           goto out;    /* No parent?  No problem. */

. . .

>> @@ -4605,7 +4612,7 @@ static int rbd_dev_v1_probe(struct rbd_device
>> *rbd_dev)
>>       /* Version 1 images have no parent (no layering) */
>>
>>       rbd_dev->parent_spec = NULL;
>> -    rbd_dev->parent_overlap = 0;
>> +    rbd_dev->parent_overlap =40;
> 
> extraneous 4

Yes I spotted this and already fixed it in my own branch but
thought it was too small and obvious to warrant a re-post.

>>
>>       rbd_dev->image_format = 1;
. . .

Re: [PATCH] rbd: have rbd_obj_method_sync() return transfer count

[Josh Durgin]

On 04/22/2013 04:39 PM, Alex Elder wrote:
> On 04/22/2013 04:57 PM, Josh Durgin wrote:
>> A couple small things below. With those fixed:
>>
>> Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
>>
>> On 04/21/2013 02:17 PM, Alex Elder wrote:
>>> Callers of rbd_obj_method_sync() don't know how many bytes of data
>>> got returned by the class method call.  As a result, they have been
>>> assuming enough got returned to decode whatever was expected.
>>>
>>> This isn't safe.  We know how many bytes got transferred, so have
>>> rbd_obj_method_sync() return that amount (rather than just 0) if
>>> the call is successful.
>
> . . .
>
>>> @@ -3709,9 +3714,9 @@ static int rbd_dev_v2_parent_info(struct
>>> rbd_device *rbd_dev)
>>>        if (ret < 0)
>>>            goto out_err;
>>>
>>> -    ret = -ERANGE;
>>>        p = reply_buf;
>>>        end = reply_buf + size;
>
> You didn't mention this, but now that I look at this I
> managed to screw up the point of this patch in this spot.
> The above line should (and will) be:
>           end = reply_buf + ret;
> (That was why the "ret = -ERANGE" had to be moved to begin
> with.)  I did it right in one similar spot elsewhere in
> the patch.
>
>>> +    ret = -ERANGE;
>>
>> maybe check for the full parent_spec here? even if there's no parent,
>> a complete parent_spec should be returned.
>
> I'm not sure I understand this comment.
>
> As it stands, we allocate a local parent_spec structure, and
> that gets filled based on what comes back from the "get_parent"
> method call.  If anything goes wrong, we discard the parent
> spec and return an error.  Only when all goes well do we assign
>      rbd_dev->parent_spec = parent_spec;
> (and the same goes for the overlap).  ceph_extract_encoded_string()
> will return an error if it ends up short, and the other things
> that extract values also check for length.

Ok, that makes sense now.

> Is that what you were referring to--return either a complete
> one or none?  Or was there something else?

I meant that the class method should always return a complete 
parent_spec, so the response length should be checked. Letting
ceph_extract_encoded_string() check the correct length like you 
mentioned is fine.