RE: [PATCH 1/4] mdev: Introduce sha1 based mdev alias

[Parav Pandit]

> -----Original Message-----
> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Tuesday, August 27, 2019 7:15 AM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: Jiri Pirko <jiri@xxxxxxxxxxxx>; kwankhede@xxxxxxxxxx;
> cohuck@xxxxxxxxxx; davem@xxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH 1/4] mdev: Introduce sha1 based mdev alias
> 
> On Mon, 26 Aug 2019 15:41:16 -0500
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> 
> > Whenever a parent requests to generate mdev alias, generate a mdev
> > alias.
> > It is an optional attribute that parent can request to generate for
> > each of its child mdev.
> > mdev alias is generated using sha1 from the mdev name.
> >
> > Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
> > ---
> >  drivers/vfio/mdev/mdev_core.c    | 98
> +++++++++++++++++++++++++++++++-
> >  drivers/vfio/mdev/mdev_private.h |  5 +-
> >  drivers/vfio/mdev/mdev_sysfs.c   | 13 +++--
> >  include/linux/mdev.h             |  4 ++
> >  4 files changed, 111 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/vfio/mdev/mdev_core.c
> > b/drivers/vfio/mdev/mdev_core.c index b558d4cfd082..e825ff38b037
> > 100644
> > --- a/drivers/vfio/mdev/mdev_core.c
> > +++ b/drivers/vfio/mdev/mdev_core.c
> > @@ -10,9 +10,11 @@
> >  #include <linux/module.h>
> >  #include <linux/device.h>
> >  #include <linux/slab.h>
> > +#include <linux/mm.h>
> >  #include <linux/uuid.h>
> >  #include <linux/sysfs.h>
> >  #include <linux/mdev.h>
> > +#include <crypto/hash.h>
> >
> >  #include "mdev_private.h"
> >
> > @@ -27,6 +29,8 @@ static struct class_compat *mdev_bus_compat_class;
> > static LIST_HEAD(mdev_list);  static DEFINE_MUTEX(mdev_list_lock);
> >
> > +static struct crypto_shash *alias_hash;
> > +
> >  struct device *mdev_parent_dev(struct mdev_device *mdev)  {
> >  	return mdev->parent->dev;
> > @@ -164,6 +168,18 @@ int mdev_register_device(struct device *dev, const
> struct mdev_parent_ops *ops)
> >  		goto add_dev_err;
> >  	}
> >
> > +	if (ops->get_alias_length) {
> > +		unsigned int digest_size;
> > +		unsigned int aligned_len;
> > +
> > +		aligned_len = roundup(ops->get_alias_length(), 2);
> > +		digest_size = crypto_shash_digestsize(alias_hash);
> > +		if (aligned_len / 2 > digest_size) {
> > +			ret = -EINVAL;
> > +			goto add_dev_err;
> > +		}
> > +	}
> 
> This looks like a sanity check, it could be done outside of the
> parent_list_lock, even before we get a parent device reference.
>
Yes.
 
> I think we're using a callback for get_alias_length() rather than a fixed field
> to support the mtty module option added in patch 4, right?
Right.
I will move the check outside.

> Its utility is rather limited with no args.  I could imagine that if a parent
> wanted to generate an alias that could be incorporated into a string with the
> parent device name that it would be useful to call this with the parent
> device as an arg.  I guess we can save that until a user comes along though.
>
Right. We save until user arrives.
I suggest you review the extra complexity I added here for vendor driven alias length, which I think we should do when an actual user comes along.

 > There doesn't seem to be anything serializing use of alias_hash.
> 
Each sha1 calculation is happening on the new descriptor allocated and initialized using crypto_shash_init().
So it appears to me that each hash calculation can occur in parallel on the individual desc.

> > +
> >  	parent = kzalloc(sizeof(*parent), GFP_KERNEL);
> >  	if (!parent) {
> >  		ret = -ENOMEM;
> > @@ -259,6 +275,7 @@ static void mdev_device_free(struct mdev_device
> *mdev)
> >  	mutex_unlock(&mdev_list_lock);
> >
> >  	dev_dbg(&mdev->dev, "MDEV: destroying\n");
> > +	kvfree(mdev->alias);
> >  	kfree(mdev);
> >  }
> >
> > @@ -269,18 +286,86 @@ static void mdev_device_release(struct device
> *dev)
> >  	mdev_device_free(mdev);
> >  }
> >
> > -int mdev_device_create(struct kobject *kobj,
> > -		       struct device *dev, const guid_t *uuid)
> > +static const char *
> > +generate_alias(const char *uuid, unsigned int max_alias_len) {
> > +	struct shash_desc *hash_desc;
> > +	unsigned int digest_size;
> > +	unsigned char *digest;
> > +	unsigned int alias_len;
> > +	char *alias;
> > +	int ret = 0;
> > +
> > +	/* Align to multiple of 2 as bin2hex will generate
> > +	 * even number of bytes.
> > +	 */
> 
> Comment style for non-networking code please.
Ack.

> 
> > +	alias_len = roundup(max_alias_len, 2);
> > +	alias = kvzalloc(alias_len + 1, GFP_KERNEL);
> 
> The size we're generating here should be small enough to just use kzalloc(),
Ack.

> probably below too.
> 
Descriptor size is 96 bytes long. kvzalloc is more optimal.

> > +	if (!alias)
> > +		return NULL;
> > +
> > +	/* Allocate and init descriptor */
> > +	hash_desc = kvzalloc(sizeof(*hash_desc) +
> > +			     crypto_shash_descsize(alias_hash),
> > +			     GFP_KERNEL);
> > +	if (!hash_desc)
> > +		goto desc_err;
> > +
> > +	hash_desc->tfm = alias_hash;
> > +
> > +	digest_size = crypto_shash_digestsize(alias_hash);
> > +
> > +	digest = kvzalloc(digest_size, GFP_KERNEL);
> > +	if (!digest) {
> > +		ret = -ENOMEM;
> > +		goto digest_err;
> > +	}
> > +	crypto_shash_init(hash_desc);
> > +	crypto_shash_update(hash_desc, uuid, UUID_STRING_LEN);
> > +	crypto_shash_final(hash_desc, digest);
> > +	bin2hex(&alias[0], digest,
> 
> &alias[0], ie. alias
Ack.

> 
> > +		min_t(unsigned int, digest_size, alias_len / 2));
> > +	/* When alias length is odd, zero out and additional last byte
> > +	 * that bin2hex has copied.
> > +	 */
> > +	if (max_alias_len % 2)
> > +		alias[max_alias_len] = 0;
> 
> Doesn't this give us a null terminated string for odd numbers but not even
> numbers?  Probably best to define that we always provide a null terminated
> string then we could do this unconditionally.
> 
> > +
> > +	kvfree(digest);
> > +	kvfree(hash_desc);
> > +	return alias;
> > +
> > +digest_err:
> > +	kvfree(hash_desc);
> > +desc_err:
> > +	kvfree(alias);
> > +	return NULL;
> > +}
> > +
> > +int mdev_device_create(struct kobject *kobj, struct device *dev,
> > +		       const char *uuid_str, const guid_t *uuid)
> >  {
> >  	int ret;
> >  	struct mdev_device *mdev, *tmp;
> >  	struct mdev_parent *parent;
> >  	struct mdev_type *type = to_mdev_type(kobj);
> > +	unsigned int alias_len = 0;
> > +	const char *alias = NULL;
> >
> >  	parent = mdev_get_parent(type->parent);
> >  	if (!parent)
> >  		return -EINVAL;
> >
> > +	if (parent->ops->get_alias_length)
> > +		alias_len = parent->ops->get_alias_length();
> > +	if (alias_len) {
> 
> Why isn't this nested into the branch above?
>
I will nest it. No specific reason to not nest it.
 
> > +		alias = generate_alias(uuid_str, alias_len);
> > +		if (!alias) {
> > +			ret = -ENOMEM;
> 
> Could use an ERR_PTR and propagate an errno.
> 
generate_alias() only returns one error type ENOMEM.
When we add more error types, ERR_PTR is useful.
 
> > +			goto alias_fail;
> > +		}
> > +	}
> > +
> >  	mutex_lock(&mdev_list_lock);
> >
> >  	/* Check for duplicate */
> > @@ -300,6 +385,8 @@ int mdev_device_create(struct kobject *kobj,
> >  	}
> >
> >  	guid_copy(&mdev->uuid, uuid);
> > +	mdev->alias = alias;
> > +	alias = NULL;
> 
> A comment justifying this null'ing might help prevent it getting culled as
> some point.  It appears arbitrary at first look.  Thanks,
>
Ack. I will add it.
 
> Alex