Re: [PATCH v3 3/3] blk-crypto: show crypto capabilities in sysfs

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Dec 09, 2021 at 02:51:59PM -0800, Bart Van Assche wrote:
> On 12/7/21 5:35 PM, Eric Biggers wrote:
> > +What:		/sys/block/<disk>/queue/crypto/modes/<mode>
> > +Date:		December 2021
> > +Contact:	linux-block@vger.kernel.org
> > +Description:
> > +		[RO] For each crypto mode (i.e., encryption/decryption
> > +		algorithm) the device supports with inline encryption, a file
> > +		will exist at this location.  It will contain a hexadecimal
> > +		number that is a bitmask of the supported data unit sizes, in
> > +		bytes, for that crypto mode.
> > +
> > +		Currently, the crypto modes that may be supported are:
> > +
> > +		   * AES-256-XTS
> > +		   * AES-128-CBC-ESSIV
> > +		   * Adiantum
> > +
> > +		For example, if a device supports AES-256-XTS inline encryption
> > +		with data unit sizes of 512 and 4096 bytes, the file
> > +		/sys/block/<disk>/queue/crypto/modes/AES-256-XTS will exist and
> > +		will contain "0x1200".
> 
> So a bitmask is used to combine multiple values into a single number?

You could think of it that way, yes.

> Has it been considered to report each value separately, e.g. 512\n4096\n
> instead of 0x1200\n?  I think the former approach is more friendly for shell
> scripts.

I don't think that would be acceptable to the sysfs folks, as they only allow
one value per file.  I suppose a bitmask could be viewed as unacceptable too,
but it seemed to make sense here, given that the data unit sizes are always
powers of 2, and the hardware reports them as bitmasks.

Greg already reviewed this patch, but maybe he wasn't looking at this part.

Greg, any opinion on the best way to report a set of power-of-2 values via
sysfs?

> 
> > +struct blk_crypto_attr {
> > +	struct attribute attr;
> > +	ssize_t (*show)(struct blk_crypto_profile *profile,
> > +			struct blk_crypto_attr *attr, char *page);
> > +};
> 
> It is not clear to me why this structure has been introduced instead of using the
> existing kobj_attribute structure?

kobj_attribute isn't strongly-typed to the specific type of kobject.  It also
includes a store function pointer, which is not necessary here.

What I'm doing here is very common; take a look at some other code that
implements sysfs attributes.  Even block/blk-sysfs.c.

> > +static int __init blk_crypto_sysfs_init(void)
> > +{
> > +	int i;
> > +
> > +	BUILD_BUG_ON(BLK_ENCRYPTION_MODE_INVALID != 0);
> > +	for (i = 1; i < BLK_ENCRYPTION_MODE_MAX; i++) {
> > +		struct blk_crypto_attr *attr = &__blk_crypto_mode_attrs[i];
> > +
> > +		attr->attr.name = blk_crypto_modes[i].name;
> > +		attr->attr.mode = 0444;
> > +		attr->show = blk_crypto_mode_show;
> > +		blk_crypto_mode_attrs[i - 1] = &attr->attr;
> > +	}
> > +	return 0;
> > +}
> > +subsys_initcall(blk_crypto_sysfs_init);
> 
> Would it be possible to remove the use of subsys_initcall() if the crypto attributes and
> blk_crypto_modes[] would be defined in the same file?

I want to make it so that all crypto modes show up in sysfs without having to
write extra code every time a new crypto mode is added.  That's not possible if
the attributes are defined statically.  Defining them in the same array would
come close, since then it would be hard for people to forgot to update one place
and not the other.  But that would mix together the sysfs support and the core
blk-crypto support, which seems undesirable.

- Eric