[PATCH 04/10] nvme: add nvme_auth_derive_tls_psk()

[Hannes Reinecke]

On 1/28/25 09:58, Christoph Hellwig wrote:
>> +	info_len = strlen(psk_digest) + strlen(psk_prefix) + 5;
> 
> Where does the magic 5 come from?
> 
Two bytes for the HASH ID, two bytes for the length field from
HKDF-Expand-Label, and one byte for the space separator.

>>
>> +	info = kzalloc(info_len, GFP_KERNEL);
>> +	if (!info) {
>> +		ret = -ENOMEM;
>> +		goto out_free_prk;
>> +	}
>> +
>> +	put_unaligned_be16(psk_len, info);
>> +	memcpy(info + 2, psk_prefix, strlen(psk_prefix));
>> +	sprintf(info + 2 + strlen(psk_prefix), "%02d %s", hmac_id, psk_digest);
> 
> And this feels like we'd want a basic struct for this, even if it
> just is the len field and a flexible array.  Which will also help the
> compiler to to do bounds checking using __counted_by.
> 
It would, if the were counting the full 'info' string.
But the 'length' field is just the length of the string 'tls13 ' + 
'nvme-tls-psk' (cf the definition of HKDF-Expand-Label in RFC 8446).
So having a _counted_by thingie would only count the first part
of the full 'info' string.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare at suse.de                                +49 911 74053 688
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