Re: [PATCH v6 1/7] tpm: dynamically allocate the allocated_banks array

From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Roberto Sassu <roberto.sassu@huawei.com>
Cc: zohar@linux.ibm.com, david.safford@ge.com, monty.wiseman@ge.com,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, silviu.vlasceanu@huawei.com
Subject: Re: [PATCH v6 1/7] tpm: dynamically allocate the allocated_banks array
Date: Tue, 4 Dec 2018 15:18:48 -0800	[thread overview]
Message-ID: <20181204231848.GA1233@linux.intel.com> (raw)
In-Reply-To: <20181204082138.24600-2-roberto.sassu@huawei.com>

On Tue, Dec 04, 2018 at 09:21:32AM +0100, Roberto Sassu wrote:
> tpm2_get_pcr_allocation() determines if a PCR bank is allocated by checking
> the mask in the TPML_PCR_SELECTION structure returned by the TPM for
> TPM2_Get_Capability(). One PCR bank with algorithm set to SHA1 is always
> allocated for TPM 1.x.

...

> +		for (j = 0; j < pcr_selection.size_of_select; j++)
> +			if (pcr_selection.pcr_select[j])
> +				break;
> +
> +		if (j < pcr_selection.size_of_select) {
> +			chip->allocated_banks[nr_alloc_banks] = hash_alg;
> +			nr_alloc_banks++;
> +		}
> +

Why was this needed? Can CAP_PCRS return completely unallocated banks?

Kind of out-of-context for the rest of the changes.

Should this be a bug fix of its own because it looks like as this is a
bug fix for existing code, and not a new feature? Just asking because
I don't yet fully understand this change.

Anyway, I believe that you can streamline this by:

/* Check that at least some of the PCRs have been allocated. This is
 * required because CAP_PCRS ...
 */
if (memchr_inv(pcr_selection.pcr_select, 0, pcr_selection.size_of_select))
	nr_allocated_banks++;

[yeah, comment would be awesome about CAP_PCRS. Did not finish up the
comment because I don't know the answer]

In addition, it would be consistent to call the local variable also
nr_allocated_banks (not nr_alloc_banks).

/Jarkko