Re: [PATCH v4 1/6] tpm: dynamically allocate active_banks array

[Mimi Zohar]

On Thu, 2018-11-08 at 17:21 +0200, Jarkko Sakkinen wrote:
> On Thu, Nov 08, 2018 at 07:20:51PM +0530, Nayna Jain wrote:
> > Based on a discussion with Ken, the count in the TPML_PCR_SELECTION returns
> > the number of possible algorithms supported. In the example below, two
> > possible algorithms - SHA1 and SHA256 - are returned.
> > 
> > # /usr/local/bin/tssgetcapability -cap 5
> > 2 PCR selections
> > ÂÂÂÂhash TPM_ALG_SHA1
> > ÂÂÂÂTPMS_PCR_SELECTION length 3
> > ÂÂÂÂff ff ff
> > ÂÂÂÂhash TPM_ALG_SHA256
> > ÂÂÂÂTPMS_PCR_SELECTION length 3
> > ÂÂÂÂ00 00 00
> > 
> > The pcr_select fields - "ff ff ff" and "00 00 00" - are bit masks for the
> > enabled PCRs. The SHA1 bank is enabled for all PCRs (0-23), while the SHA256
> > bank is not enabled.
> > 
> > The current code works, but it unnecessarily extends some banks. Instead of
> > basing the number of active banks on the number of algorithms returned, it
> > should be based on the pcr_select field.
> > 
> > ÂÂ - Mimi & Nayna
> 
> I would just allocate array of the size of possible banks and grow
> nr_active_banks for active algorithms to keep the code simple because
> we are talking about insignificant amount of wasted space (might be
> even zero bytes given how kernel allocators works)>

That's fine. ÂRemember the memory is just one concern, but the other
concerns are the performance of calculating the unneeded hash and the
TPM performance of including it in the PCR extend.

Mimi