Re: [PATCH v4 1/1] tpm: add sysfs exports for all banks of PCR registers

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Aug 18, 2020 at 09:44:30AM -0700, James Bottomley wrote:
> On Tue, 2020-08-18 at 19:19 +0300, Jarkko Sakkinen wrote:
> > On Tue, Aug 18, 2020 at 07:12:09PM +0300, Jarkko Sakkinen wrote:
> > > On Mon, Aug 17, 2020 at 02:35:06PM -0700, James Bottomley wrote:
> > > > Create sysfs per hash groups with 24 PCR files in them one group,
> > > > named pcr-<hash>, for each agile hash of the TPM.  The files are
> > > > plugged in to a PCR read function which is TPM version agnostic,
> > > > so this works also for TPM 1.2 but the hash is only sha1 in that
> > > > case.
> > > > 
> > > > Note: the macros used to create the hashes emit spurious
> > > > checkpatch warnings.  Do not try to "fix" them as checkpatch
> > > > recommends, otherwise they'll break.
> > > > 
> > > > Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership
> > > > .com>
> > > > Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
> > > > Tested-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
> > > 
> > > I have hard time understanding why this is required.
> > > 
> > > You can grab the information through /dev/tpm0 just fine.
> > 
> > I just think it is principally wrong to add sysfs files if they don't
> > have any measurable value other than perhaps some convenience.
> 
> That's pretty much the whole point of sysfs (and procfs): to add
> convenient extraction of information even if it could potentially be
> obtained by other sources.  For instance, the whole reason we add a lot
> of the broken out inquiry data in SCSI via sysfs is precisely so users
> don't have to go prodding devices with direct SCSI commands, which are
> pretty much analagous to TPM device commands.
> 
> The question you should be asking isn't whether the information *could*
> be obtained by other means, but whether providing it in this form
> facilitates current operations and whether the interface would have
> users.

Usually users use some appropriate applications to do their work, not
talk directly to the kernel.

Grabbing PCRs is a trivial program to write and I don't get the logic.

My email program is useful for me but I definitely do not want it to be
part of the Linux kernel. One great reason for that is that it would
involve a tedious process to update it later on.

/Jarkko