Re: [PATCH v5 0/5] Lazy flush for the auth session

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Sep 24, 2024 at 7:33 PM EEST, James Bottomley wrote:
> On Tue, 2024-09-24 at 19:29 +0300, Jarkko Sakkinen wrote:
> > On Tue Sep 24, 2024 at 4:48 PM EEST, James Bottomley wrote:
> [...]
> > > Patch 3 is completely unnecessary: the null key is only used to
> > > salt the session and is not required to be resident while the
> > > session is used (so can be flushed after session creation)
> > > therefore keeping it around serves no purpose once the session is
> > > created and simply clutters up the TPM volatile handle slots. (I
> > > don't know of a case where we use all the slots in a kernel
> > > operation, but since we don't need it lets not find out when we get
> > > one).  So I advise dropping patch 3.
> > 
> > Let's go this through just to check I'm understanding.
> > 
> > Holding null key had radical effect on boot time: it cut it down by
> > 5 secons down to 15 seconds:
> > 
> > https://lore.kernel.org/linux-integrity/CALSz7m1WG7fZ9UuO0URgCZEDG7r_wB4Ev_4mOHJThH_d1Ed1nw@mail.gmail.com/
> > 
> > Then in subsequent version I implemented lazy auth session and boot
> > time went down to 9.7 seconds.
> > 
> > So is the point you're trying to make that since auth session is 
> > already held as long as we can and they flushed in synchronous
> > point too, I can just as well drop patch 3?
>
> Yes, because the null key is only used in session generation which is
> now lazy, it adds or subtracts nothing from the timings.  When you're
> forced to flush the session, the null key goes too, so you again have
> to restore it from the context.  When you can keep the session you
> don't need the null key because you're not regenerating it.

Yeah, OK, then we're in sync with this. It's evolutionary cruft.

Just had to check that the logic matches how I projected your earlier
comment because these are sensitive changes.

BR, Jarkko