linux-kernel - Re: [PATCH v8 2/3] tpm_crb_ffa:Introduce memzero macro to replace memset

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Message-ID: <aGffUrDSjNH6w6rB@kernel.org>
Date: Fri, 4 Jul 2025 17:04:02 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Prachotan Bathi <prachotan.bathi@....com>,
	Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Stuart Yoder <stuart.yoder@....com>,
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 2/3] tpm_crb_ffa:Introduce memzero macro to replace
 memset

On Fri, Jul 04, 2025 at 11:40:10AM +0100, David Laight wrote:
> On Fri, 4 Jul 2025 05:56:50 +0300
> Jarkko Sakkinen <jarkko@...nel.org> wrote:
> 
> > On Fri, Jul 04, 2025 at 05:45:11AM +0300, Jarkko Sakkinen wrote:
> ...
> > > Well, that was some truly misguided advice from my side so all the shame
> > > here is on me :-) There's no global memzero() and neither explicit
> > > version makes much sense here. Sorry about that.
> > > 
> > > I gave it now (actual) thought, and here's what I'd propose:
> > > 
> > > diff --git a/drivers/char/tpm/tpm_crb_ffa.c b/drivers/char/tpm/tpm_crb_ffa.c
> > > index 96746d5b03e3..e769f6143a7c 100644
> > > --- a/drivers/char/tpm/tpm_crb_ffa.c
> > > +++ b/drivers/char/tpm/tpm_crb_ffa.c
> > > @@ -203,26 +203,20 @@ static int __tpm_crb_ffa_try_send_receive(unsigned long func_id,
> > >  	msg_ops = tpm_crb_ffa->ffa_dev->ops->msg_ops;
> > >  
> > >  	if (ffa_partition_supports_direct_req2_recv(tpm_crb_ffa->ffa_dev)) {
> > > -		memzero(&tpm_crb_ffa->direct_msg_data2,
> > > -		       sizeof(struct ffa_send_direct_data2));
> > > -
> > > -		tpm_crb_ffa->direct_msg_data2.data[0] = func_id;
> > > -		tpm_crb_ffa->direct_msg_data2.data[1] = a0;
> > > -		tpm_crb_ffa->direct_msg_data2.data[2] = a1;
> > > -		tpm_crb_ffa->direct_msg_data2.data[3] = a2;
> > > +		tpm_crb_ffa->direct_msg_data2 = (struct ffa_send_direct_data2){
> > > +			.data = { func_id, a0, a1, a2 },
> > > +		};
> 
> clang has a habit of compiling that as an un-named on-stack structure that
> is initialised and then memcpy() used to copy it into place.
> Often not was intended and blows the stack when the structure is large.
> 
> So probably not a pattern that should be encouraged.

This is interesting observation so I had to do some compilation tests to
verify the claim just to see how it plays out (and for the sake of
learning while doing it).

Note that I use GCC for the examples but I have high doubts that clang
would do worse. Please share the insight if that is a wrong assumption.

OK, so... here's the dissembly (using objdump) for the  unchanged version:

ffff8000801805a0:	8b020260 	add	x0, x19, x2
ffff8000801805a4:	94011819 	bl	ffff8000801c6608 <__memset>
ffff8000801805a8:	a9035a75 	stp	x21, x22, [x19, #48]
ffff8000801805ac:	aa1a03e1 	mov	x1, x26
ffff8000801805b0:	aa1903e0 	mov	x0, x25
ffff8000801805b4:	a9047e77 	stp	x23, xzr, [x19, #64]

[ Off-topic: note that how a2 gets optimized out with the zero register
  so that it is probably a parameter that we don't need at all in the
  first place? ]

However, in the changed version the matching snippet looks factors
better:

ffff800080180620:	a9017c7f 	stp	xzr, xzr, [x3, #16]
ffff800080180624:	f900107f 	str	xzr, [x3, #32]

Further, look at the stack size in the original version:

ffff800080180524 <__tpm_crb_ffa_send_receive.constprop.0>:
ffff800080180524:	a9ba7bfd 	stp	x29, x30, [sp, #-96]!

On the other hand, in the changed version:

ffff800080180524 <__tpm_crb_ffa_send_receive.constprop.0>:
ffff800080180524:	a9bb7bfd 	stp	x29, x30, [sp, #-80]!

I don't know, at least the figures I'm able to measure with my limited
ARM assembly knowledge look way better.

BR, Jarkko`