Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for encl_stack

[Jarkko Sakkinen <jarkko@kernel.org>]

On Wed, Mar 30, 2022 at 05:54:18PM +0300, Jarkko Sakkinen wrote:
> On Mon, Mar 28, 2022 at 02:49:04PM -0700, Reinette Chatre wrote:
> > Hi Jarkko,
> > 
> > On 3/22/2022 12:43 AM, Jarkko Sakkinen wrote:
> > > Simplify the test_encl_bootstrap.S flow by using rip-relative addressing.
> > > Compiler does the right thing here, and this removes dependency on where
> > > TCS entries need to be located in the binary, i.e. allows the binary layout
> > > changed freely in the future.
> > > 
> > > Cc: Reinette Chatre <reinette.chatre@intel.com>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > > ---
> > >  tools/testing/selftests/sgx/test_encl_bootstrap.S | 6 +-----
> > >  1 file changed, 1 insertion(+), 5 deletions(-)
> > > 
> > > diff --git a/tools/testing/selftests/sgx/test_encl_bootstrap.S b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > > index 82fb0dfcbd23..1c1b5c6c4ffe 100644
> > > --- a/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > > +++ b/tools/testing/selftests/sgx/test_encl_bootstrap.S
> > > @@ -40,11 +40,7 @@
> > >  	.text
> > >  
> > >  encl_entry:
> > > -	# RBX contains the base address for TCS, which is the first address
> > > -	# inside the enclave for TCS #1 and one page into the enclave for
> > > -	# TCS #2. By adding the value of encl_stack to it, we get
> > > -	# the absolute address for the stack.
> > > -	lea	(encl_stack)(%rbx), %rax
> > > +	lea	(encl_stack)(%rip), %rax
> > >  	xchg	%rsp, %rax
> > >  	push	%rax
> > >  
> > 
> > The goal of the above snippet is to set RSP to ensure that each thread has its own stack.
> > 
> > Since EENTER computes RIP as EnclaveBase + TCS.OENTRY, by using offset from RIP this
> > would result in all TCS with OENTRY of encl_entry to use the same stack, no?
> > 
> > Could you please consider the following as an alternative:
> > https://lore.kernel.org/lkml/65c137c875bd4da675eaba35316ff43d7cfd52f8.1644274683.git.reinette.chatre@intel.com/
> > 
> > The idea in that patch is that a new TCS would always need to be accompanied by a
> > dedicated stack so, at least for testing purposes, the TCS and stack can be dynamically
> > allocated together with the TCS page following its stack.  This seems much simpler
> > to me and also makes the following patch unnecessary.
> 
> There's no better alternative than use rip. Compiler will fix it up.
> 
> So, no, I won't consider that. This a dead obvious change.

How you organize TCS and stack is completely unrelated topic.

BR, Jarkko