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

BR, Jarkko

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