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Message-ID: <d44b3fc7ce9f053f978645dfe19018885d372d87.camel@kernel.org>
Subject: Re: [PATCH v2 1/2] selftests/sgx: Use rip relative addressing for
 encl_stack
From:   Jarkko Sakkinen <jarkko@kernel.org>
To:     Reinette Chatre <reinette.chatre@intel.com>
Cc:     Shuah Khan <skhan@linuxfoundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Shuah Khan <shuah@kernel.org>,
        "open list:INTEL SGX" <linux-sgx@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Date:   Wed, 30 Mar 2022 23:40:56 +0300
In-Reply-To: <ef991f0d-d0d2-f422-96d8-0951d593d2a5@intel.com>
References: <20220322074313.7444-1-jarkko@kernel.org>
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         <0031a4f0-75f6-3104-1825-dcc2528f61b0@intel.com>
         <f68d472877b7136c32d8770603a3de38de59c322.camel@kernel.org>
         <ef991f0d-d0d2-f422-96d8-0951d593d2a5@intel.com>
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On Wed, 2022-03-30 at 13:05 -0700, Reinette Chatre wrote:
>=20
>=20
> On 3/30/2022 12:03 PM, Jarkko Sakkinen wrote:
> > On Wed, 2022-03-30 at 10:40 -0700, Reinette Chatre wrote:
> > > Could you please elaborate how the compiler will fix it up?
> >=20
> > Sure.
> >=20
> > Here's the disassembly of the RBX version:
> >=20
> > [0x000021a9]> pi 1
> > lea rax, [rbx + loc.encl_stack]
> >=20
> > Here's the same with s/RBX/RIP/:
> >=20
> > [0x000021a9]> pi 5
> > lea rax, loc.encl_stack
> >=20
> > Compiler will substitute correct offset relative to the RIP,
> > well, because it can and it makes sense.
>=20
> It does not make sense to me because, as proven with my test,
> the two threads end up sharing the same stack memory.

I see, I need to correct my patch, thanks!

RBX gives correct results because of the binary organization,
i.e. TCS's are placed to zero offset and forward, and=20
unrelocated symbol is just compiled in as an untranslated
offset.

RPI is given correct results but how the semantics work
right now is incompatible.

Still, even for kselftest, I would consider a switch
because that way:

1. You can layout binary however you wan and things
   won't break.
2. You can point to any symbol not just stack, if
   ever need.
  =20
I admit it works semantically but it just super
unrobust.

BR, Jarkko