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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Thu, 3 Sep 2020 09:25:51 -0700
Message-ID: <CALCETrXnzOwLSHWOtKTLuybCTZ=w+cBVMhs2uCzd+LSdbijQ3g@mail.gmail.com>
Subject: Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>, Jann Horn <jannh@google.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
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        Cyrill Gorcunov <gorcunov@gmail.com>,
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On Thu, Sep 3, 2020 at 9:21 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 9/3/2020 9:11 AM, Dave Hansen wrote:
> > On 9/3/20 9:09 AM, Yu, Yu-cheng wrote:
> >> If the debugger is going to write an MSR, only in the third case would
> >> this make a slight sense.  For example, if the system has CET enabled,
> >> but the task does not have CET enabled, and GDB is writing to a CET MSR.
> >>   But still, this is strange to me.
> >
> > If this is strange, then why do we even _implement_ writes?
> >
>
> For example, if the task has CET enabled, and it is in a control
> protection fault, the debugger can clear the task's IBT state, or unwind
> the shadow stack, etc.  But if the task does not have CET enabled (its
> CET MSRs are in INIT state), it would make sense for the PTRACE call to
> return failure than doing something else, right?

What do you mean "something else"?  I assume that, if GDB tells
ptrace() to write some value to the CET MSR, then it should get that
value.  If GDB writes to it on a task that is not currently using CET,
I don't see why it should fail.

--Andy