linux-kernel - Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

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Date:   Mon, 9 Mar 2020 16:11:03 -0700
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>, x86-patch-review@...el.com
Subject: Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

On Mon, Mar 9, 2020 at 3:19 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 3/9/20 2:12 PM, H.J. Lu wrote:
> >> But what are the rules for clone()?  Should there be rules for
> >> mismatches for CET enabling between threads if a process (not child
> >> processes)?
> > What did you mean? A threaded application is either CET enabled or not
> > CET enabled.   A new thread from clone makes no difference.
>
> Stacks are fundamentally thread-local resources.  The registers that
> point to them and MSRs that manage shadow stacks are all CPU-thread
> local.  Nothing is fundamentally tied to the address space shared across
> the process.
>
> A thread might also share *no* control flow with its child.  It might
> ask the thread to start in code that the parent can never even reach.
>
> It sounds like you've picked a Linux implementation that has
> restrictions on top of the fundamentals.  That's not wrong per se, but
> it does deserve explanation and deliberate, not experimental design.
>
> Could you go back to the folks at Intel and try to figure out what this
> was designed to *do*?  Yes, I'm probably one of those folks.  You know
> where to find me. :)

A threaded application is loaded from disk.  The object file on disk is
either CET enabled or not CET enabled.

-- 
H.J.