linux-kernel - Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

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Date:   Mon, 10 Jun 2019 15:59:29 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        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>,
        "H.J. Lu" <hjl.tools@...il.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>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function

On 6/10/19 3:40 PM, Yu-cheng Yu wrote:
> Ok, we will go back to do_mmap() with MAP_PRIVATE, MAP_NORESERVE and
> VM_DONTDUMP.  The bitmap will cover only 48-bit address space.

Could you make sure to discuss the downsides of only doing a 48-bit
address space?

What are the reasons behind and implications of VM_DONTDUMP?

> We then create PR_MARK_CODE_AS_LEGACY.  The kernel will set the bitmap, but it
> is going to be slow.

Slow compared to what?  We're effectively adding one (quick) system call
to a path that, today, has at *least* half a dozen syscalls and probably
a bunch of page faults.  Heck, we can probably avoid the actual page
fault to populate the bitmap if we're careful.  That alone would put a
syscall on equal footing with any other approach.  If the bit setting
crossed a page boundary it would probably win.

> Perhaps we still let the app fill the bitmap?

I think I'd want to see some performance data on it first.