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

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Message-ID: <cf0d1470e95e0a8b88742651d06601a53d6655c1.camel@intel.com>
Date:   Fri, 14 Jun 2019 08:25:16 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...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 Mon, 2019-06-10 at 15:59 -0700, Dave Hansen wrote:
> 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?

The downside is that we cannot load legacy lib's above 48-bit address space, but
currently ld-linux does not do that.  Should ld-linux do that in the future,
dlopen() fails.  Considering CRIU migration, we probably need to do this anyway?

> What are the reasons behind and implications of VM_DONTDUMP?

The bitmap is very big.

In GDB, it should be easy to tell why a control-protection fault occurred
without the bitmap.

Yu-cheng