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

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Date:   Fri, 14 Jun 2019 13:57:40 -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 6/14/19 10:13 AM, Yu-cheng Yu wrote:
> On Fri, 2019-06-14 at 09:13 -0700, Dave Hansen wrote:
>> On 6/14/19 8:25 AM, Yu-cheng Yu wrote:
>>> The bitmap is very big.
>>
>> Really?  It's actually, what, 8*4096=32k, so 1/32,768th of the size of
>> the libraries legacy libraries you load?  Do our crash dumps really not
>> know how to represent or deal with sparse mappings?
> 
> Ok, even the core dump is not physically big, its size still looks odd, right?

Hell if I know.

Could you please go try this in practice so that we're designing this
thing fixing real actual problems instead of phantoms that we're
anticipating?

> Could this also affect how much time for GDB to load it.

I don't know.  Can you go find out for sure, please?

> I have a related question:
> 
> Do we allow the application to read the bitmap, or any fault from the
> application on bitmap pages?

We have to allow apps to read it.  Otherwise they can't execute
instructions.

We don't have to allow them to (popuating) fault on it.  But, if we
don't, we need some kind of kernel interface to avoid the faults.