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Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
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        Arnd Bergmann <arnd@arndb.de>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
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        Kees Cook <keescook@chromium.org>,
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Date:   Fri, 14 Jun 2019 08:25:16 -0700
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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