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Message-ID: <5aa98999b1343f34828414b74261201886ec4591.camel@intel.com>
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,
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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>,
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        Florian Weimer <fweimer@redhat.com>,
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        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>
Date:   Mon, 10 Jun 2019 12:38:09 -0700
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On Mon, 2019-06-10 at 11:02 -0700, Dave Hansen wrote:
> On 6/10/19 8:22 AM, Yu-cheng Yu wrote:
> > > How does glibc know the linear address space size?  We don’t want LA64 to
> > > break old binaries because the address calculation changed.
> > 
> > When an application starts, its highest stack address is determined.
> > It uses that as the maximum the bitmap needs to cover.
> 
> Huh, I didn't think we ran code from the stack. ;)
> 
> Especially given the way that we implemented the new 5-level-paging
> address space, I don't think that expecting code to be below the stack
> is a good universal expectation.

Yes, you make a good point.  However, allowing the application manage the bitmap
is the most efficient and flexible.  If the loader finds a legacy lib is beyond
the bitmap can cover, it can deal with the problem by moving the lib to a lower
address; or re-allocate the bitmap.  If the loader cannot allocate a big bitmap
to cover all 5-level address space (the bitmap will be large), it can put all
legacy lib's at lower address.  We cannot do these easily in the kernel.

Yu-cheng