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From: Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup
 function
Date: Mon, 10 Jun 2019 11:02:45 -0700
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To: Yu-cheng Yu <yu-cheng.yu@intel.com>, Andy Lutomirski <luto@amacapital.net>
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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.