linux-kernel - Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp

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Message-ID: <a3584dd1-c030-cebe-3f3d-eadc0744580a@intel.com>
Date:   Fri, 2 Nov 2018 12:44:14 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Waiman Long <longman@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp

On 11/2/18 12:40 PM, Waiman Long wrote:
> The 64k+ limit check is kind of arbitrary. So the check is now removed
> to just let expand_stack() decide if a segmentation fault should happen.

With the 64k check removed, what's the next limit that we bump into?  Is
it just the stack_guard_gap space above the next-lowest VMA?