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

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Message-ID: <CALCETrVqe2BhHTMG0joSC3eTk6CibAtn1c5gd7Ou6zLXQrgO3w@mail.gmail.com>
Date:   Fri, 2 Nov 2018 13:11:50 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp

On Fri, Nov 2, 2018 at 12:50 PM Waiman Long <longman@...hat.com> wrote:
>
> On 11/02/2018 03:44 PM, Dave Hansen wrote:
> > 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?
>
> I think it is both the stack_guard_gap space above the next lowest VMA
> and the rlimit(RLIMIT_STACK).
>

Did the non-working programs ever work?  Because, if not, I say let them fail.