linux-kernel - Re: [RFC PATCH 0/4] x86: Improve Minimum Alternate Stack Size

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Message-ID: <20201006170020.GB6642@arm.com>
Date:   Tue, 6 Oct 2020 18:00:21 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "H.J. Lu" <hjl.tools@...il.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] x86: Improve Minimum Alternate Stack Size

On Tue, Oct 06, 2020 at 08:33:47AM -0700, Dave Hansen wrote:
> On 10/6/20 8:25 AM, Dave Martin wrote:
> > Or are people reporting real stack overruns on x86 today?
> 
> We have real overruns.  We have ~2800 bytes of XSAVE (regisiter) state
> mostly from AVX-512, and a 2048 byte MINSIGSTKSZ.

Right.  Out of interest, do you believe that's a direct consequence of
the larger kernel-generated signal frame, or does the expansion of
userspace stack frames play a role too?

In practice software just assumes SIGSTKSZ and then ignores the problem
until / unless an actual stack overflow is seen.

There's probably a lot of software out there whose stack is
theoretically too small even without AVX-512 etc. in the mix, especially
when considering the possibility of nested signals...

Cheers
---Dave