linux-kernel - Re: [patch V4 02/15] entry: Provide generic syscall entry functionality

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Date:   Wed, 22 Jul 2020 09:54:55 +0200
From:   peterz@...radead.org
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        linux-arch@...r.kernel.org, Will Deacon <will@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Mark Rutland <mark.rutland@....com>,
        Keno Fischer <keno@...iacomputing.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Subject: Re: [patch V4 02/15] entry: Provide generic syscall entry
 functionality

On Tue, Jul 21, 2020 at 02:38:16PM -0700, Kees Cook wrote:
> One thing I noticed while doing syscall entry timings for the kernel
> stack base offset randomization was that the stack protector was being
> needlessly enabled in certain paths (seccomp, audit) due to seeing a
> register array being declared on the stack. As part of that series I
> suggested down-grading the stack protector. Since then, Peter's changes
> entirely disabled the stack protector on the entry code, which I
> grudgingly accept (I'd rather have a way to mark a variable as "ignore
> this for stack protector detection", but ... there isn't, so fine.)

I don't think I'd like to have that per variable, but a function
attribute to disable stack protector would be awesome, except our
GCC-besties forgot to create that function attribute :-(

If/when we get such a function attribute, we can add it to noinstr.

Also see this here:

  https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@gentoo.org