linux-kernel - Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

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Date:   Tue, 28 Jul 2020 10:16:32 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
Cc:     David Laight <David.Laight@...lab.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman
<madvenka@...ux.microsoft.com> wrote:
>
> Thanks. See inline..
>
> On 7/28/20 10:13 AM, David Laight wrote:
> > From:  madvenka@...ux.microsoft.com
> >> Sent: 28 July 2020 14:11
> > ...
> >> The kernel creates the trampoline mapping without any permissions. When
> >> the trampoline is executed by user code, a page fault happens and the
> >> kernel gets control. The kernel recognizes that this is a trampoline
> >> invocation. It sets up the user registers based on the specified
> >> register context, and/or pushes values on the user stack based on the
> >> specified stack context, and sets the user PC to the requested target
> >> PC. When the kernel returns, execution continues at the target PC.
> >> So, the kernel does the work of the trampoline on behalf of the
> >> application.
> > Isn't the performance of this going to be horrid?
>
> It takes about the same amount of time as getpid(). So, it is
> one quick trip into the kernel. I expect that applications will
> typically not care about this extra overhead as long as
> they are able to run.

What did you test this on?  A page fault on any modern x86_64 system
is much, much, much, much slower than a syscall.

--Andy