Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200728131050.24443-1-madvenka@linux.microsoft.com>
 <c23de6ec47614f489943e1a89a21dfa3@AcuMS.aculab.com> <f5cfd11b-04fe-9db7-9d67-7ee898636edb@linux.microsoft.com>
In-Reply-To: <f5cfd11b-04fe-9db7-9d67-7ee898636edb@linux.microsoft.com>
From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 28 Jul 2020 10:16:32 -0700
Message-ID: <CALCETrUta5-0TLJ9-jfdehpTAp2Efmukk2npYadFzz9ozOrG2w@mail.gmail.com>
Subject: Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor
To:     "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>
Cc:     David Laight <David.Laight@aculab.com>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
        "linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-integrity@vger.kernel.org" <linux-integrity@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-security-module@vger.kernel.org" 
        <linux-security-module@vger.kernel.org>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman
<madvenka@linux.microsoft.com> wrote:
>
> Thanks. See inline..
>
> On 7/28/20 10:13 AM, David Laight wrote:
> > From:  madvenka@linux.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