Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 29 Mar 2018 14:02:33 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andrew Hunter <ahh@google.com>,
        Andi Kleen <andi@firstfloor.org>, Chris Lameter <cl@linux.com>,
        Ben Maurer <bmaurer@fb.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Message-ID: <21903915.856.1522346553810.JavaMail.zimbra@efficios.com>
In-Reply-To: <20180329122439.4a909c72@gandalf.local.home>
References: <20180327160542.28457-1-mathieu.desnoyers@efficios.com> <20180328174935.GK4082@hirez.programming.kicks-ass.net> <181076499.279.1522268382303.JavaMail.zimbra@efficios.com> <alpine.DEB.2.21.1803282305030.1496@nanos.tec.linutronix.de> <87410797.545.1522331641598.JavaMail.zimbra@efficios.com> <20180329142338.GD4043@hirez.programming.kicks-ass.net> <544124089.623.1522337940950.JavaMail.zimbra@efficios.com> <20180329122439.4a909c72@gandalf.local.home>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable
 sequences system call (v12)
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Thread-Topic: rseq: Introduce restartable sequences system call (v12)
Thread-Index: qvRuBK4nMzc3YgRr7lnp2KqNAoq3Dw==
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

----- On Mar 29, 2018, at 12:24 PM, rostedt rostedt@goodmis.org wrote:

> On Thu, 29 Mar 2018 11:39:00 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> Enforcing SIGSEGV on syscall entry when nested in a rseq critical section
>> will not be free both in terms of syscall overhead, and in terms of code
>> maintenance: we'd need to add those checks into entry.S for each architecture
>> supported, which pretty much doubles the amount of architecture-specific
>> code we need to implement for rseq. Currently, all we need is to hook in
>> signal delivery and wire up the system call numbers.
> 
> Why not have the check on syscall exit? Then we could use the ptrace
> type mechanism to only go that path when a rseq exists for the program.

Currently, anyone using ptrace on a process has pretty much given up all
hopes of performance. Processes will use rseq to gain performance, not the
opposite, so this deterioration will be unwelcome.

One thing I would find more acceptable is to only compile in this check under
a CONFIG_DEBUG_RSEQ option or something similar. This means we can then put
the check at the most convenient location without caring too much about its
performance impact.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com