linux-kernel - Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

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Date:   Thu, 29 Mar 2018 14:46:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable
 sequences system call (v12)

On Thu, 29 Mar 2018 14:35:16 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> ----- On Mar 29, 2018, at 2:07 PM, rostedt rostedt@...dmis.org wrote:
> 
> > On Thu, 29 Mar 2018 14:02:33 -0400 (EDT)
> > Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> >   
> >> 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.  
> > 
> > The ptrace path has nothing to do with ptrace anymore, and probably be
> > hard to notice the performance hit. You simply set a TIF flag, and on
> > exit of the syscall it jumps to a path that checks special cases
> > (tracing system calls being one of them). It's called the ptrace path
> > because ptrace was the first one to use it (I'm guessing, I haven't
> > actually looked at the history).  
> 
> Last time I checked, it's not only a jump, it's actually saving/restoring
> tons of registers. Did this change recently ?
> 
> I use it for LTTng syscall tracing too. My experience so far is that it's really
> terribly slow. I've been waiting on Andy Lutomirski to complete his changes in that
> area to look into making this faster for syscall tracepoints.

This gives us more incentive to help Andy make it faster ;-)

-- Steve

> 
> > 
> > This is used to add any system call checks that are not done during
> > normal operation. And this certainly falls under that category.  
> 
> I know it's used for stuff like seccomp too. My guess has always been that security
> people care much more about robustness than performance.
> 
> Thanks,
> 
> Mathieu
> 
>