linux-kernel - Re: [PATCH 07/14] powerpc: Add support for restartable sequences

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Date:   Thu, 17 May 2018 09:43:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>, rostedt <rostedt@...dmis.org>,
        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>,
        Joel Fernandes <joelaf@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 07/14] powerpc: Add support for restartable sequences

On Thu, May 17, 2018 at 09:19:49AM +0800, Boqun Feng wrote:
> On Wed, May 16, 2018 at 04:13:16PM -0400, Mathieu Desnoyers wrote:

> > and that x86 calls it from syscall_return_slowpath() (which AFAIU is
> > now used in the fast-path since KPTI), I wonder where we should call
> 
> So we actually detect this after the syscall takes effect, right? I
> wonder whether this could be problematic, because "disallowing syscall"
> in rseq areas may means the syscall won't take effect to some people, I
> guess?

It doesn't really matter I suspect, the important part is the program
getting killed.

I agree that doing it on sysenter is slightly nicer, but I'll take
sysexit if that's what it takes.

> > this on PowerPC ?  I was under the impression that PowerPC return to
> > userspace fast-path was not calling C code unless work flags were set,
> > but I might be wrong.
> > 
> 
> I think you're right. So we have to introduce callsite to rseq_syscall()
> in syscall path, something like:
> 
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 51695608c68b..a25734a96640 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -222,6 +222,9 @@ system_call_exit:
>  	mtmsrd	r11,1
>  #endif /* CONFIG_PPC_BOOK3E */
>  
> +	addi    r3,r1,STACK_FRAME_OVERHEAD
> +	bl	rseq_syscall
> +
>  	ld	r9,TI_FLAGS(r12)
>  	li	r11,-MAX_ERRNO
>  	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
> 
> But I think it's important for us to first decide where (before or after
> the syscall) we do the detection.

The important thing is the processed getting very dead. Either sysenter
or sysexit gets that done.