Subject: Re: [PATCH 07/14] powerpc: Add support for restartable sequences

From: Boqun Feng <[email-masked]>
Date: Thu, 17 May 2018 09:19:49 +0800

On Wed, May 16, 2018 at 04:13:16PM -0400, Mathieu Desnoyers wrote:
> ----- On May 16, 2018, at 12:18 PM, Peter Zijlstra [email-masked] wrote:
> 
> > On Mon, Apr 30, 2018 at 06:44:26PM -0400, Mathieu Desnoyers wrote:
> >> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> >> index c32a181a7cbb..ed21a777e8c6 100644
> >> --- a/arch/powerpc/Kconfig
> >> +++ b/arch/powerpc/Kconfig
> >> @@ -223,6 +223,7 @@ config PPC
> >>  	select HAVE_SYSCALL_TRACEPOINTS
> >>  	select HAVE_VIRT_CPU_ACCOUNTING
> >>  	select HAVE_IRQ_TIME_ACCOUNTING
> >> +	select HAVE_RSEQ
> >>  	select IRQ_DOMAIN
> >>  	select IRQ_FORCED_THREADING
> >>  	select MODULES_USE_ELF_RELA
> >> diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
> >> index 61db86ecd318..d3bb3aaaf5ac 100644
> >> --- a/arch/powerpc/kernel/signal.c
> >> +++ b/arch/powerpc/kernel/signal.c
> >> @@ -133,6 +133,8 @@ static void do_signal(struct task_struct *tsk)
> >>  	/* Re-enable the breakpoints for the signal stack */
> >>  	thread_change_pc(tsk, tsk->thread.regs);
> >>  
> >> +	rseq_signal_deliver(tsk->thread.regs);
> >> +
> >>  	if (is32) {
> >>          	if (ksig.ka.sa.sa_flags & SA_SIGINFO)
> >>  			ret = handle_rt_signal32(&ksig, oldset, tsk);
> >> @@ -164,6 +166,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long
> >> thread_info_flags)
> >>  	if (thread_info_flags & _TIF_NOTIFY_RESUME) {
> >>  		clear_thread_flag(TIF_NOTIFY_RESUME);
> >>  		tracehook_notify_resume(regs);
> >> +		rseq_handle_notify_resume(regs);
> >>  	}
> >>  
> >>  	user_enter();
> > 
> > Again no rseq_syscall().
> 
> Same question for PowerPC as for ARM:
> 
> Considering that rseq_syscall is implemented as follows:
> 
> +void rseq_syscall(struct pt_regs *regs)
> +{
> +       unsigned long ip = instruction_pointer(regs);
> +       struct task_struct *t = current;
> +       struct rseq_cs rseq_cs;
> +
> +       if (!t->rseq)
> +               return;
> +       if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
> +           rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
> +               force_sig(SIGSEGV, t);
> +}
> 
> 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?

> 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.

Regards,
Boqun

> Thoughts ?
> Thanks!
> Mathieu
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com