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From:   Boqun Feng <boqun.feng@gmail.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Will Deacon <will.deacon@arm.com>
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Subject: Re: [PATCH 07/14] powerpc: Add support for restartable sequences
Date:   Fri, 18 May 2018 07:50:43 +0800
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On Thu, May 17, 2018, at 11:28 PM, Mathieu Desnoyers wrote:
> ----- On May 16, 2018, at 9:19 PM, Boqun Feng boqun.feng@gmail.com wrote:
> 
> > On Wed, May 16, 2018 at 04:13:16PM -0400, Mathieu Desnoyers wrote:
> >> ----- On May 16, 2018, at 12:18 PM, Peter Zijlstra peterz@infradead.org 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.
> 
> As Peter said, we don't really care whether it's on syscall entry or 
> exit, as
> long as the process gets killed when the erroneous use is detected. I 
> think doing
> it on syscall exit is a bit easier because we can clearly access the 
> userspace
> TLS, which AFAIU may be less straightforward on syscall entry.
>

Fair enough.
 
> We may want to add #ifdef CONFIG_DEBUG_RSEQ / #endif around the code you
> proposed above, so it's only compiled in if CONFIG_DEBUG_RSEQ=y.
> 

OK.

> On the ARM leg of the email thread, Will Deacon suggests to test whether 
> current->rseq
> is non-NULL before calling rseq_syscall(). I wonder if this added check 
> is justified
> as the assembly level, considering that this is just a debugging option. 
> We already do
> that check at the very beginning of rseq_syscall().
> 

Yes, I think it's better to do the check in rseq_syscall(), leaving the asm
code a bit cleaner.

Regards,
Boqun

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