linux-kernel - Re: [PATCH 2/4] MIPS: Add syscall detection for restartable sequences

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Date:   Fri, 15 Jun 2018 11:43:17 -0700
From:   Paul Burton <paul.burton@...s.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     linux-mips <linux-mips@...ux-mips.org>,
        Peter Zijlstra <peterz@...radead.org>,
        James Hogan <jhogan@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 2/4] MIPS: Add syscall detection for restartable sequences

Hi Mathieu,

On Fri, Jun 15, 2018 at 01:41:25PM -0400, Mathieu Desnoyers wrote:
> > diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
> > index 38a302919e6b..d7de8adcfcc8 100644
> > --- a/arch/mips/kernel/entry.S
> > +++ b/arch/mips/kernel/entry.S
> > @@ -79,6 +79,10 @@ FEXPORT(ret_from_fork)
> > 	jal	schedule_tail		# a0 = struct task_struct *prev
> > 
> > FEXPORT(syscall_exit)
> > +#ifdef CONFIG_DEBUG_RSEQ
> > +	move	a0, sp
> > +	jal	rseq_syscall
> > +#endif
> > 	local_irq_disable		# make sure need_resched and
> > 					# signals dont change between
> > 					# sampling and return
> > @@ -141,6 +145,10 @@ work_notifysig:				# deal with pending signals and
> > 	j	resume_userspace_check
> > 
> > FEXPORT(syscall_exit_partial)
> > +#ifdef CONFIG_DEBUG_RSEQ
> > +	move	a0, sp
> > +	jal	rseq_syscall
> > +#endif
> > 	local_irq_disable		# make sure need_resched doesn't
> > 					# change between and return
> > 	LONG_L	a2, TI_FLAGS($28)	# current->work
> 
> Just to double-check: you did test with CONFIG_DEBUG_RSEQ=y, right ?

Yes, I did. Although I only ran the selftests, which I don't believe
would actually trigger the SIGSEGV condition.

Side-note: maybe it'd be useful to have a test that does intentionally
perform a syscall within a restartable sequence & checks that it
actually receives a SIGSEGV?.

> Are there any live registers that need to be saved before calling
> rseq_syscall ?

No - we just need gp/$28 & sp/$29, and the calling convention means
rseq_syscall() should return with those unmodified. Everything else that
we or userland care about is about to be loaded from the stack anyway.

Thanks,
    Paul