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

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 23 May 2018 16:14:39 -0400 (EDT)

----- On May 20, 2018, at 10:08 AM, Boqun Feng [email-masked] wrote:

> On Fri, May 18, 2018 at 02:17:17PM -0400, Mathieu Desnoyers wrote:
>> ----- On May 17, 2018, at 7:50 PM, Boqun Feng [email-masked] wrote:
>> [...]
>> >> > 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)
>> >> > 
>> 
>> By the way, I think this is not the right spot to call rseq_syscall, because
>> interrupts are disabled. I think we should move this hunk right after
>> system_call_exit.
>> 
> 
> Good point.
> 
>> Would you like to implement and test an updated patch adding those calls for ppc
>> 32 and 64 ?
>> 
> 
> I'd like to help, but I don't have a handy ppc environment for test...
> So I made the below patch which has only been build-tested, hope it
> could be somewhat helpful.

Hi Boqun,

I tried your patch in a ppc64 le environment, and it does not survive boot
with CONFIG_DEBUG_RSEQ=y. init gets killed right away.

Moreover, I'm not sure that the r3 register don't contain something worth
saving before the call on ppc32. Just after there is a "mr" instruction
which AFAIU takes r3 as input register.

Can you look into it ?

Thanks,

Mathieu

> 
> Regards,
> Boqun
> 
> --------------------------------->8
> Subject: [PATCH] powerpc: Add syscall detection for restartable sequences
> 
> Syscalls are not allowed inside restartable sequences, so add a call to
> rseq_syscall() at the very beginning of system call exiting path for
> CONFIG_DEBUG_RSEQ=y kernel. This could help us to detect whether there
> is a syscall issued inside restartable sequences.
> 
> Signed-off-by: Boqun Feng <[email-masked]>
> ---
> arch/powerpc/kernel/entry_32.S | 5 +++++
> arch/powerpc/kernel/entry_64.S | 5 +++++
> 2 files changed, 10 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index eb8d01bae8c6..2f134eebe7ed 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -365,6 +365,11 @@ syscall_dotrace_cont:
> 	blrl			/* Call handler */
> 	.globl	ret_from_syscall
> ret_from_syscall:
> +#ifdef CONFIG_DEBUG_RSEQ
> +	/* Check whether the syscall is issued inside a restartable sequence */
> +	addi    r3,r1,STACK_FRAME_OVERHEAD
> +	bl      rseq_syscall
> +#endif
> 	mr	r6,r3
> 	CURRENT_THREAD_INFO(r12, r1)
> 	/* disable interrupts so current_thread_info()->flags can't change */
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 2cb5109a7ea3..2e2d59bb45d0 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -204,6 +204,11 @@ system_call:			/* label this so stack traces look sane */
>  * This is blacklisted from kprobes further below with _ASM_NOKPROBE_SYMBOL().
>  */
> system_call_exit:
> +#ifdef CONFIG_DEBUG_RSEQ
> +	/* Check whether the syscall is issued inside a restartable sequence */
> +	addi    r3,r1,STACK_FRAME_OVERHEAD
> +	bl      rseq_syscall
> +#endif
> 	/*
> 	 * Disable interrupts so current_thread_info()->flags can't change,
> 	 * and so that we don't get interrupted after loading SRR0/1.
> --
> 2.16.2

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com