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

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 23 May 2018 17:29:45 -0400 (EDT)

----- On May 23, 2018, at 4:14 PM, Mathieu Desnoyers [email-masked] wrote:

> ----- 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.

The following fixup gets ppc64 to work:

--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -208,6 +208,7 @@ system_call_exit:
        /* Check whether the syscall is issued inside a restartable sequence */
        addi    r3,r1,STACK_FRAME_OVERHEAD
        bl      rseq_syscall
+       ld      r3,RESULT(r1)
 #endif
        /*
         * Disable interrupts so current_thread_info()->flags can't change,

> 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.

I'll start testing on ppc32 now.

Thanks,

Mathieu

> 
> 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

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