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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Boqun Feng <boqun.feng@gmail.com>
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Subject: Re: [PATCH 07/14] powerpc: Add support for restartable sequences
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Date:   Thu, 24 May 2018 11:03:29 +1000
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Mathieu Desnoyers <mathieu.desnoyers@efficios.com> writes:
> ----- On May 23, 2018, at 4:14 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
...
>> 
>> 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.


Sorry this code is super gross and hard to deal with.

> 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,

I don't think that's safe.

If you look above that, we have r3, r8 and r12 all live:

.Lsyscall_exit:
	std	r3,RESULT(r1)
	CURRENT_THREAD_INFO(r12, r1)

	ld	r8,_MSR(r1)
#ifdef CONFIG_PPC_BOOK3S
	/* No MSR:RI on BookE */
	andi.	r10,r8,MSR_RI
	beq-	.Lunrecov_restore
#endif


They're all volatile across function calls:

  http://openpowerfoundation.org/wp-content/uploads/resources/leabi/content/dbdoclet.50655240_68174.html


The system_call_exit symbol is actually there for kprobes and cosmetic
purposes. The actual syscall return flow starts at .Lsyscall_exit.

So I think this would work:

diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
index db4df061c33a..e19f377a25e0 100644
--- a/arch/powerpc/kernel/entry_64.S
+++ b/arch/powerpc/kernel/entry_64.S
@@ -184,6 +184,14 @@ system_call:			/* label this so stack traces look sane */
 
 .Lsyscall_exit:
 	std	r3,RESULT(r1)
+
+#ifdef CONFIG_DEBUG_RSEQ
+	/* Check whether the syscall is issued inside a restartable sequence */
+	addi    r3,r1,STACK_FRAME_OVERHEAD
+	bl      rseq_syscall
+	ld	r3,RESULT(r1)
+#endif
+
 	CURRENT_THREAD_INFO(r12, r1)
 
 	ld	r8,_MSR(r1)


I'll try and get this series into my test setup at some point, been a
bit busy lately :)

cheers