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

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