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

Mathieu Desnoyers [off-list ref] writes:

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----- On May 23, 2018, at 4:14 PM, Mathieu Desnoyers
mathieu.desnoyers@efficios.com wrote:

...

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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.

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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 :)