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

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

----- On May 20, 2018, at 10:08 AM, Boqun Feng boqun.feng@gmail.com wrote:

quoted

On Fri, May 18, 2018 at 02:17:17PM -0400, Mathieu Desnoyers wrote:

quoted

----- On May 17, 2018, at 7:50 PM, Boqun Feng boqun.feng@gmail.com wrote:
[...]

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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.

quoted

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

quoted

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 <redacted>
---
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