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

On Wed, May 16, 2018 at 04:13:16PM -0400, Mathieu Desnoyers wrote:

----- On May 16, 2018, at 12:18 PM, Peter Zijlstra peterz@infradead.org wrote:

quoted

On Mon, Apr 30, 2018 at 06:44:26PM -0400, Mathieu Desnoyers wrote:

quoted

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c32a181a7cbb..ed21a777e8c6 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -223,6 +223,7 @@ config PPC
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HAVE_IRQ_TIME_ACCOUNTING
+	select HAVE_RSEQ
 	select IRQ_DOMAIN
 	select IRQ_FORCED_THREADING
 	select MODULES_USE_ELF_RELA

diff --git a/arch/powerpc/kernel/signal.c b/arch/powerpc/kernel/signal.c
index 61db86ecd318..d3bb3aaaf5ac 100644
--- a/arch/powerpc/kernel/signal.c
+++ b/arch/powerpc/kernel/signal.c

@@ -133,6 +133,8 @@ static void do_signal(struct task_struct *tsk)
 	/* Re-enable the breakpoints for the signal stack */
 	thread_change_pc(tsk, tsk->thread.regs);
 
+	rseq_signal_deliver(tsk->thread.regs);
+
 	if (is32) {
         	if (ksig.ka.sa.sa_flags & SA_SIGINFO)
 			ret = handle_rt_signal32(&ksig, oldset, tsk);

@@ -164,6 +166,7 @@ void do_notify_resume(struct pt_regs *regs, unsigned long

thread_info_flags)
 	if (thread_info_flags & _TIF_NOTIFY_RESUME) {
 		clear_thread_flag(TIF_NOTIFY_RESUME);
 		tracehook_notify_resume(regs);
+		rseq_handle_notify_resume(regs);
 	}
 
 	user_enter();

Again no rseq_syscall().

Same question for PowerPC as for ARM:

Considering that rseq_syscall is implemented as follows:

+void rseq_syscall(struct pt_regs *regs)
+{
+       unsigned long ip = instruction_pointer(regs);
+       struct task_struct *t = current;
+       struct rseq_cs rseq_cs;
+
+       if (!t->rseq)
+               return;
+       if (!access_ok(VERIFY_READ, t->rseq, sizeof(*t->rseq)) ||
+           rseq_get_rseq_cs(t, &rseq_cs) || in_rseq_cs(ip, &rseq_cs))
+               force_sig(SIGSEGV, t);
+}

and that x86 calls it from syscall_return_slowpath() (which AFAIU is
now used in the fast-path since KPTI), I wonder where we should call

So we actually detect this after the syscall takes effect, right? I
wonder whether this could be problematic, because "disallowing syscall"
in rseq areas may means the syscall won't take effect to some people, I
guess?

this on PowerPC ?  I was under the impression that PowerPC return to
userspace fast-path was not calling C code unless work flags were set,
but I might be wrong.

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)

But I think it's important for us to first decide where (before or after
the syscall) we do the detection.

Regards,
Boqun

Thoughts ?

Thanks!

Mathieu

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