Re: [PATCH 03/14] arm: Add restartable sequences support

On Wed, May 16, 2018 at 04:13:13PM -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 06:44:22PM -0400, Mathieu Desnoyers wrote:

quoted

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a7f8e7f4b88f..4f5c386631d4 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig

@@ -91,6 +91,7 @@ config ARM
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_RCU_TABLE_FREE if (SMP && ARM_LPAE)
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_RSEQ
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UID16
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN

diff --git a/arch/arm/kernel/signal.c b/arch/arm/kernel/signal.c
index bd8810d4acb3..5879ab3f53c1 100644
--- a/arch/arm/kernel/signal.c
+++ b/arch/arm/kernel/signal.c

@@ -541,6 +541,12 @@ static void handle_signal(struct ksignal *ksig, struct

pt_regs *regs)
 	int ret;
 
 	/*
+	 * Increment event counter and perform fixup for the pre-signal
+	 * frame.
+	 */
+	rseq_signal_deliver(regs);
+
+	/*
 	 * Set up the stack frame
 	 */
 	if (ksig->ka.sa.sa_flags & SA_SIGINFO)

@@ -660,6 +666,7 @@ do_work_pending(struct pt_regs *regs, unsigned int

thread_flags, int syscall)
 			} else {
 				clear_thread_flag(TIF_NOTIFY_RESUME);
 				tracehook_notify_resume(regs);
+				rseq_handle_notify_resume(regs);
 			}
 		}
 		local_irq_disable();

I think you forgot to hook up rseq_syscall() checking.

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

Thoughts ?

Since this only matters for CONFIG_DEBUG_RSEQ, can we just force the
slowpath for rseq tasks when that option is set?

Will