[PATCH V2 2/3] riscv: Add support for restartable sequence

[Vincent Chen <vincent.chen@sifive.com>]

Add calls to rseq_signal_deliver(), rseq_handle_notify_resume() and
rseq_syscall() to introduce RSEQ support.

1. Call the rseq_handle_notify_resume() function on return to userspace
   if TIF_NOTIFY_RESUME thread flag is set.

2. Call the rseq_signal_deliver() function to fixup on the pre-signal
   frame when a signal is delivered on top of a restartable sequence
   critical section.

3. Check that system calls are not invoked from within rseq critical
   sections by invoking rseq_signal() from ret_from_syscall(). With
   CONFIG_DEBUG_RSEQ, such behavior results in termination of the
   process with SIGSEGV.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
---
 arch/riscv/Kconfig         | 1 +
 arch/riscv/kernel/entry.S  | 4 ++++
 arch/riscv/kernel/signal.c | 2 ++
 3 files changed, 7 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index fa98b279257e..ded32979d33d 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -67,6 +67,7 @@ config RISCV
 	select HAVE_COPY_THREAD_TLS
 	select HAVE_ARCH_KASAN if MMU && 64BIT
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_RSEQ
 
 config ARCH_MMAP_RND_BITS_MIN
 	default 18 if 64BIT
diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index bad4d85b5e91..89d3713b0aef 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -214,6 +214,10 @@ ENTRY(handle_exception)
 handle_syscall:
 	 /* save the initial A0 value (needed in signal handlers) */
 	REG_S a0, PT_ORIG_A0(sp)
+#ifdef CONFIG_RSEQ_DEBUG
+	move a0, sp
+	call rseq_syscall
+#endif
 	/*
 	 * Advance SEPC to avoid executing the original
 	 * scall instruction on sret
diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
index 17ba190e84a5..d939c5de41c4 100644
--- a/arch/riscv/kernel/signal.c
+++ b/arch/riscv/kernel/signal.c
@@ -234,6 +234,7 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 	sigset_t *oldset = sigmask_to_save();
 	int ret;
 
+	rseq_signal_deliver(ksig, regs);
 	/* Are we from a system call? */
 	if (regs->cause == EXC_SYSCALL) {
 		/* Avoid additional syscall restarting via ret_from_exception */
@@ -316,5 +317,6 @@ asmlinkage __visible void do_notify_resume(struct pt_regs *regs,
 	if (thread_info_flags & _TIF_NOTIFY_RESUME) {
 		clear_thread_flag(TIF_NOTIFY_RESUME);
 		tracehook_notify_resume(regs);
+		rseq_handle_notify_resume(NULL, regs);
 	}
 }
-- 
2.7.4