RISC-V: Add support for restartable sequence

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

1. 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.

2. 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>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -101,6 +101,7 @@ config RISCV
 	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
+	select HAVE_RSEQ
 	select IRQ_DOMAIN
 	select IRQ_FORCED_THREADING
 	select MODULES_USE_ELF_RELA if MODULES

arch/riscv/kernel/entry.S

@@ -225,6 +225,10 @@ ret_from_syscall:
 	 * (If it was configured with SECCOMP_RET_ERRNO/TRACE)
 	 */
 ret_from_syscall_rejected:
+#ifdef CONFIG_DEBUG_RSEQ
+	move a0, sp
+	call rseq_syscall
+#endif
 	/* Trace syscalls, but only if requested by the user. */
 	REG_L t0, TASK_TI_FLAGS(tp)
 	andi t0, t0, _TIF_SYSCALL_WORK

arch/riscv/kernel/signal.c

@@ -258,6 +258,8 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 		}
 	}
 
+	rseq_signal_deliver(ksig, regs);
+
 	/* Set up the stack frame */
 	ret = setup_rt_frame(ksig, oldset, regs);