xtensa: fix system_call interaction with ptrace

Don't overwrite return value if system call was cancelled at entry by
ptrace. Return status code from do_syscall_trace_enter so that
pt_regs::syscall doesn't need to be changed to skip syscall.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/entry.S

@@ -1892,6 +1892,7 @@ ENTRY(system_call)
 
 	mov	a6, a2
 	call4	do_syscall_trace_enter
+	beqz	a6, .Lsyscall_exit
 	l32i	a7, a2, PT_SYSCALL
 
 1:
@@ -1904,8 +1905,6 @@ ENTRY(system_call)
 
 	addx4	a4, a7, a4
 	l32i	a4, a4, 0
-	movi	a5, sys_ni_syscall;
-	beq	a4, a5, 1f
 
 	/* Load args: arg0 - arg5 are passed via regs. */
 
@@ -1925,6 +1924,7 @@ ENTRY(system_call)
 
 	s32i	a6, a2, PT_AREG2
 	bnez	a3, 1f
+.Lsyscall_exit:
 	abi_ret(4)
 
 1:

arch/xtensa/kernel/ptrace.c

@@ -542,14 +542,28 @@ long arch_ptrace(struct task_struct *child, long request,
 	return ret;
 }
 
-void do_syscall_trace_enter(struct pt_regs *regs)
+void do_syscall_trace_leave(struct pt_regs *regs);
+int do_syscall_trace_enter(struct pt_regs *regs)
 {
+	if (regs->syscall == NO_SYSCALL)
+		regs->areg[2] = -ENOSYS;
+
 	if (test_thread_flag(TIF_SYSCALL_TRACE) &&
-	    tracehook_report_syscall_entry(regs))
+	    tracehook_report_syscall_entry(regs)) {
+		regs->areg[2] = -ENOSYS;
 		regs->syscall = NO_SYSCALL;
+		return 0;
+	}
+
+	if (regs->syscall == NO_SYSCALL) {
+		do_syscall_trace_leave(regs);
+		return 0;
+	}
 
 	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))
 		trace_sys_enter(regs, syscall_get_nr(current, regs));
+
+	return 1;
 }
 
 void do_syscall_trace_leave(struct pt_regs *regs)