arm64: ptrace: add PTRACE_SET_SYSCALL

Note: This patch is from v6 of Takahiro's proposed
"arm64: add seccomp support" patchset (leecam@google.com)

To allow tracer to be able to change/skip a system call by re-writing
a syscall number, there are several approaches:

(1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
    later on in syscall_trace_enter(), or
(2) support ptrace(PTRACE_SET_SYSCALL) as on arm

Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
tracer as well as that secure_computing() expects a changed syscall number
to be visible, especially case of -1, before this function returns in
syscall_trace_enter(), we'd better take (2).

Signed-off-by: AKASHI Takahiro <takahiro.akashi <at> linaro.org>

arch/arm64/include/uapi/asm/ptrace.h

@@ -23,6 +23,7 @@
 
 #include <asm/hwcap.h>
 
+#define PTRACE_SET_SYSCALL	23
 
 /*
  * PSR bits

arch/arm64/kernel/ptrace.c

@@ -1064,7 +1064,19 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
 long arch_ptrace(struct task_struct *child, long request,
 		 unsigned long addr, unsigned long data)
 {
-	return ptrace_request(child, request, addr, data);
+	int ret;
+
+	switch (request) {
+		case PTRACE_SET_SYSCALL:
+			task_pt_regs(child)->syscallno = data;
+			ret = 0;
+			break;
+		default:
+			ret = ptrace_request(child, request, addr, data);
+			break;
+	}
+
+	return ret;
 }
 
 enum ptrace_syscall_dir {