Arm64 holds a syscall number in w8(x8) register. Ptrace tracer may change
its value either to:
   * any valid syscall number to alter a system call, or
   * -1 to skip a system call

This patch implements this behavior by reloading that value into syscallno
in struct pt_regs after tracehook_report_syscall_entry() or
secure_computing(). In case of '-1', a return value of system call can also
be changed by the tracer setting the value to x0 register, and so
sys_ni_nosyscall() should not be called.

See also:
     42309ab4, ARM: 8087/1: ptrace: reload syscall number after
	      secure_computing() check

Signed-off-by: AKASHI Takahiro <redacted>
---
  arch/arm64/kernel/entry.S  |    2 ++
  arch/arm64/kernel/ptrace.c |   13 +++++++++++++
  2 files changed, 15 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5141e79..de8bdbc 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -628,6 +628,8 @@ ENDPROC(el0_svc)
  __sys_trace:
  	mov	x0, sp
  	bl	syscall_trace_enter
+	cmp	w0, #-1				// skip syscall?
+	b.eq	ret_to_user