arm64: ptrace: allow tracer to skip a system call

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

If tracer specifies -1 as a syscall number, this traced system call should
be skipped with a value in x0 used as a return value.

This patch enables this semantics, but there is a restriction here:

   when syscall(-1) is issued by user, tracer cannot skip this system call
   and modify a return value at syscall entry.

In order to ease this flavor, we need to treat whatever value in x0 as
a return value, but this might result in a bogus value being returned,
especially when tracer doesn't do anything at this syscall.
So we always return ENOSYS instead, while we have another chance to change
a return value at syscall exit.

Please also note:
* syscall entry tracing and syscall exit tracing (ftrace tracepoint and
  audit) are always executed, if enabled, even when skipping a system call
  (that is, -1).
  In this way, we can avoid a potential bug where audit_syscall_entry()
  might be called without audit_syscall_exit() at the previous system call
  being called, that would cause OOPs in audit_syscall_entry().

* syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected
  in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS)
  is not used in this case, we may neglect the case.

Signed-off-by: AKASHI Takahiro <takahiro.akashi <at> linaro.org>
Git-commit: feb28436457d33fef9f264635291432df4b74122
Git-repo: https://android.googlesource.com/kernel/common.git


[imaund@codeaurora.org: Resolved context conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>
Signed-off-by: Ravi Kumar Siddojigari <rsiddoji@codeaurora.org>
Change-Id: I7daf084bd5a28dde5a6c5e7fe95b4b7db8df928a
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>