[PATCH v9 2/6] arm64: ptrace: allow tracer to skip a system call

On Thu, Nov 27, 2014 at 06:46:29AM +0000, AKASHI Takahiro wrote:

On 11/26/2014 10:02 PM, Will Deacon wrote:

quoted

On Wed, Nov 26, 2014 at 04:49:47AM +0000, AKASHI Takahiro wrote:

quoted

If tracer modifies a syscall number to -1, this traced system call should
be skipped with a return value specified in x0.
This patch implements this semantics.

Please 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().

Signed-off-by: AKASHI Takahiro <redacted>
---
  arch/arm64/kernel/entry.S |   10 +++++++++-
  1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 726b910..946ec52 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -161,6 +161,7 @@
   */
  sc_nr	.req	x25		// number of system calls
  scno	.req	x26		// syscall number
+scno_w	.req	w26		// syscall number (lower 32 bits)
  stbl	.req	x27		// syscall table pointer
  tsk	.req	x28		// current thread_info

@@ -668,8 +669,14 @@ ENDPROC(el0_svc)
  	 * switches, and waiting for our parent to respond.
  	 */
  __sys_trace:
-	mov	x0, sp
+	cmp     scno_w, #-1			// set default errno for

I hate that we have to use scno_w, but the only alternative I can think of
is using w8 directly, which isn't any better and doesn't work for compat.
Ho-hum, I guess we'll stick with what you have.

The possible approaches might be:
* use 32-bit registers for scno & sc_nr and use "sxtw scno, w8," or
* use an extra reg like
     __sys_trace:
         mov x0, #0xffffffff

or even better: mov w0, #-1

         cmp scno, x0
         b.ne 1f

Good thinking, I prefer that.

Will