On Thu, Nov 20, 2014 at 05:52:34AM +0000, AKASHI Takahiro wrote:

On 11/20/2014 02:13 PM, AKASHI Takahiro wrote:

quoted

On 11/20/2014 04:06 AM, Will Deacon wrote:

quoted

Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
whereas it would usually see whatever the trace put in x0, right?

Yes.
If you don't really like this behavior, how about this patch instead of my [2/6] patch?

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 726b910..1ef57d0 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

@@ -668,8 +668,15 @@ ENDPROC(el0_svc)
          * switches, and waiting for our parent to respond.
          */
  __sys_trace:
+       cmp     w8, #-1                         // default errno for invalid

I needed to correct the code here:
w8 should be w26, thinking of compat syscalls.

quoted

+       b.ne    1f                              // system call
+       mov     x0, #-ENOSYS
+       str     x0, [sp, #S_X0]
+1:

and this part might better be generalized like the following:

__sys_trace:
	cmp	w26, w25	// cannot use x26 and x25 here
	b.hs	1f		// scno > sc_nr || scno < 0
	b	2f
1:
	mov	x0, #-ENOSYS
	str	x0, [sp, #S_X0]
2:

If you will be comfortable, I will submit a new patch soon.

Yes, please send a new series including this change.

Will