On Wed, Jul 16, 2014 at 1:21 PM, Kees Cook [off-list ref] wrote:

On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski [off-list ref] wrote:

quoted

+
+               ret = seccomp_phase1(&sd);
+               if (ret == SECCOMP_PHASE1_SKIP) {
+                       regs->orig_ax = -ENOSYS;

Before, seccomp didn't touch orig_ax on a skip. I don't see any
problem with this, and it's probably more clear this way, but are you
sure there aren't unexpected side-effects from this?

It's necessary to cause the syscall to be skipped -- see syscall_trace_enter.

That being said, setting it to -ENOSYS is nonsense and probably
confused you at least as much as it confused me.  It should be
regs->orig_ax = -1.

--Andy