Re: [PATCH v5 3/3] arm64: Add seccomp support

[Will Deacon]

On Tue, Aug 12, 2014 at 12:17:53PM +0100, AKASHI Takahiro wrote:
> On 08/12/2014 06:40 PM, Will Deacon wrote:
> > On Tue, Aug 12, 2014 at 07:57:25AM +0100, AKASHI Takahiro wrote:
> >>
> >>   >     case SECCOMP_MODE_FILTER:
> >>   >         case SECCOMP_RET_TRACE:
> >>   >             ...
> >>   >             if (syscall_get_nr(current, regs) < 0)
> >>   >                 goto skip;
> >>
> >> This implies that we should modify syscallno *before* __secure_computing()
> >> returns.
> >
> > Why does it imply that? There are four competing entities here:
> >
> >   - seccomp
> >   - tracehook
> >   - ftrace (trace_sys_*)
> >   - audit
> >
> > With the exception of ftrace, they can all potentially rewrite the pt_regs
> > (the code you cite above is just below a ptrace_event call), so we have
> > to choose some order in which to call them.
> 
> (audit won't change registers.)

Sorry, you're quite right.

> > On entry, x86 and arm call them in the order I listed above, so it seems
> > sensible to follow that.
> 
> Right, but as far as I understand, ptrace_event() in __secure_computing()
> calls ptrace_notify(), and eventually executes ptrace_stop(), which can
> be stopped while tracer runs (until ptrace(PTRACE_CONT)?).
> So syscall_get_nr() is expected to return -1 if trace changes a syscall number to -1
> (as far as sycall_get_nr() refers to syscallno in pt_regs).
> 
> That is why I think we should have PTRACE_SET_SYSCALL.

Gotcha, yeah that looks like the cleanest approach after all. Thanks for the
explanation.

Will
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