[PATCH v5 3/3] arm64: Add seccomp support
will.deacon at arm.com
Tue Aug 12 02:40:58 PDT 2014
On Tue, Aug 12, 2014 at 07:57:25AM +0100, AKASHI Takahiro wrote:
> On 08/11/2014 06:24 PM, Will Deacon wrote:
> > On Fri, Aug 08, 2014 at 08:35:42AM +0100, AKASHI Takahiro wrote:
> >> As discussed in a few weeks ago, aarch64 won't support PTRACE_SET_SYSCALL.
> > Well, I don't think anything was set in stone. If you have a compelling
> > reason why adding the new request gives you something over setting w8
> > directly, then we can extend ptrace.
> Yeah, I think I may have to change my mind. Looking into __secure_computing(),
> I found the code below:
> > 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()
Why does it imply that? There are four competing entities here:
- ftrace (trace_sys_*)
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.
On entry, x86 and arm call them in the order I listed above, so it seems
sensible to follow that.
> I assumed, in my next version, we could skip a system call by overwriting
> syscallno with x8 in syscall_trace_enter() after __secure_computing()
> returns 0, and it actually works.
Why does overwriting the syscallno with x8 skip the syscall?
I thought the idea was that we would save w8 prior to each call that could
change the pt_regs, then if it was changed to -1 we would replace it with
the saved value and return -1? The only confusion I have is whether we
should call the exit hooks after skipping a syscall. I *think* x86 does
call them, but ARM doesn't. Andy says this can trigger an OOPs:
so we should fix that for ARM while we're here.
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