Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Mon, Oct 29, 2018 at 3:40 PM, Tycho Andersen [off-list ref] wrote:

    * switch to a flags based future-proofing mechanism for struct
      seccomp_notif and seccomp_notif_resp, thus avoiding version issues
      with structure length (Kees)

[...]

+struct seccomp_notif {
+       __u64 id;
+       __u32 pid;
+       __u32 flags;
+       struct seccomp_data data;
+};
+
+struct seccomp_notif_resp {
+       __u64 id;
+       __s64 val;
+       __s32 error;
+       __u32 flags;
+};

Hrm, so, what's the plan for when struct seccomp_data changes size?
I'm realizing that it might be "too late" for userspace to discover
it's running on a newer kernel. i.e. it gets a user notification, and
discovers flags it doesn't know how to handle. Do we actually need
both flags AND a length? Designing UAPI is frustrating! :)

Do we need another ioctl to discover the seccomp_data size maybe?

-- 
Kees Cook