On Sep 10, 2015 5:22 PM, "Tycho Andersen" [off-list ref] wrote:

This patch adds a way for a process that is "real root" to access the
seccomp filters of another process. The process first does a
PTRACE_SECCOMP_GET_FILTER_FD to get an fd with that process' seccomp filter
attached, and then iterates on this with PTRACE_SECCOMP_NEXT_FILTER using
bpf(BPF_PROG_DUMP) to dump the actual program at each step.

+
+       fd = bpf_new_fd(filter->prog, O_RDONLY);
+       if (fd > 0)
+               atomic_inc(&filter->prog->aux->refcnt);

Why isn't this folded into bpf_new_fd?

+
+       return fd;
+}
+
+long seccomp_next_filter(struct task_struct *child, u32 fd)
+{
+       struct seccomp_filter *cur;
+       struct bpf_prog *prog;
+       long ret = -ESRCH;
+
+       if (!capable(CAP_SYS_ADMIN))
+               return -EACCES;
+
+       if (child->seccomp.mode != SECCOMP_MODE_FILTER)
+               return -EINVAL;
+
+       prog = bpf_prog_get(fd);
+       if (IS_ERR(prog)) {
+               ret = PTR_ERR(prog);
+               goto out;
+       }
+
+       for (cur = child->seccomp.filter; cur; cur = cur->prev) {
+               if (cur->prog == prog) {
+                       if (!cur->prev)
+                               ret = -ENOENT;
+                       else
+                               ret = bpf_prog_set(fd, cur->prev->prog);

This lets you take an fd pointing to one prog and point it elsewhere.
I'm not sure that's a good idea.

--Andy