[PATCH v5 0/6] seccomp: add PR_SECCOMP_EXT and SECCOMP_EXT_ACT_TSYNC

[Kees Cook]

This adds the ability for threads to request seccomp filter
synchronization across their thread group (either at filter attach time
or later). (For example, for Chrome to make sure graphic driver threads
are fully confined after seccomp filters have been attached.)

To support this, seccomp locking on writes is introduced, along with
refactoring of no_new_privs. Races with thread creation are handled via
the tasklist_list.

I think all the concerns raised during the discussion[1] of the first
version of this patch have been addressed. However, the races involved
have tricked me before. :)

Thanks!

-Kees

[1] https://lkml.org/lkml/2014/1/13/795

v5:
 - move includes around (drysdale)
 - drop set_nnp return value (luto)
 - use smp_load_acquire/store_release (luto)
 - merge nnp changes to seccomp always, fewer ifdef (luto)
v4:
 - cleaned up locking further, as noticed by David Drysdale
v3:
 - added SECCOMP_EXT_ACT_FILTER for new filter install options
v2:
 - reworked to avoid clone races

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