seccomp: Filter flag to log all actions except SECCOMP_RET_ALLOW

seccomp: Filter flag to log all actions except SECCOMP_RET_ALLOW

Add a new filter flag, SECCOMP_FILTER_FLAG_LOG, that enables logging for
all actions except for SECCOMP_RET_ALLOW for the given filter.

SECCOMP_RET_KILL actions are always logged, when "kill" is in the
actions_logged sysctl, and SECCOMP_RET_ALLOW actions are never logged,
regardless of this flag.

This flag can be used to create noisy filters that result in all
non-allowed actions to be logged. A process may have one noisy filter,
which is loaded with this flag, as well as a quiet filter that's not
loaded with this flag. This allows for the actions in a set of filters
to be selectively conveyed to the admin.

Since a system could have a large number of allocated seccomp_filter
structs, struct packing was taken in consideration. On 64 bit x86, the
new log member takes up one byte of an existing four byte hole in the
struct. On 32 bit x86, the new log member creates a new four byte hole
(unavoidable) and consumes one of those bytes.

Unfortunately, the tests added for SECCOMP_FILTER_FLAG_LOG are not
capable of inspecting the audit log to verify that the actions taken in
the filter were logged.

With this patch, the logic for deciding if an action will be logged is:

if action == RET_ALLOW:
  do not log
else if action == RET_KILL && RET_KILL in actions_logged:
  log
else if filter-requests-logging && action in actions_logged:
  log
else if audit_enabled && process-is-being-audited:
  log
else:
  do not log

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Kees Cook <keescook@chromium.org>