Current implementation of seccomp.IsEnabled (rooted in runc) is not
too good.
First, it parses the whole /proc/self/status, adding each key: value
pair into the map (lots of allocations and future work for garbage
collector), when using a single key from that map.
Second, the presence of "Seccomp" key in /proc/self/status merely means
that kernel option CONFIG_SECCOMP is set, but there is a need to _also_
check for CONFIG_SECCOMP_FILTER (the code for which exists but never
executed in case /proc/self/status has Seccomp key).
Replace all this with a single call to prctl; see the long comment in
the code for details.
While at it, improve the IsEnabled documentation.
NOTE historically, parsing /proc/self/status was added after a concern
was raised in https://github.com/opencontainers/runc/pull/471 that
prctl(PR_GET_SECCOMP, ...) can result in the calling process being
killed with SIGKILL. This is a valid concern, so the new code here
does not use PR_GET_SECCOMP at all.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
