seccomp_user_notif.2: EXAMPLE: ensure path read() by the supervisor is null-terminated

seccomp_user_notif.2: EXAMPLE: ensure path read() by the supervisor is null-terminated

From a conversation with Jann Horn:

    >> We should probably make sure here that the value we read is actually
    >> NUL-terminated?
    >
    > So, I was curious about that point also. But, (why) are we not
    > guaranteed that it will be NUL-terminated?

    Because it's random memory filled by another process, which we don't
    necessarily trust. While seccomp notifiers aren't usable for applying
    *extra* security restrictions, the supervisor will still often be more
    privileged than the supervised process.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>