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Add '-p' flag to get signalled when parent dies #114

Merged
merged 3 commits into from
Apr 21, 2018
Merged

Add '-p' flag to get signalled when parent dies #114

merged 3 commits into from
Apr 21, 2018

Commits on Apr 19, 2018

  1. Add '-p' flag to get signalled when parent dies 

    Add a new flag '-p', which sets up the parent death signal to `SIGKILL`.
This will cause the kernel to send us a `SIGKILL` as soon as the direct
parent process dies. This is useful e.g. in combination with unshare(1)
from util-linux when using PID namespaces. When unshare forks the child,
which is about to become PID 1, killing the unshare parent will not
cause the child to exit. When executing the command

    $ unshare --pid --fork tini -- <prog>

then killing unshare will not cause tini to be killed. Since util-linux
v2.32, unshare has an option "--kill-child=<SIGNAL>" that will set up
the parent death signal for the forked process. This does not help
though in case either SELinux or AppArmor are in use and credentials of
the forked process change (e.g. by changing its UID), as these LSMs will
clear the parent death signal again. The following example would trigger
that situation:

    $ unshare --pid --fork setpriv --reuid user tini -s -- <prog>

The parent death signal will get reset by the LSMs as soon as `setpriv`
switchets its user ID to that of "user", and killing unshare will again
not result in tini being killed. The new '-p' flag helps that exact
scenario:

    $ unshare --pid --fork setpriv --reuid user tini -s -p SIGKILL -- <prog>

As soon as unshare is getting killed, tini will get signalled SIGKILL
and exit as well, tearing down <prog> with it.
    @pks-t
    pks-t committed Apr 19, 2018
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  2. Exercise pdeathsignal in tests 

    See: #114 (comment)
    @krallin @pks-t
    krallin authored and pks-t committed Apr 19, 2018
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  3. Update README for parent death signal

    @krallin
    krallin committed Apr 19, 2018
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