[Devel] lxc-start: Invalid argument - failed to remove CAP_SYS_BOOT capability

[Michael Tokarev]

The message in $subj is displayed (and the utility fails) when
trying to start a container on any of my systems.  I traced it
to failing prctl(PR_CAPBSET_DROP, CAP_SYS_BOOT).  According to
the manpage:

   The call fails with the error: EPERM if the calling thread does
   not have the CAP_SETPCAP; EINVAL if arg2 does not represent a
   valid capability; or EINVAL if file capabilities are not
   enabled in the kernel, in which case bounding sets are not sup‐
   ported.

and the corresponding kernel config is SECURITY_FILE_CAPABILITIES,
which is in "Security options" menu named "File POSIX Capabilities".

This is a config option that's not checked by lxc-checkconfig, but
since not setting it entirely prevents lxc from working, I think it
should be checked too.  In any way, I don't think I've seen any
references to that option anywhere.

So here it goes, if not only for reference so that others who will
come to this issue in the future will know what to do.

Thanks!

/mjt
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