[Bug libc/26371] New: [RFE] please add clone3() wrapper (in particular the CLONE_INTO_CGROUP feature of it)

["mztyvop at 0pointer dot net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26371

            Bug ID: 26371
           Summary: [RFE] please add clone3() wrapper (in particular the
                    CLONE_INTO_CGROUP feature of it)
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: mztyvop at 0pointer dot net
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

In systemd we'd really like to make use of the new CLONE_INTO_CGROUP kernel
feature available for clone3(). It would allows us removing some really ugly
code in systemd: right now whenever PID 1 forks off a service process we
manually migrate the process into the destination cgroup and we do this twice
(once from the parent/PID 1 and once from the child, since we need to be sure
the migration is complete before we continue in either). 

Unfortunately I don't see how we can properly make use of this in systemd,
without glibc providing some form of support for clone3() for us.

The problem is that if we issue clone3() with our own wrapper, glibc won't know
about the call. Then, when in the child we issue setgroups(), setuid() and so
on, the setxuid propagation stuff in glibc tries to propagate these credential
changes to other threads — threads that don't actually exist in the child...

Since issuing setuid() in some form is very typical during service invocation
this means this pretty systematically deadlocks if we use clone3(). And that
sucks.

I am not aware of any work-around for this: we cannot tell glibc that a
clone/fork has happened, we cannot tell glibc that all threads that existed in
the parent suddenly vanished. 

Hence, it would be great if glibc would gain a clone3() wrapper. Or
alternatively some way how we can turn off setxid propagation in the child? Or
some way to tell glibc that a clone/fork/clone3 happened and it should forget
about all its thread info?

Here's a systemd PR that implements things in systemd with a manual wrapper for
clone3(), and that then triggers the setxid deadlock as mentioned in the child:

https://github.com/systemd/systemd/pull/16706

Ideas, suggestions how to deal with this with current glibc would be very
welcome. I'd be quite sad if we'd have to wait for glibc() to wrap clone3()
natively to make use of this new concept.

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