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07c3ef5822
Pull clone and pidfs updates from Christian Brauner:
"Add three new clone3() flags for pidfd-based process lifecycle
management.
CLONE_AUTOREAP:
CLONE_AUTOREAP makes a child process auto-reap on exit without ever
becoming a zombie. This is a per-process property in contrast to
the existing auto-reap mechanism via SA_NOCLDWAIT or SIG_IGN for
SIGCHLD which applies to all children of a given parent.
Currently the only way to automatically reap children is to set
SA_NOCLDWAIT or SIG_IGN on SIGCHLD. This is a parent-scoped
property affecting all children which makes it unsuitable for
libraries or applications that need selective auto-reaping of
specific children while still being able to wait() on others.
CLONE_AUTOREAP stores an autoreap flag in the child's
signal_struct. When the child exits do_notify_parent() checks this
flag and causes exit_notify() to transition the task directly to
EXIT_DEAD. Since the flag lives on the child it survives
reparenting: if the original parent exits and the child is
reparented to a subreaper or init the child still auto-reaps when
it eventually exits. This is cleaner than forcing the subreaper to
get SIGCHLD and then reaping it. If the parent doesn't care the
subreaper won't care. If there's a subreaper that would care it
would be easy enough to add a prctl() that either just turns back
on SIGCHLD and turns off auto-reaping or a prctl() that just
notifies the subreaper whenever a child is reparented to it.
CLONE_AUTOREAP can be combined with CLONE_PIDFD to allow the parent
to monitor the child's exit via poll() and retrieve exit status via
PIDFD_GET_INFO. Without CLONE_PIDFD it provides a fire-and-forget
pattern. No exit signal is delivered so exit_signal must be zero.
CLONE_THREAD and CLONE_PARENT are rejected: CLONE_THREAD because
autoreap is a process-level property, and CLONE_PARENT because an
autoreap child reparented via CLONE_PARENT could become an
invisible zombie under a parent that never calls wait().
The flag is not inherited by the autoreap process's own children.
Each child that should be autoreaped must be explicitly created
with CLONE_AUTOREAP.
CLONE_NNP:
CLONE_NNP sets no_new_privs on the child at clone time. Unlike
prctl(PR_SET_NO_NEW_PRIVS) which a process sets on itself,
CLONE_NNP allows the parent to impose no_new_privs on the child at
creation without affecting the parent's own privileges.
CLONE_THREAD is rejected because threads share credentials.
CLONE_NNP is useful on its own for any spawn-and-sandbox pattern
but was specifically introduced to enable unprivileged usage of
CLONE_PIDFD_AUTOKILL.
CLONE_PIDFD_AUTOKILL:
This flag ties a child's lifetime to the pidfd returned from
clone3(). When the last reference to the struct file created by
clone3() is closed the kernel sends SIGKILL to the child. A pidfd
obtained via pidfd_open() for the same process does not keep the
child alive and does not trigger autokill - only the specific
struct file from clone3() has this property. This is useful for
container runtimes, service managers, and sandboxed subprocess
execution - any scenario where the child must die if the parent
crashes or abandons the pidfd or just wants a throwaway helper
process.
CLONE_PIDFD_AUTOKILL requires both CLONE_PIDFD and CLONE_AUTOREAP.
It requires CLONE_PIDFD because the whole point is tying the
child's lifetime to the pidfd. It requires CLONE_AUTOREAP because a
killed child with no one to reap it would become a zombie - the
primary use case is the parent crashing or abandoning the pidfd so
no one is around to call waitpid(). CLONE_THREAD is rejected
because autokill targets a process not a thread.
If CLONE_NNP is specified together with CLONE_PIDFD_AUTOKILL an
unprivileged user may spawn a process that is autokilled. The child
cannot escalate privileges via setuid/setgid exec after being
spawned. If CLONE_PIDFD_AUTOKILL is specified without CLONE_NNP the
caller must have have CAP_SYS_ADMIN in its user namespace"
* tag 'vfs-7.1-rc1.pidfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
selftests: check pidfd_info->coredump_code correctness
pidfds: add coredump_code field to pidfd_info
kselftest/coredump: reintroduce null pointer dereference
selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests
selftests/pidfd: add CLONE_NNP tests
selftests/pidfd: add CLONE_AUTOREAP tests
pidfd: add CLONE_PIDFD_AUTOKILL
clone: add CLONE_NNP
clone: add CLONE_AUTOREAP
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