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Date: Mon, 6 May 2019 12:45:14 +0200
From: Jann Horn <>
To: Solar Designer <>
Cc: Kernel Hardening <>, 
	Christian Brauner <>
Subject: Re: race-free process signaling

+cc Christian Brauner (author of pidfd code)

On Mon, May 6, 2019 at 12:22 PM Solar Designer <> wrote:
> I totally missed the recent work in this area (I'm not on LKML),

FWIW, you don't usually need to actually read LKML to see when major
developments happen - I read LWN, which seems to work pretty well for
that purpose.

> and am
> now wondering whether the solution that got in ("use /proc/<pid> fds as
> stable handles on struct pid"):
> is better or worse than what I had proposed in 1999 and 2005 ("locking"
> of pids for the caller's own visibility only):
> [Subject starts with "PID reuse safety for userspace apps", in case MARC
> is ever gone and someone wants to look this up in another archive.

(The kernel people now have as an email archive, which
is much nicer to use IMO - it has search, it has a nice thread view,
and you can download raw mbox files if you want to reply to a mail:

> I proposed a lockpid syscall back then, but I'd use a mere prctl now.]
> I still like my proposal much better - no dependency on procfs, much
> simpler implementation - but perhaps I'm missing the context here.

Actually, there is ongoing development of pidfd stuff, including
procfs-less pidfds. You may want to look through
<>, or something like that.

The following series adds anon-inode-based pidfds that can be returned
from sys_clone():
"[PATCH v3 2/4] clone: add CLONE_PIDFD"
"[PATCH v3 3/4] signal: support CLONE_PIDFD with pidfd_send_signal"

This patch adds process exit notifications that can be received
through the normal file polling syscalls (epoll/poll/select/...):
"[PATCH v2 1/2] Add polling support to pidfd"

> Maybe I should have sent a patch back then.  Oh well.

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