Re: [RFC PATCH] Minimal non-child process exit notification support

[Daniel Colascione <dancol@google.com>]

On Wed, Oct 31, 2018 at 12:27 PM, David Laight <David.Laight@aculab.com> wrote:
> From: Daniel Colascione
>> Sent: 29 October 2018 17:53
>>
>> This patch adds a new file under /proc/pid, /proc/pid/exithand.
>> Attempting to read from an exithand file will block until the
>> corresponding process exits, at which point the read will successfully
>> complete with EOF.  The file descriptor supports both blocking
>> operations and poll(2). It's intended to be a minimal interface for
>> allowing a program to wait for the exit of a process that is not one
>> of its children.
>
> Why do you need an extra file?

Because no current file suffices.

> It ought to be possible to use poll() to wait for POLLERR having set
> 'events' to zero on any of the nodes in /proc/pid - or even on
> the directory itself.

That doesn't actually work today. And waiting on a directory with
POLLERR would be very weird, since directories in general don't do
things like blocking reads or poll support. A separate file with
self-contained, well-defined semantics is cleaner.

> Indeed, to avoid killing the wrong process you need to have opened
> some node of /proc/pid/* (maybe cmdline) before sending the kill
> signal.

The kernel really needs better documentation of the semantics of
procfs file descriptors. You're not the only person to think,
mistakenly, that keeping a reference to a /proc/$PID/something FD
reserves $PID and prevents it being used for another process. Procfs
FDs do no such thing. kill(2) is unsafe whether or not
/proc/pid/cmdline or any other /proc file is open.