Re: [PATCH] clone.2: add CLONE_PIDFD entry

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Christian,

On 9/19/19 8:47 AM, Christian Brauner wrote:
> On Thu, Sep 19, 2019 at 06:04:55AM +0200, Michael Kerrisk (man-pages) wrote:

[...]

>>>> Thanks for that info. One other questions springs to mind.
>>>> I haven't looked at the source or tried testing this,
>>>> but can anything actually be read() from a PIDFD? Presumably,
>>>
>>> We had discussed this but decided to not implement this right away.
>>> Mainly, because we did not have a clear picture what the semantics
>>> should be. But it is something that we will probably want in the
>>> future...
>>
>> That makes sense.
>>
>> A further question... We now have three ways of getting a
>> process file descriptor [*]:
>>
>> open() of /proc/PID
>> pidfd_open()
>> clone()/clone3() with CLONE_PIDFD
>>
>> I thought the FD was supposed to be equivalent in all three cases.
>> However, if I try (on kernel 5.3) poll() an FD returned by opening
>> /proc/PID, poll() tells me POLLNVAL for the FD. Is that difference
>> intentional? (I am guessing it is not.)
> 
> It's intentional.
> The short answer is that /proc/<pid> is a convenience for sending
> signals.
> The longer answer is that this stems from a heavy debate about what a
> process file descriptor was supposed to be and some people pushing for
> at least being able to use /proc/<pid> dirfds while ignoring security
> problems as soon as you're talking about returning those fds from
> clone(); not to mention the additional problems discovered when trying
> to implementing this.
> A "real" pidfd is one from CLONE_PIDFD or pidfd_open() and all features
> such as exit notification, read, and other future extensions will only
> be implemented on top of them.
> As much as we'd have liked to get rid of two different file descriptor
> types it doesn't hurt us much and is not that much different from what
> we will e.g. see with fsinfo() in the new mount api which needs to work
> on regular fds gotten via open()/openat() and mountfds gotten from
> fsopen() and fspick(). The mountfds will also allow for advanced
> operations that the other ones will not. There's even an argument to be
> made that fds you will get from open()/openat() and openat2() are
> different types since they have very different behavior; openat2()
> returning fds that are non arbitrarily upgradable etc.

Okay. So, it would be fair to say (in the man pages) that
pidfd_open() is the preferred way of obtaining a PID file
descriptor for an already existing process?

>> [*} By the way, going forward, can we call these things
>> "process FDs", rather than "PID FDs"? The API names are what
>> they are, an that's okay, but these just as we have socket
>> FDs that refer to sockets, directory FDs that refer to 
>> directories, and timer FDs that refer to timers, and so on,
>> these are FDs that refer to *processes*, not "process IDs".
>> It's a little thing, but I think the naming better, and
>> it's what I propose to use in the manual pages.
> 
> The naming was another debate and we ended with this compromise.
> I would just clarify that a pidfd is a process file descriptor. I
> wouldn't make too much of a deal of hiding the shortcut "pidfd". People
> are already using it out there in the wild and it's never proven a good
> idea to go against accepted practice.

Okay.

I have a draft pidfd_open(2) page that I will send out soon.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/