On Mon, Sep 23, 2019 at 10:11:45AM +0200, Michael Kerrisk (man-pages) wrote: > 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? Sure, or just not make a big thing about /proc/<pid> being useable. > > >> [*} 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! I will review it soon! Christian
