linux-kernel - Re: For review: pidfd

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Message-ID: <20190923144711.ssbrg6bdquhewo7q@wittgenstein>
Date:   Mon, 23 Sep 2019 16:47:13 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        Daniel Colascione <dancol@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Linux API <linux-api@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: For review: pidfd_open(2) manual page

On Mon, Sep 23, 2019 at 12:53:09PM +0200, Florian Weimer wrote:
> * Michael Kerrisk:
> 
> > SYNOPSIS
> >        int pidfd_open(pid_t pid, unsigned int flags);
> 
> Should this mention <sys/types.h> for pid_t?
> 
> > ERRORS
> >        EINVAL flags is not 0.
> >
> >        EINVAL pid is not valid.
> >
> >        ESRCH  The process specified by pid does not exist.
> 
> Presumably, EMFILE and ENFILE are also possible errors, and so is
> ENOMEM.

So, error codes that could surface are:
EMFILE: too many open files
ENODEV: the anon inode filesystem is not available in this kernel (unlikely)
ENOMEM: not enough memory (to allocate the backing struct file)
ENFILE: you're over the max_files limit which can be set through proc

I think that should be it.

> 
> >        A  PID  file descriptor can be monitored using poll(2), select(2),
> >        and epoll(7).  When the process that it refers to terminates,  the
> >        file descriptor indicates as readable.  Note, however, that in the
> >        current implementation, nothing can be read from the file descrip‐
> >        tor.
> 
> “is indicated as readable” or “becomes readable”?  Will reading block?
> 
> >        The  pidfd_open()  system call is the preferred way of obtaining a
> >        PID file descriptor.  The alternative is to obtain a file descrip‐
> >        tor by opening a /proc/[pid] directory.  However, the latter tech‐
> >        nique is possible only if the proc(5) file system is mounted; fur‐
> >        thermore,  the  file  descriptor  obtained in this way is not pol‐
> >        lable.
> 
> One question is whether the glibc wrapper should fall back back to the
> /proc subdirectory if it is not available.  Probably not.

No, that would not be transparent to userspace. Especially because both
fds differ in what can be done with them.

> 
> >        static
> >        int pidfd_open(pid_t pid, unsigned int flags)
> >        {
> >            return syscall(__NR_pidfd_open, pid, flags);
> >        }
> 
> Please call this function something else (not pidfd_open), so that the
> example continues to work if glibc provides the system call wrapper.

Agreed!