linux-kernel - Re: For review: pidfd

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Message-ID: <87tv939td6.fsf@mid.deneb.enyo.de>
Date:   Mon, 23 Sep 2019 12:53:09 +0200
From:   Florian Weimer <fw@...eb.enyo.de>
To:     "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Cc:     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

* 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.

>        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.

>        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.