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Date:   Sun, 31 Mar 2019 14:09:03 -0600
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian@...uner.io>,
        Daniel Colascione <dancol@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Jonathan Kowalski <bl0pbl33p@...il.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Nagarathnam Muthusamy <nagarathnam.muthusamy@...cle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH v2 0/5] pid: add pidfd_open()



> On Mar 30, 2019, at 11:24 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@...uner.io> wrote:
>> 
>> 
>> To clarify, what the Android guys really wanted to be part of the api is
>> a way to get race-free access to metadata associated with a given pidfd.
>> And the idea was that *if and only if procfs is mounted* you could do:
>> 
>> int pidfd = pidfd_open(1234, 0);
>> 
>> int procfd = open("/proc", O_RDONLY | O_CLOEXEC);
>> int procpidfd = ioctl(pidfd, PIDFD_TO_PROCFD, procfd);
> 
> And my claim is that this is three system calls - one of them very
> hacky - to just do
> 
>    int pidfd = open("/proc/%d", O_PATH);

Hi Linus-

I want to re-check this because I think Christian’s example was bad.  I proposed these ioctls, but that wasn’t the intended use.  The real point is:

int pidfd = new_improved_clone(...);

To be useful, this type of API *must* work without proc mounted.

And, later:

openat(fd to pidfd’s proc directory, “status”, ...);

And we want a non-utterly-crappy way to do this.  The ioctl is certainly ugly, but it *works*.

Another approach is:

pid_t pid = pidfd_get_pid(pidfd);
sprintf(buf, “/proc/%d”, pid);
int procfd = open(buf, O_PATH);
if (pidfd_get_pid(pidfd) != pid) {
  we lose;
}

But this is clunky.

Do you think the clunky version is okay, or do you have a suggestion for making it better?

—Andy

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