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Message-Id: <132107F4-F56B-4D6E-9E00-A6F7C092E6BD@amacapital.net>
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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