Re: [PATCH v2 0/5] pid: add pidfd_open()

[Andy Lutomirski <luto@amacapital.net>]

> On Mar 30, 2019, at 11:24 AM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>> On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@brauner.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