> On Mar 30, 2019, at 11:24 AM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> On Sat, Mar 30, 2019 at 10:12 AM Christian Brauner <christian@xxxxxxxxxx> 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
