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

[Christian Brauner]

On Sat, Mar 30, 2019 at 10:04:33AM -0700, Linus Torvalds wrote:
> On Sat, Mar 30, 2019 at 9:34 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> >
> > Just to be clear, I'm not proposing granting secret access to procfs,
> > and as far as I can see, nobody else is either. We've been talking
> > about making it easier to avoid races when you happen to want a pidfd
> > and a procfs fd that point to the same process
> 
> So I thought that was the whole point of just opening /proc/<pid>.
> Exactly because that way you can then use openat() from there on.

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 then we internally verify that the struct pid that the pidfd is
refering to, is still the same as the one that /proc/<pid> is refering
to and only then do we return an fd for the process /proc/<pid>
directory which would then allow you to do, e.g.:

int statusfd = openat(procpidfd, "status", O_RDONLY | O_CLOEXEC);

this would provide race-free access to metadat but again, only if /proc
is mounted and available to the user. But if that's an instant NAK we
will definitely *not* do this.