Re: [PATCH 2/4] pid: add pidfd_open()

[Christian Brauner <christian@brauner.io>]

On Wed, Mar 27, 2019 at 06:07:54PM +0100, Jann Horn wrote:
> On Wed, Mar 27, 2019 at 5:22 PM Christian Brauner <christian@brauner.io> wrote:
> > pidfd_open() allows to retrieve pidfds for processes and removes the
> > dependency of pidfd on procfs. Multiple people have expressed a desire to
> > do this even when pidfd_send_signal() was merged. It is even recorded in
> [...]
> > IF PROCFD_TO_PIDFD is passed as a flag together with a file descriptor to a
> > /proc mount in a given pid namespace and a pidfd pidfd_open() will return a
> > file descriptor to the corresponding /proc/<pid> directory in procfs
> > mounts' pid namespace. pidfd_open() is very careful to verify that the pid
> 
> nit: s/mounts'/mount's/

Thanks.

> 
> > hasn't been recycled in between.
> > IF PIDFD_TO_PROCFD is passed as a flag together with a file descriptor
> > referencing a /proc/<pid> directory a pidfd referencing the struct pid
> > stashed in /proc/<pid> of the process will be returned.
> 
> nit: s/of the process //?

Yes.

> 
> > The pidfd_open() syscalls in that manner resembles openat() as it uses a
> 
> nit: s/syscalls/syscall/

Thanks.

> 
> [...]
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 20881598bdfa..c9e24e726aba 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> [...]
> > +static struct file *pidfd_open_proc_pid(const struct file *procf, pid_t pid,
> > +                                       const struct pid *pidfd_pid)
> > +{
> > +       char name[11]; /* int to strlen + \0 */
> 
> nit: The comment is a bit off; an unconstrained int needs 1+10+1
> bytes, I think? minus sign, 10 digits, nullbyte? But of course that
> can't actually happen here.

Yes, the comment is misleading.

> 
> > +       struct file *file;
> > +       struct pid *proc_pid;
> > +
> > +       snprintf(name, sizeof(name), "%d", pid);
> > +       file = file_open_root(procf->f_path.dentry, procf->f_path.mnt, name,
> > +                             O_DIRECTORY | O_NOFOLLOW, 0);
> 
> Maybe explicitly write the implied O_RDONLY (which is 0) here for clarity?

Yes.

> 
> [...]
> > +static int pidfd_to_procfd(pid_t pid, int procfd, int pidfd)
> > +{
> > +       long fd;
> > +       pid_t ns_pid;
> > +       struct fd fdproc, fdpid;
> > +       struct file *file = NULL;
> > +       struct pid *pidfd_pid = NULL;
> > +       struct pid_namespace *proc_pid_ns = NULL;
> > +
> > +       fdproc = fdget(procfd);
> > +       if (!fdproc.file)
> > +               return -EBADF;
> > +
> > +       fdpid = fdget(pidfd);
> > +       if (!fdpid.file) {
> > +               fdput(fdpid);
> 
> Typo: s/fdput(fdpid)/fdput(fdproc)/

Good catch!

> 
> [...]
> > +SYSCALL_DEFINE4(pidfd_open, pid_t, pid, int, procfd, int, pidfd, unsigned int,
> > +               flags)
> [...]
> > +       if (!flags) {
> [...]
> > +               rcu_read_lock();
> > +               pidfd_pid = get_pid(find_pid_ns(pid, task_active_pid_ns(current)));
> > +               rcu_read_unlock();
> 
> The previous three lines are equivalent to `pidfd_pid = find_get_pid(pid)`.

Perfect, will replace.

> 
> > +               fd = pidfd_create_fd(pidfd_pid, O_CLOEXEC);
> 
> Nit: You could hardcode O_CLOEXEC in pidfd_create_fd() and get rid of
> the second function argument if you want to.

Hm, let me rename this to pidfd_create_cloexec(pidfd_pid) then.

> 
> > +               put_pid(pidfd_pid);
> > +       } else if (flags & PIDFD_TO_PROCFD) {
> [...]
> > +               fd = pidfd_to_procfd(pid, procfd, pidfd);
> 
> The `pid` argument of pidfd_to_procfd() looks unused, maybe it makes
> sense to get rid of that?

Yes.