Re: [PATCH 1/2] pidfd_create(): remove static qualifier and declare pidfd_create() in linux/pid.h

[Matthew Bobrowski <repnop@google.com>]

On Mon, Apr 19, 2021 at 02:50:33PM +0200, Christian Brauner wrote:
> On Fri, Apr 16, 2021 at 09:22:09AM +1000, Matthew Bobrowski wrote:
> > With the idea to have fanotify(7) return pidfds within a `struct
> > fanotify_event_metadata`, pidfd_create()'s scope is to increased so
> > that it can be called from other subsystems within the Linux
> > kernel. The current `static` qualifier from its definition is to be
> > removed and a new function declaration for pidfd_create() is to be
> > added to the linux/pid.h header file.
> > 
> > Signed-off-by: Matthew Bobrowski <repnop@google.com>
> > ---
> >  include/linux/pid.h | 1 +
> >  kernel/pid.c        | 2 +-
> >  2 files changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/pid.h b/include/linux/pid.h
> > index fa10acb8d6a4..af308e15f174 100644
> > --- a/include/linux/pid.h
> > +++ b/include/linux/pid.h
> > @@ -78,6 +78,7 @@ struct file;
> >  
> >  extern struct pid *pidfd_pid(const struct file *file);
> >  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
> > +int pidfd_create(struct pid *pid, unsigned int flags);
> >  
> >  static inline struct pid *get_pid(struct pid *pid)
> >  {
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index ebdf9c60cd0b..91c4b6891c15 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -553,7 +553,7 @@ struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
> >   * Return: On success, a cloexec pidfd is returned.
> >   *         On error, a negative errno number will be returned.
> >   */
> > -static int pidfd_create(struct pid *pid, unsigned int flags)
> 
> > +int pidfd_create(struct pid *pid, unsigned int flags)
> 
> Can you please add a comment to the kernel doc mentioning that this
> helper is _not_ intended to be exported to modules? I don't want drivers
> to get the idea that it's ok to start returning pidfds from everywhere
> just yet.

Sure, this makes sense to me. 
 
> And I think we should add sm like
> 
> 	if (flags & ~(O_NONBLOCK | O_CLOEXEC | O_RDWR))
> 		return -EINVAL;
> 
> in pidfd_open() so future callers don't accidently create pidfds with
> random flags we don't support.

In the context of exporting pidfd_create() to the rest of the kernel,
presumably we should be adding this flag check to pidfd_create() and
not pidfd_open(), right? I gather that's what you actually meant.

/M