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

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:

quoted

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)

quoted

+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