Re: [PATCH] file: always lock position

[Christian Brauner <brauner@kernel.org>]

On Mon, Jul 24, 2023 at 08:53:32AM -0700, Linus Torvalds wrote:
> Is it too late to just fix pidfd_getfd() to duplicate the 'struct
> file', and act like a new open, and act like /proc/<pid>/fd/<xyz>?

So thinking a little about it I think that doesn't work.
/proc/<pid>/fd/<xyz> does a reopen and for good reasons. The original
open will have gone through the module's/subsytem's ->open() method
which might stash additional refcounted data in e.g., file->private_data
if we simply copy that file or sm then we risk UAFs. If we don't skip
->open() though and effectively emulate /proc/<pid>/fd/<xyz> completely
then we break any such use-cases where a socket or something else is
shared as they cannot be reopened. So the module/subsystem really needs
to be informed that a new struct file is created and not simply refd.