anonfd: split interface into file creation and install

Split the anonfd interface into a bare file pointer creation one, and a
file pointer creation plus install one.

There are cases, like the usage of eventfds inside other kernel
interfaces, where the file pointer created by anonfd needs to be used
inside the initialization of other structures.

As it is right now, as soon as anon_inode_getfd() returns, the kenrle can
race with userspace closing the newly installed file descriptor.

This patch, while keeping the old anon_inode_getfd(), introduces a new
anon_inode_getfile() (whose services are reused in anon_inode_getfd())
that allows to split the file creation phase and the fd install one.

Once all the kernel structures are initialized, the code can call the
proper fd_install().

Gregory manifested the need for something like this inside KVM.

Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: James Morris <jmorris@namei.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Gregory Haskins <ghaskins@novell.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index 3b85ba6479f..94dd10366a7 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -27,6 +27,7 @@
 
 #ifdef CONFIG_EVENTFD
 
+struct file *eventfd_file_create(unsigned int count, int flags);
 struct eventfd_ctx *eventfd_ctx_get(struct eventfd_ctx *ctx);
 void eventfd_ctx_put(struct eventfd_ctx *ctx);
 struct file *eventfd_fget(int fd);
@@ -40,6 +41,11 @@ int eventfd_signal(struct eventfd_ctx *ctx, int n);
  * Ugly ugly ugly error layer to support modules that uses eventfd but
  * pretend to work in !CONFIG_EVENTFD configurations. Namely, AIO.
  */
+static inline struct file *eventfd_file_create(unsigned int count, int flags)
+{
+	return ERR_PTR(-ENOSYS);
+}
+
 static inline struct eventfd_ctx *eventfd_ctx_fdget(int fd)
 {
 	return ERR_PTR(-ENOSYS);