memfd: export alloc_file()

Patch series "mm: memfd_luo hotfixes".

This series contains a couple of fixes for memfd preservation using LUO.


This patch (of 3):

The Live Update Orchestrator's (LUO) memfd preservation works by
preserving all the folios of a memfd, re-creating an empty memfd on the
next boot, and then inserting back the preserved folios.

Currently it creates the file by directly calling shmem_file_setup().
This leaves out other work done by alloc_file() like setting up the file
mode, flags, or calling the security hooks.

Export alloc_file() to let memfd_luo use it. Rename it to
memfd_alloc_file() since it is no longer private and thus needs a
subsystem prefix.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Pratyush Yadav (Google) <[e-mail]>
Reviewed-by: Mike Rapoport (Microsoft) <[e-mail]>
Reviewed-by: Pasha Tatashin <[e-mail]>
Cc: Baolin Wang <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/memfd.h

···
  * to by vm_flags_ptr.
  */
 int memfd_check_seals_mmap(struct file *file, vm_flags_t *vm_flags_ptr);
+struct file *memfd_alloc_file(const char *name, unsigned int flags);
 #else
 static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned int a)
 {
···
 					 vm_flags_t *vm_flags_ptr)
 {
 	return 0;
+}
+
+static inline struct file *memfd_alloc_file(const char *name, unsigned int flags)
+{
+	return ERR_PTR(-EINVAL);
 }
 #endif
 

mm/memfd.c

···
 	return ERR_PTR(error);
 }
 
-static struct file *alloc_file(const char *name, unsigned int flags)
+struct file *memfd_alloc_file(const char *name, unsigned int flags)
 {
 	unsigned int *file_seals;
 	struct file *file;
···
 		return PTR_ERR(name);
 
 	fd_flags = (flags & MFD_CLOEXEC) ? O_CLOEXEC : 0;
-	return FD_ADD(fd_flags, alloc_file(name, flags));
+	return FD_ADD(fd_flags, memfd_alloc_file(name, flags));
 }