expose a low-level variant of fd_install() for binder

Similar situation to that of __alloc_fd(); do not use unless you
really have to.  You should not touch any descriptor table other
than your own; it's a sure sign of a really bad API design.

As with __alloc_fd(), you *must* use a first-class reference to
struct files_struct; something obtained by get_files_struct(some task)
(let alone direct task->files) will not do.  It must be either
current->files, or obtained by get_files_struct(current) by the
owner of that sucker and given to you.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/staging/android/binder.c

@@ -386,17 +386,8 @@ int task_get_unused_fd_flags(struct binder_proc *proc, int flags)
 static void task_fd_install(
 	struct binder_proc *proc, unsigned int fd, struct file *file)
 {
-	struct files_struct *files = proc->files;
-	struct fdtable *fdt;
-
-	if (files == NULL)
-		return;
-
-	spin_lock(&files->file_lock);
-	fdt = files_fdtable(files);
-	BUG_ON(fdt->fd[fd] != NULL);
-	rcu_assign_pointer(fdt->fd[fd], file);
-	spin_unlock(&files->file_lock);
+	if (proc->files)
+		__fd_install(proc->files, fd, file);
 }
 
 /*

fs/file.c

@@ -599,11 +599,18 @@ EXPORT_SYMBOL(put_unused_fd);
  *
  * It should never happen - if we allow dup2() do it, _really_ bad things
  * will follow.
+ *
+ * NOTE: __fd_install() variant is really, really low-level; don't
+ * use it unless you are forced to by truly lousy API shoved down
+ * your throat.  'files' *MUST* be either current->files or obtained
+ * by get_files_struct(current) done by whoever had given it to you,
+ * or really bad things will happen.  Normally you want to use
+ * fd_install() instead.
  */
 
-void fd_install(unsigned int fd, struct file *file)
+void __fd_install(struct files_struct *files, unsigned int fd,
+		struct file *file)
 {
-	struct files_struct *files = current->files;
 	struct fdtable *fdt;
 	spin_lock(&files->file_lock);
 	fdt = files_fdtable(files);
@@ -612,4 +619,9 @@ void fd_install(unsigned int fd, struct file *file)
 	spin_unlock(&files->file_lock);
 }
 
+void fd_install(unsigned int fd, struct file *file)
+{
+	__fd_install(current->files, fd, file);
+}
+
 EXPORT_SYMBOL(fd_install);

include/linux/fdtable.h

@@ -121,6 +121,8 @@ struct files_struct *dup_fd(struct files_struct *, int *);
 
 extern int __alloc_fd(struct files_struct *files,
 		      unsigned start, unsigned end, unsigned flags);
+extern void __fd_install(struct files_struct *files,
+		      unsigned int fd, struct file *file);
 
 extern struct kmem_cache *files_cachep;