tomoyo: reduce mmap_sem hold for mm->exe_file

The mm->exe_file is currently serialized with mmap_sem (shared) in order
to both safely (1) read the file and (2) compute the realpath by calling
tomoyo_realpath_from_path, making it an absolute overkill.  Good users
will, on the other hand, make use of the more standard get_mm_exe_file(),
requiring only holding the mmap_sem to read the value, and relying on
reference

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

security/tomoyo/util.c

@@ -948,15 +948,18 @@ bool tomoyo_path_matches_pattern(const struct tomoyo_path_info *filename,
  */
 const char *tomoyo_get_exe(void)
 {
+	struct file *exe_file;
+	const char *cp;
 	struct mm_struct *mm = current->mm;
-	const char *cp = NULL;
 
 	if (!mm)
 		return NULL;
-	down_read(&mm->mmap_sem);
-	if (mm->exe_file)
-		cp = tomoyo_realpath_from_path(&mm->exe_file->f_path);
-	up_read(&mm->mmap_sem);
+	exe_file = get_mm_exe_file(mm);
+	if (!exe_file)
+		return NULL;
+
+	cp = tomoyo_realpath_from_path(&exe_file->f_path);
+	fput(exe_file);
 	return cp;
 }