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>

1 files changed, 8 insertions, 5 deletions

diff --git a/security/tomoyo/util.c b/security/tomoyo/util.c
index 2952ba576fb9..b974a6997d7f 100644
--- a/security/tomoyo/util.c
+++ b/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;
 }