mm_for_maps: shift down_read(mmap_sem) to the caller

mm_for_maps() takes ->mmap_sem after security checks, this looks
strange and obfuscates the locking rules. Move this lock to its
single caller, m_start().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 3 insertions, 5 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 917f338a6739..f3c2e4085fed 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -235,9 +235,8 @@ static int check_mem_permission(struct task_struct *task)
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
         struct mm_struct *mm = get_task_mm(task);
-        if (!mm)
-                return NULL;
-        if (mm != current->mm) {
+
+        if (mm && mm != current->mm) {
                 /*
                  * task->mm can be changed before security check,
                  * in that case we must notice the change after.
@@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
                 if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
                     mm != task->mm) {
                         mmput(mm);
-                        return NULL;
+                        mm = NULL;
                 }
         }
-        down_read(&mm->mmap_sem);
         return mm;
 }