[PATCH RFC 2/7] kernel/fork: factor out atomcially replacing the current MM exe_file

[David Hildenbrand <david@xxxxxxxxxx>]

Let's factor the main logic out into atomic_set_mm_exe_file(), such that
all mm->exe_file logic is contained in kernel/fork.c.

While at it, perform some simple cleanups that are possible now that
we're simplifying the individual functions.

Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 kernel/fork.c      | 35 +++++++++++++++++++++++++++++++++--
 kernel/sys.c       | 33 +--------------------------------
 3 files changed, 36 insertions(+), 34 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 8ba434287387..043702972e5f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2540,6 +2540,8 @@ extern int mm_take_all_locks(struct mm_struct *mm);
 extern void mm_drop_all_locks(struct mm_struct *mm);
 
 extern void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
+extern int atomic_set_mm_exe_file(struct mm_struct *mm,
+				  struct file *new_exe_file);
 extern struct file *get_mm_exe_file(struct mm_struct *mm);
 extern struct file *get_task_exe_file(struct task_struct *task);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 426cd0c51f9e..199463625adc 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1138,8 +1138,8 @@ void mmput_async(struct mm_struct *mm)
  * Main users are mmput() and sys_execve(). Callers prevent concurrent
  * invocations: in mmput() nobody alive left, in execve task is single
  * threaded. sys_prctl(PR_SET_MM_MAP/EXE_FILE) also needs to set the
- * mm->exe_file, but does so without using set_mm_exe_file() in order
- * to do avoid the need for any locks.
+ * mm->exe_file, but uses atomic_set_mm_exe_file(), avoiding the need
+ * for any locks.
  */
 void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 {
@@ -1159,6 +1159,37 @@ void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 		fput(old_exe_file);
 }
 
+int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
+{
+	struct vm_area_struct *vma;
+	struct file *old_exe_file;
+	int ret = 0;
+
+	/* Forbid mm->exe_file change if old file still mapped. */
+	old_exe_file = get_mm_exe_file(mm);
+	if (old_exe_file) {
+		mmap_read_lock(mm);
+		for (vma = mm->mmap; vma && !ret; vma = vma->vm_next) {
+			if (!vma->vm_file)
+				continue;
+			if (path_equal(&vma->vm_file->f_path,
+				       &old_exe_file->f_path))
+				ret = -EBUSY;
+		}
+		mmap_read_unlock(mm);
+		fput(old_exe_file);
+		if (ret)
+			return ret;
+	}
+
+	/* set the new file, lockless */
+	get_file(new_exe_file);
+	old_exe_file = xchg(&mm->exe_file, new_exe_file);
+	if (old_exe_file)
+		fput(old_exe_file);
+	return 0;
+}
+
 /**
  * get_mm_exe_file - acquire a reference to the mm's executable file
  *
diff --git a/kernel/sys.c b/kernel/sys.c
index 2e2e3f378d97..7dcd9fb3153c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1828,7 +1828,6 @@ SYSCALL_DEFINE1(umask, int, mask)
 static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 {
 	struct fd exe;
-	struct file *old_exe, *exe_file;
 	struct inode *inode;
 	int err;
 
@@ -1851,40 +1850,10 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	if (err)
 		goto exit;
 
-	/*
-	 * Forbid mm->exe_file change if old file still mapped.
-	 */
-	exe_file = get_mm_exe_file(mm);
-	err = -EBUSY;
-	if (exe_file) {
-		struct vm_area_struct *vma;
-
-		mmap_read_lock(mm);
-		for (vma = mm->mmap; vma; vma = vma->vm_next) {
-			if (!vma->vm_file)
-				continue;
-			if (path_equal(&vma->vm_file->f_path,
-				       &exe_file->f_path))
-				goto exit_err;
-		}
-
-		mmap_read_unlock(mm);
-		fput(exe_file);
-	}
-
-	err = 0;
-	/* set the new file, lockless */
-	get_file(exe.file);
-	old_exe = xchg(&mm->exe_file, exe.file);
-	if (old_exe)
-		fput(old_exe);
+	err = atomic_set_mm_exe_file(mm, exe.file);
 exit:
 	fdput(exe);
 	return err;
-exit_err:
-	mmap_read_unlock(mm);
-	fput(exe_file);
-	goto exit;
 }
 
 /*
-- 
2.30.2