ANDROID: sdcardfs: implement vm_ops->page_mkwrite

This comes from wrapfs
commit 3dfec0ffe5e2 ("Wrapfs: implement vm_ops->page_mkwrite")

Some file systems (e.g., ext4) require it.  Reported by Ted Ts'o.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 34133558
Change-Id: I1a389b2422c654a6d3046bb8ec3e20511aebfa8e

diff --git a/fs/sdcardfs/mmap.c b/fs/sdcardfs/mmap.c
index ac5f3deae088e355de3bd88436477a4bc162b8bf..28db1d11c99e0de7215d9786a4fa6bd75c4ec228 100644 (file)
--- a/fs/sdcardfs/mmap.c
+++ b/fs/sdcardfs/mmap.c
@@ -48,6 +48,39 @@ static int sdcardfs_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
        return err;
 }
 
+static int sdcardfs_page_mkwrite(struct vm_area_struct *vma,
+                              struct vm_fault *vmf)
+{
+       int err = 0;
+       struct file *file, *lower_file;
+       const struct vm_operations_struct *lower_vm_ops;
+       struct vm_area_struct lower_vma;
+
+       memcpy(&lower_vma, vma, sizeof(struct vm_area_struct));
+       file = lower_vma.vm_file;
+       lower_vm_ops = SDCARDFS_F(file)->lower_vm_ops;
+       BUG_ON(!lower_vm_ops);
+       if (!lower_vm_ops->page_mkwrite)
+               goto out;
+
+       lower_file = sdcardfs_lower_file(file);
+       /*
+        * XXX: vm_ops->page_mkwrite may be called in parallel.
+        * Because we have to resort to temporarily changing the
+        * vma->vm_file to point to the lower file, a concurrent
+        * invocation of sdcardfs_page_mkwrite could see a different
+        * value.  In this workaround, we keep a different copy of the
+        * vma structure in our stack, so we never expose a different
+        * value of the vma->vm_file called to us, even temporarily.
+        * A better fix would be to change the calling semantics of
+        * ->page_mkwrite to take an explicit file pointer.
+        */
+       lower_vma.vm_file = lower_file;
+       err = lower_vm_ops->page_mkwrite(&lower_vma, vmf);
+out:
+       return err;
+}
+
 static ssize_t sdcardfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
        /*
@@ -77,4 +110,5 @@ const struct address_space_operations sdcardfs_aops = {
 
 const struct vm_operations_struct sdcardfs_vm_ops = {
        .fault          = sdcardfs_fault,
+       .page_mkwrite   = sdcardfs_page_mkwrite,
 };