ANDROID: sdcardfs: implement vm_ops->page_mkwrite

This comes from the wrapfs patch
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

1 files changed, 34 insertions, 0 deletions

diff --git a/fs/sdcardfs/mmap.c b/fs/sdcardfs/mmap.c
index e21f64675a80..7dd715875ef1 100644
--- 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, loff_t pos)
 {
@@ -78,4 +111,5 @@ const struct address_space_operations sdcardfs_aops = {
 
 const struct vm_operations_struct sdcardfs_vm_ops = {
 	.fault		= sdcardfs_fault,
+	.page_mkwrite	= sdcardfs_page_mkwrite,
 };