fs: get mnt_writers count for an open backing file's real path

A writeable mapped backing file can perform writes to the real inode.
Therefore, the real path mount must be kept writable so long as the
writable map exists.

This may not be strictly needed for ovelrayfs private upper mount,
but it is correct to take the mnt_writers count in the vfs helper.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231009153712.1566422-2-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 25 insertions, 6 deletions

diff --git a/fs/open.c b/fs/open.c
index a65ce47810cf..fe63e236da22 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -870,6 +870,30 @@ SYSCALL_DEFINE3(fchown, unsigned int, fd, uid_t, user, gid_t, group)
 	return ksys_fchown(fd, user, group);
 }
 
+static inline int file_get_write_access(struct file *f)
+{
+	int error;
+
+	error = get_write_access(f->f_inode);
+	if (unlikely(error))
+		return error;
+	error = mnt_get_write_access(f->f_path.mnt);
+	if (unlikely(error))
+		goto cleanup_inode;
+	if (unlikely(f->f_mode & FMODE_BACKING)) {
+		error = mnt_get_write_access(backing_file_real_path(f)->mnt);
+		if (unlikely(error))
+			goto cleanup_mnt;
+	}
+	return 0;
+
+cleanup_mnt:
+	mnt_put_write_access(f->f_path.mnt);
+cleanup_inode:
+	put_write_access(f->f_inode);
+	return error;
+}
+
 static int do_dentry_open(struct file *f,
 			  struct inode *inode,
 			  int (*open)(struct inode *, struct file *))
@@ -892,14 +916,9 @@ static int do_dentry_open(struct file *f,
 	if ((f->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
 		i_readcount_inc(inode);
 	} else if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
-		error = get_write_access(inode);
+		error = file_get_write_access(f);
 		if (unlikely(error))
 			goto cleanup_file;
-		error = mnt_get_write_access(f->f_path.mnt);
-		if (unlikely(error)) {
-			put_write_access(inode);
-			goto cleanup_file;
-		}
 		f->f_mode |= FMODE_WRITER;
 	}