[PATCH 4/9] vfs: intercept reads to overlay files

Miklos Szeredi
2017-02-17T16:09:51Z

...in order to handle the corner case when the file is copyied up after
being opened read-only.

Can be verified with the following script:

- 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
cd /
rm -rf /tmp/ovl-rorw-test
mkdir /tmp/ovl-rorw-test
cd /tmp/ovl-rorw-test
mkdir -p mnt lower upper work
echo baba > lower/foo
mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work mnt
exec 3< mnt/foo
echo bubu > mnt/foo
cat <&3
exec 3>&-
umount mnt
- 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -

Correct output is "bubu", incorrect output is "baba".

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
fs/Makefile | 2 +-
fs/open.c | 2 ++
fs/overlay_util.c | 39 +++++++++++++++++++++++++++++++++++++++
include/linux/fs.h | 13 +++++++++++++
include/linux/overlay_util.h | 13 +++++++++++++
5 files changed, 68 insertions(+), 1 deletion(-)
create mode 100644 fs/overlay_util.c
create mode 100644 include/linux/overlay_util.h

diff --git a/fs/Makefile b/fs/Makefile
index 7bbaca9c67b1..8c8f197d7c75 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -11,7 +11,7 @@ obj-y := open.o read_write.o file_table.o super.o \
attr.o bad_inode.o file.o filesystems.o namespace.o \
seq_file.o xattr.o libfs.o fs-writeback.o \
pnode.o splice.o sync.o utimes.o \
- stack.o fs_struct.o statfs.o fs_pin.o nsfs.o
+ stack.o fs_struct.o statfs.o fs_pin.o nsfs.o overlay_util.o

ifeq ($(CONFIG_BLOCK),y)
obj-y += buffer.o block_dev.o direct-io.o mpage.o
diff --git a/fs/open.c b/fs/open.c
index 9921f70bc5ca..4916ccff29f5 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -762,6 +762,8 @@ static int do_dentry_open(struct file *f,
if ((f->f_mode & FMODE_WRITE) &&
likely(f->f_op->write || f->f_op->write_iter))
f->f_mode |= FMODE_CAN_WRITE;
+ if (unlikely(d_inode(f->f_path.dentry) != inode))
+ f->f_mode |= FMODE_OVERLAY;

f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);

diff --git a/fs/overlay_util.c b/fs/overlay_util.c
new file mode 100644
index 000000000000..0daff19bad0b
--- /dev/null
+++ b/fs/overlay_util.c
@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published by
+ * the Free Software Foundation.
+ */
+#if IS_ENABLED(CONFIG_OVERLAY_FS)
+
+#include <linux/overlay_util.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include "internal.h"
+
+static bool overlay_file_consistent(struct file *file)
+{
+ return d_real_inode(file->f_path.dentry) == file_inode(file);
+}
+
+ssize_t overlay_read_iter(struct file *file, struct kiocb *kio,
+ struct iov_iter *iter)
+{
+ ssize_t ret;
+
+ if (likely(overlay_file_consistent(file)))
+ return file->f_op->read_iter(kio, iter);
+
+ file = filp_clone_open(file);
+ if (IS_ERR(file))
+ return PTR_ERR(file);
+
+ ret = vfs_iter_read(file, iter, &kio->ki_pos);
+ fput(file);
+
+ return ret;
+}
+EXPORT_SYMBOL(overlay_read_iter);
+
+#endif /* IS_ENABLED(CONFIG_OVERLAY_FS) */
diff --git a/include/linux/fs.h b/include/linux/fs.h
index efdaad954b70..d186d5390e99 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -31,6 +31,7 @@
#include <linux/workqueue.h>
#include <linux/percpu-rwsem.h>
#include <linux/delayed_call.h>
+#include <linux/overlay_util.h>

#include <asm/byteorder.h>
#include <uapi/linux/fs.h>
@@ -143,6 +144,9 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
/* File was opened by fanotify and shouldn't generate fanotify events */
#define FMODE_NONOTIFY ((__force fmode_t)0x4000000)

+/* File comes from overlay filesystem */
+#define FMODE_OVERLAY ((__force fmode_t)0x8000000)
+
/*
* Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
* that indicates that they should check the contents of the iovec are
@@ -1712,9 +1716,18 @@ struct inode_operations {
int (*set_acl)(struct inode *, struct posix_acl *, int);
} ____cacheline_aligned;

+
+static inline bool is_overlay_file(struct file *file)
+{
+ return IS_ENABLED(CONFIG_OVERLAY_FS) && file->f_mode & FMODE_OVERLAY;
+}
+
static inline ssize_t call_read_iter(struct file *file, struct kiocb *kio,
struct iov_iter *iter)
{
+ if (unlikely(is_overlay_file(file)))
+ return overlay_read_iter(file, kio, iter);
+
return file->f_op->read_iter(kio, iter);
}

diff --git a/include/linux/overlay_util.h b/include/linux/overlay_util.h
new file mode 100644
index 000000000000..886be9003bf3
--- /dev/null
+++ b/include/linux/overlay_util.h
@@ -0,0 +1,13 @@
+#ifndef _LINUX_OVERLAY_FS_H
+#define _LINUX_OVERLAY_FS_H
+
+#include <linux/types.h>
+
+struct file;
+struct kiocb;
+struct iov_iter;
+
+extern ssize_t overlay_read_iter(struct file *file, struct kiocb *kio,
+ struct iov_iter *iter);
+
+#endif /* _LINUX_OVERLAY_FS_H */
--
2.5.5