um: fix ubd cow size

ubd_file_size() cannot use ubd_dev->cow.file because at this time
ubd_dev->cow.file is not initialized.
Therefore, ubd_file_size() will always report a wrong disk size when
COW files are used.
Reading from /dev/ubd* would crash the kernel.

We have to read the correct disk size from the COW file's backing
file.

Signed-off-by: Richard Weinberger <richard@nod.at>
CC: stable@kernel.org

1 files changed, 30 insertions, 1 deletions

diff --git a/arch/um/drivers/ubd_kern.c b/arch/um/drivers/ubd_kern.c
index b3c854523e6c..944453a3ec99 100644
--- a/arch/um/drivers/ubd_kern.c
+++ b/arch/um/drivers/ubd_kern.c
@@ -511,8 +511,37 @@ __uml_exitcall(kill_io_thread);
 static inline int ubd_file_size(struct ubd *ubd_dev, __u64 *size_out)
 {
         char *file;
+        int fd;
+        int err;
+
+        __u32 version;
+        __u32 align;
+        char *backing_file;
+        time_t mtime;
+        unsigned long long size;
+        int sector_size;
+        int bitmap_offset;
+
+        if (ubd_dev->file && ubd_dev->cow.file) {
+                file = ubd_dev->cow.file;
+
+                goto out;
+        }
 
-        file = ubd_dev->cow.file ? ubd_dev->cow.file : ubd_dev->file;
+        fd = os_open_file(ubd_dev->file, global_openflags, 0);
+        if (fd < 0)
+                return fd;
+
+        err = read_cow_header(file_reader, &fd, &version, &backing_file, \
+                &mtime, &size, &sector_size, &align, &bitmap_offset);
+        os_close_file(fd);
+
+        if(err == -EINVAL)
+                file = ubd_dev->file;
+        else
+                file = backing_file;
+
+out:
         return os_file_size(file, size_out);
 }