vfs: missed source of ->f_pos races

commit 0e665d5d upstream.

compat_sys_{read,write}v() need the same "pass a copy of file->f_pos" thing
as sys_{read,write}{,v}().
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/compat.c

@@ -1174,11 +1174,14 @@ compat_sys_readv(unsigned long fd, const struct compat_iovec __user *vec,
 	struct file *file;
 	int fput_needed;
 	ssize_t ret;
+	loff_t pos;
 
 	file = fget_light(fd, &fput_needed);
 	if (!file)
 		return -EBADF;
-	ret = compat_readv(file, vec, vlen, &file->f_pos);
+	pos = file->f_pos;
+	ret = compat_readv(file, vec, vlen, &pos);
+	file->f_pos = pos;
 	fput_light(file, fput_needed);
 	return ret;
 }
@@ -1233,11 +1236,14 @@ compat_sys_writev(unsigned long fd, const struct compat_iovec __user *vec,
 	struct file *file;
 	int fput_needed;
 	ssize_t ret;
+	loff_t pos;
 
 	file = fget_light(fd, &fput_needed);
 	if (!file)
 		return -EBADF;
-	ret = compat_writev(file, vec, vlen, &file->f_pos);
+	pos = file->f_pos;
+	ret = compat_writev(file, vec, vlen, &pos);
+	file->f_pos = pos;
 	fput_light(file, fput_needed);
 	return ret;
 }