compat_ioctl: add compat_ptr_ioctl()

Many drivers have ioctl() handlers that are completely compatible between
32-bit and 64-bit architectures, except for the argument that is passed
down from user space and may have to be passed through compat_ptr()
in order to become a valid 64-bit pointer.

Using ".compat_ptr = compat_ptr_ioctl" in file operations should let
us simplify a lot of those drivers to avoid #ifdef checks, and convert
additional drivers that don't have proper compat handling yet.

On most architectures, the compat_ptr_ioctl() just passes all arguments
to the corresponding ->ioctl handler. The exception is arch/s390, where
compat_ptr() clears the top bit of a 32-bit pointer value, so user space
pointers to the second 2GB alias the first 2GB, as is the case for native
32-bit s390 user space.

The compat_ptr_ioctl() function must therefore be used only with
ioctl functions that either ignore the argument or pass a pointer to a
compatible data type.

If any ioctl command handled by fops->unlocked_ioctl passes a plain
integer instead of a pointer, or any of the passed data types is
incompatible between 32-bit and 64-bit architectures, a proper handler
is required instead of compat_ptr_ioctl.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
v3: add a better description
v2: use compat_ptr_ioctl instead of generic_compat_ioctl_ptrarg,
as suggested by Al Viro

fs/ioctl.c

@@ -8,6 +8,7 @@
 #include <linux/syscalls.h>
 #include <linux/mm.h>
 #include <linux/capability.h>
+#include <linux/compat.h>
 #include <linux/file.h>
 #include <linux/fs.h>
 #include <linux/security.h>
@@ -719,3 +720,37 @@ SYSCALL_DEFINE3(ioctl, unsigned int, fd, unsigned int, cmd, unsigned long, arg)
 {
 	return ksys_ioctl(fd, cmd, arg);
 }
+
+#ifdef CONFIG_COMPAT
+/**
+ * compat_ptr_ioctl - generic implementation of .compat_ioctl file operation
+ *
+ * This is not normally called as a function, but instead set in struct
+ * file_operations as
+ *
+ *     .compat_ioctl = compat_ptr_ioctl,
+ *
+ * On most architectures, the compat_ptr_ioctl() just passes all arguments
+ * to the corresponding ->ioctl handler. The exception is arch/s390, where
+ * compat_ptr() clears the top bit of a 32-bit pointer value, so user space
+ * pointers to the second 2GB alias the first 2GB, as is the case for
+ * native 32-bit s390 user space.
+ *
+ * The compat_ptr_ioctl() function must therefore be used only with ioctl
+ * functions that either ignore the argument or pass a pointer to a
+ * compatible data type.
+ *
+ * If any ioctl command handled by fops->unlocked_ioctl passes a plain
+ * integer instead of a pointer, or any of the passed data types
+ * is incompatible between 32-bit and 64-bit architectures, a proper
+ * handler is required instead of compat_ptr_ioctl.
+ */
+long compat_ptr_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
+{
+	if (!file->f_op->unlocked_ioctl)
+		return -ENOIOCTLCMD;
+
+	return file->f_op->unlocked_ioctl(file, cmd, (unsigned long)compat_ptr(arg));
+}
+EXPORT_SYMBOL(compat_ptr_ioctl);
+#endif

include/linux/fs.h

@@ -1727,6 +1727,13 @@ int vfs_mkobj(struct dentry *, umode_t,
 
 extern long vfs_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
 
+#ifdef CONFIG_COMPAT
+extern long compat_ptr_ioctl(struct file *file, unsigned int cmd,
+					unsigned long arg);
+#else
+#define compat_ptr_ioctl NULL
+#endif
+
 /*
  * VFS file helper functions.
  */