mtd: provide proper 32/64-bit compat_ioctl() support for BLKPG

After a bit of poking around wondering why my 32-bit user-space can't
seem to send a proper ioctl(BLKPG) to an MTD on my 64-bit kernel
(ARM64), I noticed that struct blkpg_ioctl_arg is actually pretty
unsuitable for use in the ioctl() ABI, due to its use of raw pointers,
and its lack of alignment/packing restrictions (32-bit arch'es tend to
pack the 4 fields into 4 32-bit words, whereas 64-bit arch'es would add
padding after the third int, and make this 6 32-bit words).

Anyway, this means BLKPG deserves some special compat_ioctl handling. Do
the conversion in a small shim for MTD.

block/compat_ioctl.c already has compat support for the block subsystem,
but it does so by a re-marshalling data to/from user-space (see
compat_blkpg_ioctl()). Personally, I think this approach is cleaner.

Tested only on MTD, with an ARM32 user space on an ARM64 kernel.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/include/linux/blkpg.h b/include/linux/blkpg.h
new file mode 100644
index 000000000000..bef124fde61e
--- /dev/null
+++ b/include/linux/blkpg.h
@@ -0,0 +1,21 @@
+#ifndef _LINUX_BLKPG_H
+#define _LINUX_BLKPG_H
+
+/*
+ * Partition table and disk geometry handling
+ */
+
+#include <linux/compat.h>
+#include <uapi/linux/blkpg.h>
+
+#ifdef CONFIG_COMPAT
+/* For 32-bit/64-bit compatibility of struct blkpg_ioctl_arg */
+struct blkpg_compat_ioctl_arg {
+	compat_int_t op;
+	compat_int_t flags;
+	compat_int_t datalen;
+	compat_uptr_t data;
+};
+#endif
+
+#endif /* _LINUX_BLKPG_H */