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-rw-r--r--include/uapi/asm-generic/ioctl.h10
-rw-r--r--include/uapi/linux/userfaultfd.h6
2 files changed, 15 insertions, 1 deletions
diff --git a/include/uapi/asm-generic/ioctl.h b/include/uapi/asm-generic/ioctl.h
index 7e7c11b52143..749b32fe5623 100644
--- a/include/uapi/asm-generic/ioctl.h
+++ b/include/uapi/asm-generic/ioctl.h
@@ -48,6 +48,9 @@
/*
* Direction bits, which any architecture can choose to override
* before including this file.
+ *
+ * NOTE: _IOC_WRITE means userland is writing and kernel is
+ * reading. _IOC_READ means userland is reading and kernel is writing.
*/
#ifndef _IOC_NONE
@@ -72,7 +75,12 @@
#define _IOC_TYPECHECK(t) (sizeof(t))
#endif
-/* used to create numbers */
+/*
+ * Used to create numbers.
+ *
+ * NOTE: _IOW means userland is writing and kernel is reading. _IOR
+ * means userland is reading and kernel is writing.
+ */
#define _IO(type,nr) _IOC(_IOC_NONE,(type),(nr),0)
#define _IOR(type,nr,size) _IOC(_IOC_READ,(type),(nr),(_IOC_TYPECHECK(size)))
#define _IOW(type,nr,size) _IOC(_IOC_WRITE,(type),(nr),(_IOC_TYPECHECK(size)))
diff --git a/include/uapi/linux/userfaultfd.h b/include/uapi/linux/userfaultfd.h
index 9057d7af3ae1..94046b8aa6ad 100644
--- a/include/uapi/linux/userfaultfd.h
+++ b/include/uapi/linux/userfaultfd.h
@@ -11,6 +11,12 @@
#include <linux/types.h>
+/*
+ * If the UFFDIO_API is upgraded someday, the UFFDIO_UNREGISTER and
+ * UFFDIO_WAKE ioctls should be defined as _IOW and not as _IOR. In
+ * userfaultfd.h we assumed the kernel was reading (instead _IOC_READ
+ * means the userland is reading).
+ */
#define UFFD_API ((__u64)0xAA)
/*
* After implementing the respective features it will become:

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