This adds proper documentation (inline) to avoid the risk of further misunderstandings about the semantics of _IOW/_IOR and it also reminds whoever will bump the UFFDIO_API in the future, to change the two ioctl to _IOW. This was found while implementing strace support for those ioctl, otherwise we could have never found it by just reviewing kernel code and testing it. _IOC_READ or _IOC_WRITE alters nothing but the ioctl number itself, so it's only worth fixing if the UFFDIO_API is bumped someday. Reported-by: "Dmitry V. Levin" <ldv@xxxxxxxxxxxx> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> --- include/uapi/asm-generic/ioctl.h | 10 +++++++++- include/uapi/linux/userfaultfd.h | 6 ++++++ 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/include/uapi/asm-generic/ioctl.h b/include/uapi/asm-generic/ioctl.h index 7e7c11b..749b32f 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 9057d7a..94046b8 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: -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>