From c6517b78153a1ffb401d8c3ec329effd3ee19036 Mon Sep 17 00:00:00 2001 From: Laura Abbott Date: Fri, 2 Sep 2016 15:42:24 -0700 Subject: [PATCH] doc: ioctl: Add some clarifications to botching-up-ioctls - The guide currently says to pad the structure to a multiple of 64-bits. This is not necessary in cases where the structure contains no 64-bit types. Clarify this concept to avoid unnecessary padding. - When using __u64 to hold user pointers, blindly trying to do a cast to a void __user * may generate a warning on 32-bit systems about a cast from an integer to a pointer of different size. There is a macro to deal with this which hides an ugly double cast. Add a reference to this macro. Signed-off-by: Laura Abbott Acked-by: Arnd Bergmann Signed-off-by: Jonathan Corbet --- Documentation/ioctl/botching-up-ioctls.txt | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/Documentation/ioctl/botching-up-ioctls.txt b/Documentation/ioctl/botching-up-ioctls.txt index cc30b14791cb..36138c632f7a 100644 --- a/Documentation/ioctl/botching-up-ioctls.txt +++ b/Documentation/ioctl/botching-up-ioctls.txt @@ -34,15 +34,18 @@ will need to add a a 32-bit compat layer: 64-bit platforms do. So we always need padding to the natural size to get this right. - * Pad the entire struct to a multiple of 64-bits - the structure size will - otherwise differ on 32-bit versus 64-bit. Having a different structure size - hurts when passing arrays of structures to the kernel, or if the kernel - checks the structure size, which e.g. the drm core does. + * Pad the entire struct to a multiple of 64-bits if the structure contains + 64-bit types - the structure size will otherwise differ on 32-bit versus + 64-bit. Having a different structure size hurts when passing arrays of + structures to the kernel, or if the kernel checks the structure size, which + e.g. the drm core does. * Pointers are __u64, cast from/to a uintprt_t on the userspace side and from/to a void __user * in the kernel. Try really hard not to delay this conversion or worse, fiddle the raw __u64 through your code since that - diminishes the checking tools like sparse can provide. + diminishes the checking tools like sparse can provide. The macro + u64_to_user_ptr can be used in the kernel to avoid warnings about integers + and pointres of different sizes. Basics -- 2.20.1