- 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 <labbott@xxxxxxxxxx>
---
I can split these into two separate patches if necessary but it seemed simpler
just to colapse them into a single patch.
---
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 cc30b14..36138c6 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
--
2.7.4
--
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