diff options
| author | Laura Abbott <labbott@redhat.com> | 2016-09-02 15:42:24 -0700 |
|---|---|---|
| committer | Jonathan Corbet <corbet@lwn.net> | 2016-09-06 06:00:22 -0600 |
| commit | c6517b78153a1ffb401d8c3ec329effd3ee19036 (patch) | |
| tree | 598ddd92e822e0734348d4d74dca2073182f82d3 /Documentation | |
| parent | 951499710be258b85c135ee1f7ebe5f7a4b7ac91 (diff) | |
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 <labbott@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/ioctl/botching-up-ioctls.txt | 13 |
1 files 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 |