Messages in this thread | | | Date | Mon, 04 Feb 2013 10:54:41 +0000 | From | Serban Constantinescu <> | Subject | Re: [PATCH 1/2] staging: android: ashmem: fix ashmem pin/unpin interface |
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On 04/02/13 01:41, Greg KH wrote: > On Fri, Feb 01, 2013 at 04:55:01PM +0000, Serban Constantinescu wrote: >> Hi Greg, >> >> On 01/02/13 16:18, Greg KH wrote: >>> On Fri, Feb 01, 2013 at 04:08:00PM +0000, Serban Constantinescu wrote: >>>> The values exchanged between kernel and userspace through struct >>>> ashmem_pin should be of type size_t. This change won't affect the >>>> existing interface but will stand as the basis of 64bit compat layer. >>> >>> How do you define size_t with a 64bit kernel and a 32bit userspace >>> properly? Doesn't this change open up a bunch of problems? >> >> The current ashmem pin/unpin kernel interface uses __u32 to specify >> the memory region and length in bytes. However these values should >> be of type size_t so that they are able to represent the whole range >> of possible values when compiled for a 64bit platform. > > Yes, the issue is, what size is size_t on the system if you have a 32bit > userspace and a 64bit kernel? :) > > That's why we have specific types for when we cross the user/kernel > boundry. Why not use them instead here so that you know it will work > properly in the future?
The patch set was developed using size_t to avoid the use of a #ifdef statement that would split the use for 64bit platforms using u64 and 32bit using u32. On a 64/32 system you will have a 32bit size_t in the user space and a 64bit size_t in the kernel. However in the kernel entry - compat_ashmem_ioctl we explicitly cast from compat_size_t (32bit - same as the userspace) to size_t (64bit used by the kernel).
> 695 pin.offset = (size_t)c_pin.offset; > 696 pin.len = (size_t)c_pin.len;
Same logic was applied for existing size_t ioctls:
> 46 #define ASHMEM_SET_SIZE _IOW(__ASHMEMIOC, 3, size_t)
where on a 64/32 system you are using:
> 57 #define COMPAT_ASHMEM_SET_SIZE _IOW(__ASHMEMIOC, 3, compat_size_t)
This kernel header is not intended to be exported for current Android userspace (even though we have tested this with success on one of the latest Android revisions).
>> Android API uses ashmem driver through libcutils, from where I >> attach the following snippet: >> >> <aosp>/system/core/libcutils/ashmem-dev.c >> >> 75 int ashmem_pin_region(int fd, size_t offset, size_t len) >> 76 { >> 77 struct ashmem_pin pin = { offset, len }; >> 78 return ioctl(fd, ASHMEM_PIN, &pin); >> 79 } > > Again, the 32/64 bit issue is to blame. > >> The kernel changes inline with the userspace usage and do not affect >> existing 32bit Android (we have exported the new kernel header, >> rebuilt and tested the interface with success). >> >> However this change will affect any 64bit userspace using the >> current faulty interface, but there is none that we know of. > > I'd like the Android developers to give some feedback on this, before > I'll do anything. I still think you need to change this to use the > proper kernel types.
If you or the Android developers consider the use of #ifdef u64/u32 is better I will rework the patch accordingly.
Thanks, Serban Constantinescu
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