adding 32 bit compatibility layer in custom kernel module

[anish singh]

On Tue, Jan 3, 2017 at 10:59 PM, Pradeepa Kumar <cdpradeepa at gmail.com>
wrote:

> my app is crashing as it is trying iterate cmsghdrs
> it got after call to recvmsg();
> To give some context
> below is the flow
>
> 32 bit app  <--> my kernel module <--> 64 bit app
> my kernel module implements new protocol and
> provides 'struct proto_ops'
> it never calls __sys_socket* calls.
> once 32 bit app calls recvmsg(),
> kernel module gets required data from another 64 bit app
> via netlink msg.
> issue here is the struct msghdr and strcu cmsgdr sizes
> and members are of different lenghts.
> so kernel module needs to give data to 32 bit app
> in proper struct which are of correct sizes in 32 bit mode.
> so kernel module needs a way to figure out
> it is 32 bit app that made recvmsg() call and process accordingly.
> i dont see compat_recv in struct proto_ops
>

Don't top post.
I suggest sending mail to netdev kernel mailing list
with all the relevant information as I doubt if networking
layer in kernel doesn't handle 32/64 bit properly.

>
>
>
> On Wed, Jan 4, 2017 at 12:18 PM, Anish Kumar <anish198519851985 at gmail.com>
> wrote:
>
>>
>>
>> On Jan 3, 2017, at 10:03 PM, Pradeepa Kumar <cdpradeepa at gmail.com> wrote:
>>
>> Please see inline below
>>
>> On Wed, Jan 4, 2017 at 11:07 AM, Anish Kumar <anish198519851985 at gmail.com
>> > wrote:
>>
>>>
>>>
>>> On Jan 3, 2017, at 8:04 PM, Pradeepa Kumar <cdpradeepa at gmail.com> wrote:
>>>
>>> Hi experts
>>>
>>> down votefavorite
>>> <http://stackoverflow.com/questions/41455943/adding-32-bit-compatibility-layer-in-custom-kernel-module#>
>>>
>>> in my 64 bit kernel, I have a custom kernel module providing new
>>> protocol and providing socket system calls.
>>>
>>> it works fine when 64 bit app runs.
>>>
>>> i am seeing issues when 32 bit app runs (for exp recvmsg() call does not
>>> work if msg has cmsghdrs as struct size of cmsghdr is different in 32 bit
>>> and 64 bit).
>>>
>>> This is because my custom kernel module does not have 32 bit
>>> compatibility layer ( but linux kernel has this in compat.c etc).
>>>
>>>    -
>>>
>>>    is it simple to add compatibility layer to my custom kernel module
>>>
>>>
>>>    All you have to do is add compat_ioctl call to your driver.
>>>
>>> my kernel module provides recvmsg() but i dont see any 'compat_recvmsg'
>> in struct proto_ops.
>>
>>
>> Try to wrap your text in 80 characters.
>>
>> Your application is crashing? Right?
>> Have you figured where it is crashing exactly?
>>
>> I understood that your application is crashing as
>> It not able to function with 64 bit kernel.
>>
>> This happens because of the data being passed
>> to kernel is not in the right format. You need to
>> look at what you are passing to the driver and
>> if needed write the compat_ioctl callback in your
>> driver fops.
>>
>> had there been  'compat_recvmsg' it would have been possible for me to
>> define recvmsg() for 32 bit apps.
>> please let me know if i am missing anything
>>
>>
>>>    -
>>>
>>>    how do i do this (any links ?)
>>>
>>>
>>>    You can see many drivers supports compat call.
>>>
>>> Thanks
>>>
>>> _______________________________________________
>>> Kernelnewbies mailing list
>>> Kernelnewbies at kernelnewbies.org
>>> https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>>
>>>
>>
>
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