32 bit user space compatibility

32 bit user space compatibility

[Skidanov, Alexey]

Hi,

Running 32 bit user space needs some work to be done with ioctls. I understand that there are two options to implement:
1.       Use only fixed size types. Pad IOCTLS params to multiple of 64 bits - simple; don't know if it covers all compatibility issues; 
2.       32 bit compatibility layer (through compat_ioctl, just like many drivers in kernel implement)  - just a little bit simple code with some translations; really covers all issues;
 
Which one is preferred by kernel community?

Thanks
Alexey

Re: 32 bit user space compatibility

[One Thousand Gnomes]

On Sun, 26 Oct 2014 12:25:08 +0000
"Skidanov, Alexey" <Alexey.Skidanov@amd.com> wrote:

> Hi,
> 
> Running 32 bit user space needs some work to be done with ioctls. I understand that there are two options to implement:
> 1.       Use only fixed size types. Pad IOCTLS params to multiple of 64 bits - simple; don't know if it covers all compatibility issues; 
> 2.       32 bit compatibility layer (through compat_ioctl, just like many drivers in kernel implement)  - just a little bit simple code with some translations; really covers all issues;
>  
> Which one is preferred by kernel community?

You shouldn't need to pad paramters in most cases as platform alignment
rules are usually sane for 32 and 64bit. #1 is preferred. #2 exists
because 64bit appeared after Linux was designed so it wasn't designed in
from day one.

Re: 32 bit user space compatibility

[Yann Droneaud]

Hi,

Le mardi 28 octobre 2014 à 22:34 +0000, One Thousand Gnomes a écrit :
> On Sun, 26 Oct 2014 12:25:08 +0000
> "Skidanov, Alexey" <Alexey.Skidanov@amd.com> wrote:

>  
> > Running 32 bit user space needs some work to be done with ioctls. I understand that there are two options to implement:
> > 1.       Use only fixed size types. Pad IOCTLS params to multiple of 64 bits - simple; don't know if it covers all compatibility issues; 
> > 2.       32 bit compatibility layer (through compat_ioctl, just like many drivers in kernel implement)  - just a little bit simple code with some translations; really covers all issues;
> >  
> > Which one is preferred by kernel community?
> 
> You shouldn't need to pad paramters in most cases as platform alignment
> rules are usually sane for 32 and 64bit. 

In most case, except i386 (ia32) vs amd64 (x86_64): u64 are going to be
aligned on 4 bytes boundaries for 32bits ABI and 8 bytes boundaries for
64bits ABI.

I've tried to explained this issue in a lightning talk[1][2] I'd given
at Kernel Recipes[3] this year.

[1] http://opteya.com/talks/2014/kernel-recipes/lightning-talk-kernel-userspace-ABI/
[2] https://gitorious.org/opteya/talk-kernel-userspace-abi
[3] http://kernel.recipes/

Regards.

-- 
Yann Droneaud
OPTEYA