i915 driver fails on i686 userspace + x86_64 kernel

[Pavel Roskin]

Hello!

I'm running 32-bit Ubuntu 12.04 on the latest x86_64 kernel (mainline  
git).  I'm using the latest X from the x-edgers PPA  
(2.99.904+git20131009.b9ad5b62-0ubuntu0sarvatt~precise).

The Intel Xorg driver fails with:

(EE) intel(0): No outputs and no modes.

Xorg works with the i386 kernel in the same configuration.

This is my understanding of what is happening.

sna_output_init() in the userspace driver calls  
DRM_IOCTL_MODE_GETCONNECTOR.  It sets conn.encoders_ptr to a 32-bit  
pointer (an address of enc.encoder_id in the process memory).  That  
pointer is never converted to a 64-bit pointer.

drm_mode_getconnector() in the kernel simply calls put_user() on that  
pointer to write the encoder IDs.  Debug printing shows that the  
pointer is indeed a 32-bit value.  The encoder ID goes to a wrong  
place in the memory.

Then sna_output_init() calls DRM_IOCTL_MODE_GETENCODER with the  
enc.encoder_id that wasn't written to by the kernel, so it's 0.   
drm_mode_getencoder() in the kernel returns -EINVAL.   
sna_output_init() fails.  Xorg assumes there are no valid screens and  
exits.

I believe the error is on the kernel side.  The kernel should convert  
the pointer.  compat_ptr() doesn't convert the value, only the type.   
The comment in arch/x86/include/asm/compat.h says:

/*
  * A pointer passed in from user mode. This should not
  * be used for syscall parameters, just declare them
  * as pointers because the syscall entry code will have
  * appropriately converted them already.
  */

I cannot find that conversion code.  Maybe it's in  
arch/x86/ia32/ia32entry.S, but we want architecture independent code  
in drivers/gpu/drm.

Or maybe the API for DRM_IOCTL_MODE_GETCONNECTOR is fundamentally wrong?

-- 
Regards,
Pavel Roskin