[Mesa-dev] [PATCH 2/4] i965: Fix intel_miptree_map() signature to be more 64-bit safe

Kenneth Graunke kenneth at whitecape.org
Wed Nov 19 21:18:54 PST 2014

On Wednesday, November 19, 2014 02:13:03 PM Ian Romanick wrote:
> On 11/18/2014 09:11 PM, Chad Versace wrote:
> > This patch should diminish the likelihood of pointer arithmetic overflow
> > bugs, like the one fixed by b69c7c5dac.
> > 
> > Change the type of parameter 'out_stride' from int to ptrdiff_t. The
> > logic is that if you call intel_miptree_map() and use the value of
> > 'out_stride', then you must be doing pointer arithmetic on 'out_ptr'.
> > Using ptrdiff_t instead of int should make a little bit harder to hit
> > overflow bugs.
> So... we talked about this a little bit on Monday, and I don't think we
> ever had a conclusion.  What happens if you have a 32-bit application
> running on a platform with 48-bit GPU address space?

CC'ing Ben, who knows all the gory details.

I don't really understand the problem - the GPU uses 48-bit addressing, and
can access more than 4G...but we're talking about map, which makes a buffer
available in an application's virtual address space...which is 32-bit in your
example.  It should always be placed at a < 4GB virtual address and work out

That said, I don't think the kernel ever uses >= 4GB address spaces today.
Ben wrote 4GGGTT support and had both kernel and userspace patches to make
it work, but I don't think it ever actually landed.  I'm pretty sure
shipping userspace is not quite 48-bit safe - there are a few buffers that
have to be placed < 4GB (some hardware packets only take 32-bit addresses
still), and I don't think any software is in place to make that happen.

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