Re: [PATCH v2 11/17] selftests/x86/ldt_gdt: Prepare for access bit forced

[Andy Lutomirski]

> On Dec 14, 2017, at 2:15 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
>> On Thu, Dec 14, 2017 at 2:11 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> 
>> That seems to rather defeat the point of using a VMA, though.
> 
> There never was any point in using a VMA per se.
> 
> The point was always to just map the damn thing in the user page
> tables, wasn't it?
> 
> The vma bit was just an implementation detail.

And all this is why I dislike using a VMA.  My patch puts it at a negative address. We could just as easily put it just above TASK_SIZE_MAX, but I'm a bit nervous about bugs that overrun an access_ok check by a small amount.  IIRC I found one of those in the net code once, and I didn't look very hard.