Possible use_mm() mis-uses

[Linus Torvalds]

On Wed, Aug 22, 2018 at 12:44 PM Felix Kuehling <felix.kuehling at amd.com> wrote:
>
> You're right, but that's a bit fragile and convoluted. I'll fix KFD to
> handle this more robustly. See the attached (untested) patch.

Yes, this patch that makes the whole "has to use current mm" or uses
"get_task_mm()" looks good from a VM< worry standpoint.

Thanks.

> And
> obviously that opaque pointer didn't work as intended. It just gets
> promoted to an mm_struct * without a warning from the compiler. Maybe I
> should change that to a long to make abuse easier to spot.

Using a "void *" is actually just about the worst possible type for
something that should be a cookie, because it silently translates to
any pointer.

"long" is actually not much better, becuase it will silently convert
to any integer type.

A good fairly type-safe cookie type is actually this:

    typedef volatile const struct no_such_struct *cookie_ptr_t;

and now something of type "cookie_ptr_t" is actually very  hard to
convert to other types by mistake.

Note that the "volatile const" is not just random noise - it's so that
it won't even convert without warnings to things that take a "const
void *" as an argument (like, say, the source of 'memcpy()').

So you almost _have_ to explicitly cast it to use it.

           Linus