Re: [RFC v1] implement SL*B and stack usercopy runtime checks

[Linus Torvalds]

On Sun, Jul 3, 2011 at 11:57 AM, Vasiliy Kulikov <segoon@xxxxxxxxxxxx> wrote:
>> If you seriously clean it up (that at a minimum includes things like
>> making it configurable using some pretty helper function that just
>> compiles away for all the normal cases,
>
> Hm, it is not as simple as it looks at the first glance - even if the
> object size is known at the compile time (__compiletime_object_size), it
> might be a field of a structure, which crosses the slab object
> boundaries because of an overflow.

No, I was more talking about having something like

  #ifdef CONFIG_EXPENSIVE_CHECK_USERCOPY
  extern int check_user_copy(const void *kptr, unsigned long size);
  #else
  static inline int check_user_copy(const void *kptr, unsigned long size)
  { return 0; }
  #endif

so that the actual user-copy routines end up being clean and not have
#ifdefs in them or any implementation details like what you check
(stack, slab, page cache - whatever)

If you can also make it automatically not generate any code for cases
that are somehow obviously safe, then that's an added bonus.

But my concern is that performance is a real issue, and the strict
user-copy checking sounds like mostly a "let's enable this for testing
kernels when chasing some particular issue" feature, the way
DEBUG_PAGEALLOC is. And at the same time, code cleanliness and
maintainability is a big deal, so the usercopy code itself should have
minimal impact and look nice regardless (which is why I strongly
object to that kind of "(!slab_access_ok(to, n) ||
!stack_access_ok(to, n))" crud - the internal details of what you
check are *totally* irrelevant to the usercopy code.

                           Linus
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