Re: [PATCH 20/30] microblaze_mmu_v1: uaccess MMU update

Arnd Bergmann
2009-04-29T11:02:21Z

On Wednesday 29 April 2009, Michal Simek wrote:
> Here is that change which remove address space problem.
>
> #define __clear_user(addr, n)   (memset((__force void *)(addr), 0, (n)), 0)
>
> Am I right?
>
> The same mishmash is for memset_fromio/memset_toio and maybe some others which I want to fix too.

This will work, but a better fix would be to define an inline
function that explicitly takes a __user pointer. This would give
you warnings when code accidentally calls __clear_user on a
kernel pointer (this also adds the might_sleep()):

static inline unsigned long __must_check
__clear_user(void __user *to, unsigned long n)
{
memset((__force void *)addr, 0, n);
return 0;
}

static inline unsigned long __must_check
clear_user(void __user *to, unsigned long n)
{
might_sleep();
if (unlikely(!access_ok(VERIFY_WRITE, to, n)))
return n;

return __clear_user(to, n);
}

The above is just the nommu variant. For mmu, you need to
have exception handling in __clear_user to take care of the
case where the address is part of the user mapping (access_ok)
but not currently mapped.