lkml.org 
[lkml]   [2018]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 3/3] riscv: fix __user annotation for __copy_user()
From
Date
On 6/4/18 12:09 PM, Luc Van Oostenryck wrote:
> On Mon, Jun 04, 2018 at 11:46:50AM -0700, Atish Patra wrote:
>> On 6/1/18 8:22 AM, Luc Van Oostenryck wrote:
>>> __copy_user() is a function, written in assembly, used to copy
>>> memory between kernel & user space. As such its to & from args
>>> may both take a user pointer or a kernel pointer.
>>>
>>> However the prototype for this function declare these two args
>>> as 'void __user *', which is no more & no less correct than
>>> declaring them as 'void *'. In fact theer is no possible correct
>>
>> /s/theer/there
>>
>>> annotation for such a function.
>>>
>>> The problem is worked around here by declaring these args as
>>> unsigned long and casting them to the right type in each of
>>> two callers raw_copy_{to,from}_user() as some kind of cast would
>>> be needed anyway.
>>>
>>> Note: another solution, maybe cleaner but slightly more complex,
>>> would be to declare two version of __copy_user,
>>> either in the asm file or via an alias, each having already
>>> the correct typing for raw_copy_{to,from}_user().
>>>
>>
>> I feel that would be a better solution as it is implemented similarly
>> in ARM as well.
>>
>> I am unable to understand how "unsigned long" is better than "void*".
>> x86 implementation has both arguments as void*. Can you please clarify ?
>
> "better" is quite relative and it must be understood that sparse
> allow to cast pointers o fany kinds to and from unsigned long
> without any warnings (while doing a cast between different address
> space will emit a warning unless you use '__force').
>

Got it.
> As I tried to explain here above, the fact that this function is
> declared as taking 2 __user pointers requires to use of casts
> (ugly casts with __force) to get over the __user. By declaring
> them as taking unsigned long, you still have to use casts but, IMO,
> it's cleaner
>

Thanks for the detailed explanation.

> Note: they're generic pointers/addresses anyway, they can't be
> dereferenced anyway so unsigned is as good as a plain void*
> or a void __user*
> Note: using unsigned long here, fundamentally to bypass the __user,
> is the same as casting a const pointer back to a plain pointer
> via an intermediate cast to unsigned long. People can argue
> that's kinda cheating, and they would be right of course, but
> using __force or declaring twice the function with two different
> names and prototype is also a form of cheating.
> Note: if this would be my code, I would choose the solution with
> two declarations.

I prefer that as well.

Regards,
Atish
>
>
> Best regards,
> -- Luc
>

\
 
 \ /
  Last update: 2018-06-04 21:29    [W:0.177 / U:0.444 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site