linux-kernel - Re: [PATCH 3/3] riscv: fix __user annotation for __copy

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Date:   Mon, 4 Jun 2018 21:09:28 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Atish Patra <atish.patra@....com>
Cc:     Palmer Dabbelt <palmer@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Albert Ou <albert@...ive.com>
Subject: Re: [PATCH 3/3] riscv: fix __user annotation for __copy_user()

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').

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

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.


Best regards,
-- Luc