Messages in this thread | | | Subject | Re: [PATCH 3/3] riscv: fix __user annotation for __copy_user() | From | Atish Patra <> | Date | Mon, 4 Jun 2018 12:28:47 -0700 |
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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 >
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