Re: Copy large memory regions from & to userspace

[Robert Hancock <hancockr@shaw.ca>]

Clemens Kolbitsch wrote:
> On Friday 31 August 2007 15:25:40 you wrote:
> > On 8/30/07, Clemens Kolbitsch <clemens.kol@gmx.at> wrote:
> > > Hi!
> > > Just a short question: What is the correct method of copying large areas
> > > of memory from userspace into userspace when running in kernel-mode?
> > relayfs?
>
> no... I'm copying user-memory to user-memory, not kernel-to-user, however 
> running the code in kernel-mode.
>
> what i wanted to know is how to check the access-rights...
> i didn't get any other answers, so for now i'm just using 
>
> if (access_ok(VERIFY_READ, from, PAGE_SIZE) &&
>     access_ok(VERIFY_WRITE, to, PAGE_SIZE))
> {
>     memcpy(to, from, PAGE_SIZE);
> }
>
> and hope that this is the *correct* way to do it...

No, it's not. access_ok does not guarantee that the memory region can be 
validly read or written. It only allows using __copy_to_user or 
__copy_from_user which skips the same checks that access_ok does.

I'm not aware of any code in the kernel that does userspace-to-userspace 
copies directly. Likely because there's rarely a need for it?

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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