Re: copy_to_user problem

[PRASENJIT CHAKRABORTY <pras_chakra@yahoo.com>]

Yes I tried copy_to_user as well, but to no avail as
copy_to_user is basically a call to verify_area
followed by __copy_to_user.

--- Kanoj Sarcar <kanojsarcar@yahoo.com> wrote:
> Can you try copy_to_user(), and not
> __copy_to_user().
> 
> Kanoj
> 
> --- PRASENJIT CHAKRABORTY <pras_chakra@yahoo.com>
> wrote:
> > Hello All,
> > 
> >    I posted this problem some days back but still
> > have
> > not received any solution.
> > 
> > So sorry for the resend again.
> > 
> > I am developing a driver. Though the driver is
> > functioning well but I am stuck up at the point of
> > transferring something from kernel to user space.
> > 
> > I am using __copy_to_user() and before calling
> this
> > I
> > am checking the validity of user address with
> > verify_area().
> > 
> > Now the problem is that __copy_to_user() sometimes
> > return value > 0 which indicates a failure. This
> > happens everytime the user address is not
> currently
> > present in the Physical Page i.e the when
> > __copy_to_user tries to copy to a page which is
> not
> > currently paged in then it fails.
> > 
> > Moreover if I access the user buffer from my user
> > program before passing it to the kernel for
> transfer
> > then __copy_to_user performs it happily or if I
> lock
> > that page through mlock() system call. I read the
> > documentation but nowhere I found that I need to
> do
> > something before __copy_to_user().
> > 
> > The problem for the time being has been
> workarounded
> > by putting __verify_write() before __copy_to_user.
> > 
> > So I would like to know what is wrong with my
> > approach
> > or is it due to some other issue which is unknown
> to
> > me (e.g corruption etc).
> > 
> > I shall be grateful to you all if you kindly help
> me
> > out of this.
> > 
> > Thanks n Regards,
> > 
> > Prasenjit
> > 
> > 
> > 
> > 
> > 
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> 
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