Need help in using __copy_to_user()

[PRASENJIT CHAKRABORTY <pras_chakra@yahoo.com>]

Hello All,

   I am a new addition to this list.

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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