Updating variable shared between user and kernel space

Updating variable shared between user and kernel space

[Kristian Evensen]

Hello,

I have a value I would like to share between userspace and kernelspace
(it is store in my target struct and intialized in the user space part
of my module), but when I update it in kernel space it gets the wrong
value. When for example incrementing the value by 1 every time the
rule matches, the values are all messed up (but never larger than the
number of matches/increments). When creating and initializing the
value in the module, it behaves correctly.

I always though that if you try to accesses userspace varibles from
kernel space, you will get a seg. fault or similar, and am therefore a
little confused as to why this do not work. Does anybody know why
and/or how to share variables?

-Kristian

Re: Updating variable shared between user and kernel space

[Jan Engelhardt]

On Thursday 2009-03-26 15:00, Kristian Evensen wrote:
>
>I have a value I would like to share between userspace and kernelspace
>(it is store in my target struct and intialized in the user space part
>of my module), but when I update it in kernel space it gets the wrong
>value. When for example incrementing the value by 1 every time the
>rule matches, the values are all messed up (but never larger than the
>number of matches/increments). When creating and initializing the
>value in the module, it behaves correctly.

See the "Writing Netfilter Modules" e-book rev March 26 2009,
section 4.6 "SMP problems". I just updated this because xt_quota,
which serves as an example, was changed recently to account for
the RCU patches, and so the explanation needed to change too.

Re: Updating variable shared between user and kernel space

[Kristian Evensen]

> See the "Writing Netfilter Modules" e-book rev March 26 2009,
> section 4.6 "SMP problems". I just updated this because xt_quota,
> which serves as an example, was changed recently to account for
> the RCU patches, and so the explanation needed to change too.

Thank you, that was the chapter I was using a reference. Kmalloc here I come.

-Kristian