Modify user space from kernel.

[Conrad Meyer]

On Fri, Jul 29, 2016 at 6:11 AM, Adam Starak <starak.adam at gmail.com> wrote:
> Hello!
>
> My name is Adam. I participate in Google Summer of Code this year. I came
> up with a big problem, which doesn't allow me to go further in my project.
>
> I made a new syscall, which is going to retrieve sysctl data and put it
> inside the nvlist. And here my problem is. I need to move somehow this data
> (packed nvlist) into the user space. Is there any chance to pass data from
> kernel to user space without knowing the size of it?
>
> Right now, the implementation of __sysctl() function requests void pointer
> and size in order to get data. If allocated memory is too low, it returns
> ENOMEM and you need to realloc the data. I wanted to avoid this situation.

Hey Adam,

That is the usual way to do it.  Just curious — why do you want to
avoid that situation?

Your other option might be to put an upper limit on the size of the
result, and pass a buffer of that size in from userspace.  But then
you are artificially limited to some arbitrary size and must
preallocate a large buffer even in the case that the output is small.

Best,
Conrad