Camera Up vector in vertex shader.

Hello!

I'm working on a billboard/sprite vertex shader (for snow/rain), and I wonder if there is a way I can access the camera's upvector from within the vertex shader.
I pass the vector using Shader.SetGlobalVector("_Upvec", cam.transform.up); now and it works fine, but I suppose if this vector is already available from built in parameters, or through view matrices somehow?
My hlsl knowledge is a bit rusty, so any input appreciated

Thanks!

Edit: Ok, it seems like I found the right vector. This works:

upvec = UNITY_MATRIX_IT_MV[1].xyz;

Kjetil