Rotate a ray to camera direction, in a CG shader

I've got a shader which draws a ray-casted sphere. Here's a snippet:

 // A basic ray.
 struct ray {
     float3 o; // Point of origin
     fixed3 d; // Direction
 };
 
 // Raytrace sphere
 float3 calcPixelColor(float2 pixel, float2 resolution) {
     const float fov = 2.5;
     
     // Pixel in screen space
     float2 p = (-resolution + 2.0 * pixel) / resolution.y;
     
     // Create a ray
     ray r;
     r.o = _WorldSpaceCameraPos;
     
     float3 ww = float3(0.0, 0.0, 1.0);
     fixed3 uu = normalize(cross(fixed3(0.0, -1.0, 0.0), ww));
     fixed3 vv = normalize(cross(ww, uu));
     
     // Calculate ray direction
     fixed3 er = normalize(float3(p.xy, fov));
     r.d = normalize(er.x * uu + er.y * vv + er.z * ww);
     
     // The color of a sphere, or background
     float3 col = intersectSphere(r);
     
     return col;
 }

It emulates a camera pointed into the scene: [0, 0, 1].

But it gives me kind of a weird result.

If I set up my virtual camera in the shader at [0,0,-5], looking down the z-axis like this:

 r.o = float3(0, 0, -5);
 fixed3 ww = fixed3(0, 0, 1);

My shader renders a proper sphere:

And if I feed in the Unity camera's position, like so:

 r.o = _WorldSpaceCameraPos;

Everything is fine. But I can't seem to apply the camera's orientation to the direction vector ww. I try this:

 fixed3 ww = normalize(float3(0, 0, 1) * UNITY_MATRIX_IT_MV);

But all I get is a blank blue background unless I move the camera really close to [0,0,0]. Then I see part of the sphere, but if I rotate the camera, nothing happens.