Controlling how blocks face are configured in blockstates and model [JSON] files,
In the Eclipse "Package Explorer" view, look for Referenced Libraries -> Forge Src 1.12 ... -> assets.minecraft.blockstates --> 'birch_log.json' for an Example (Birch Logs)
The models listed can be found in assets.minecraft.models.block
The base Class for logs is in net.minecraft.block.BlockLog . which you can probably use natively or extend to get the effect you are after.
Basic concepts are covered here: https://mcforge.readthedocs.io/en/latest/blocks/states/