USERSPACE I O(9)

💖 Userspace I/O 💖

okay so bestie, welcome to Userspace I/O(9) — that's section 9 of the Kernel API Manual, the kernel-dev one, not the "type man ls" one. Buckle up, this one's a banger 🎀

Userspace I/O -------------

✨ the actual vibe (description) ✨

So the whole point here is defining a shared-memory interface for LIO to pass SCSI commands and data to userspace for processing. Why? So that backends that are simply too complex for in-kernel support can actually exist. It's giving "the kernel said 'not my job' and outsourced it to you" energy, and honestly? slay 💅

It uses the UIO framework to do a whole lot of the device-creation and introspection work for us. free labor, we love to see it 🫶

Check the .h file for how the ring is laid out — that's where the deets live. But heads up: while the command ring is defined, the particulars of the data area are not. Offset values in the command entry point to other locations internal to the mmap-ed area. There's separate space outside the command ring for data buffers. And why is that iconic? Because it leaves maximum flexibility for moving buffer allocations around, or even page flipping or other allocation techniques, without altering the command ring layout. future-proof queen 👑

🚨 SECURITY (read this one, fr fr) 🚨

SECURITY: the user process must be assumed to be malicious. Like, no benefit of the doubt, no "but they seem nice." 😭

There's no way to prevent it breaking the command ring protocol if it wants to — that ship has sailed. But in order to prevent other issues, we must only ever read data from the shared memory area, not offsets or sizes. not the offsets, not the sizes, just the data. periodt.

This applies to command ring entries as well as the mailbox. Extra code needed for this may have a 'UAM' comment — so if you see one of those, that's what it's for, iykyk ⭐

👀 SEE ALSO 👀

• Kernel file ./drivers/target/target_core_user.c
• queue_cmd_ring(9)
• queue_tmr_ring(9)

and that's the whole page, bestie 🌸 this lil icon comes from ./drivers/target, dated October 2026, and it's your girl Userspace I/O(9) signing off — go read that .h file, mwah 💜🦋