ld: add public unlock-CDB seam for external consumers

Expose is_unlock_read_buffer() and UNLOCK_MARKER so consumers (bdemu)
can recognise and answer the LibreDrive unlock READ_BUFFER handshake
without open-coding the variant CDB shapes or the verification marker.
These handshake internals must live only in freemkv-unlock-ld.

ld/src/cdb.rs

@@ -0,0 +1,57 @@
+//! Public unlock-CDB seam.
+//!
+//! These items expose the *minimum* an external consumer (e.g. the bdemu
+//! drive emulator) needs to recognise and answer the LibreDrive unlock
+//! READ_BUFFER handshake, without that consumer open-coding the
+//! handshake internals. The concrete CDB shapes and the verification
+//! marker are unlock-handshake details and must live ONLY in this crate.
+
+/// The 4-byte marker the unlock READ_BUFFER response carries at bytes
+/// `[12..16]`. A consumer answering the handshake writes this at that
+/// offset; a verifier checks for it there. It is the universal
+/// "this is a real unlock reply" tag, independent of the per-drive
+/// signature at `[0..4]`.
+pub const UNLOCK_MARKER: &[u8] = b"MMkv";
+
+/// Returns `true` when a READ_BUFFER (`0x3C`) CDB with the given mode
+/// (`cdb[1] & 0x1F`) and buffer id (`cdb[2]`) is an unlock-handshake
+/// read — i.e. one of the LibreDrive unlock variants.
+///
+/// This is the single source of truth for the unlock READ_BUFFER CDB
+/// shapes; consumers must call it rather than hardcoding the mode /
+/// buffer-id pairs.
+pub fn is_unlock_read_buffer(mode: u8, buf_id: u8) -> bool {
+    matches!((mode, buf_id), (1, 0x44) | (2, 0x77))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn marker_is_four_bytes() {
+        assert_eq!(UNLOCK_MARKER, b"MMkv");
+        assert_eq!(UNLOCK_MARKER.len(), 4);
+    }
+
+    #[test]
+    fn unlock_variants_match() {
+        // Variant A and variant B.
+        assert!(is_unlock_read_buffer(1, 0x44));
+        assert!(is_unlock_read_buffer(2, 0x77));
+    }
+
+    #[test]
+    fn non_unlock_cdbs_do_not_match() {
+        // Right mode, wrong buffer id.
+        assert!(!is_unlock_read_buffer(1, 0x77));
+        assert!(!is_unlock_read_buffer(2, 0x44));
+        // Wrong mode, right buffer id.
+        assert!(!is_unlock_read_buffer(2, 0x44));
+        assert!(!is_unlock_read_buffer(0, 0x44));
+        assert!(!is_unlock_read_buffer(3, 0x77));
+        // Ordinary data reads.
+        assert!(!is_unlock_read_buffer(2, 0x00));
+        assert!(!is_unlock_read_buffer(0, 0x00));
+    }
+}

ld/src/lib.rs

@@ -20,6 +20,7 @@
 // `crate::error::*` / `crate::scsi::*` paths.
 pub(crate) use libfreemkv::{error, scsi};
 
+pub mod cdb;
 pub mod profile;
 
 mod platform;