# fingerprintd

Fingerprint daemon for the Fairphone 6 (`milos`, SM7635) on mainline Linux.

## Why a daemon

The sensor is a FocalTech FT9391 on a TrustZone-owned SPI bus. `spi@a88000` is
`disabled` in both the mainline and the stock Android device tree, and the pads
are XPU-protected — touching them from the normal world is an instant SError
reboot. Every pixel the sensor produces stays inside the TEE: capture,
preprocessing, the classifier, enrolment and matching all run in the `focal64`
trustlet, which reports a matched finger id and nothing else. A libfprint-style
driver cannot exist on this device.

So the normal world's job is narrower than usual, and none of it is per-request
work:

* **Power the sensor.** Rail on gpio29, reset on gpio74, interrupt on gpio75 —
  the same division of labour the downstream driver uses. One sensor reset buys
  exactly one trustlet init, so whatever powers the sensor must also hold the
  session open.
* **Be QTEE's filesystem.** QTEE cannot reach storage. When the trustlet saves
  or loads a template it calls back into the normal world through the `gpfile`
  (0x7000) and RPMB (0x2000) listeners, and expects them served. QTEE does the
  crypto and the anti-rollback; this side moves opaque bytes and performs the
  authenticated RPMB transactions against the UFS device.
* **Speak a biometrics API.** The daemon owns `net.reactivated.Fprint`, so
  `pam_fprintd`, the Plasma fingerprint KCM and `fprintd-enroll(1)` work
  against it unmodified.

A listener registration is held for as long as the process lives and QTEE's
listener table is global to the boot, so this has to be one long-lived process
rather than a tool spawned per request.

## Layout

```
interfaces/     Fingerprintd{,-Sfs}.cppm   the core: pure C++ modules
implementations/main.cpp                   the daemon shell
tests/                                     one suite per core module
```

`fingerprintd-core` is a static library with no GLib, no libqcomtee and no
system headers. Everything in it is a wire format or a state machine that was
recovered by reverse-engineering, so all of it is pinned by tests that run on a
dev box with no phone, no TEE and no sensor. The daemon shell holds everything
that touches hardware.

## Build

```sh
crafter-build              # bin/fingerprintd-<target>-<march>/fingerprintd
crafter-build test         # the unit suites
```

Cross-compiling for the phone:

```sh
packaging/make-sysroot.sh                        # once; no root, no qemu, no device
crafter-build -- --target=aarch64-alpine-linux-musl \
  --sysroot=~/.cache/fingerprintd/sysroot-aarch64-alpine \
  --march=armv8-a --mtune=generic
crafter-build test --target=aarch64-alpine-linux-musl --sysroot=... \
  --march=armv8-a --mtune=generic                # runs the suites under qemu-aarch64
```

The result links dynamically against the phone's own musl and libc++
(`libc++`, `libc++abi`, `libunwind`, `libgcc_s`, all already present on pmOS).
The research harness this replaces had to be built `-static`, but only because
it was built with the host's glibc toolchain — that constraint does not apply
to a real Alpine sysroot.

Verified on the device: all five suites pass cross-built and run **on the phone
itself**, not only under emulation.

## Status

**It works end to end through fprintd's own clients.** On the Fairphone 6, as a
systemd unit owning `net.reactivated.Fprint`:

```
fprintd-enroll -f right-index-finger user   ten stages, enroll-completed
fprintd-list user                            - #0: right-index-finger
fprintd-verify user   (wrong finger)         verify-no-match, on the first press
fprintd-verify user   (enrolled finger)      verify-match,    on the first press
```

Nothing in fprintd was modified; the daemon speaks its interface.

| module | what it holds |
|---|---|
| `:Sfs` | the gpfile frame — the read/write offset split, the `O_TRUNC` guard, root mapping, path-traversal rejection |
| `:Rpmb` | request/reply framing, the bytes-transferred out-parameter, JEDEC result codes, chunking, the one-time-programmable key guard |
| `:Ta` | command surface, the 740-byte event context, capture flags, save masks, enrol/auth payloads, the error table, the verdict rule |
| `:Engine` | baseline calibration, touch edges, enrolment progress, the accounting |
| `:Store` | the finger name map |
| `:Tee` | QTEE service/op numbers, the listener table, the 13-byte CBOR credentials blob |
| `:Sensor` | the pins, the timings, and the XPU guard |

Every constant that was recovered by reverse-engineering carries where it came
from, and the tests are written to fail if it is undone rather than to restate
it. Several replay real captures off the phone.

Two policy decisions live in the daemon and both were forced by measurement.
Verification is judged **per press**: any matching frame wins, only rejections
is no-match — both correct presses in the acceptance run had rejected frames
before the one that matched, so a first-frame rule would have failed them. And
the shipped config sets `max_authentication_rescan_times` to 0, because at the
stock budget a wrong finger never yields a terminal frame and a PAM client
waits forever for the `verify-no-match` it needs.

**Known about the loop, from real use.** A verify frame is four QTEE round
trips (~200 ms idle); a `REPORT_EVENT` that runs the matcher is ~300 ms, and
the rising edge pays it twice because events 5 and 7 both reach the matcher.
The first frame of a press is the finger landing and usually rejects; matches
land at frame 3, 5, 8 — so a quick tap is often a miss. Two attempts to fix
that (drop event 5; recapture after 50 ms) went in together and produced zero
matches; both are reverted and recorded. gpio75 is a ~1 ms pulse, not a
level — an IRQ-driven idle needs GPIO edge events. Changes to this loop are
made one variable at a time, on a measured baseline.

**Not done:** packaging (`provides="fprintd=…"` so this replaces the fprintd
daemon package while `fprintd-pam` stays), the shipped storage policy, polkit
(a caller-uid rule stands in), trustlet-side template removal (deletes drop the
name only), and the kernel config change — `CONFIG_QCOMTEE`, which selects the
SHM bridge — that gates any public image. A claim held by a client that leaves
the bus is dropped.

## Runtime dependencies, not carried here

The `focal64` trustlet is proprietary and is **not** in this repo. It is
extracted from the device's own stock Android partition on first boot by the
`fp6-vendor-blobs` mechanism, the same way the audio firmware is.