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

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

Cross-compiling for the phone:

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.