- C++ 88.7%
- Shell 11.3%
| Filename | Latest commit message | Latest commit date |
|---|---|---|
Jorijn van der Graaf
e2033ed26d
Template learning, the half of the algorithm this daemon never ran. Every match rate measured against this device -- 1/10, 2/10, 3/10, 7/10 -- was measured against a day-zero template that no stock user lives with, because a stock template is rewritten on every successful press and ours never moved a byte. HarvestTemplate mirrors stock's post-match loop: while the finger is still on the sensor, capture and fold, sending no event so the matcher cannot re-run and revise a verdict the client already has. A quick tap pays almost nothing, since the finger is gone by the time a verdict lands and the first capture reads the idle floor; a held press contributes the frames it was actually held for. That last property is why this subsumes the enrolment-tuning thread. The working hypothesis after the strict-threshold and 30-sample failures was that enrolment conditions have to resemble verification conditions. Learning is exactly that, done by the algorithm from real unlock presses instead of by coaching a user into positions they never use. FlushTemplate issues the SAVE_DATA, and the worker calls it after the verdict is posted rather than before. Persisting a learned template is roughly 350 ms of gpfile and RPMB traffic and it does not belong on an unlock path; stock defers it the same way, with a lazy-updater timer. --learn=0 turns the whole thing off, which is the only way the comparison is single-variable: learning is cumulative, so an A/B needs one binary and one template lineage rather than two builds. --learn-frames bounds what one press may contribute, because the template body moves as a single gpfile op against a 516084-byte listener buffer and a 30-sample body already measured 386402. --probe-learn sends one UPDATE_TEMPLATE with no finger and then ENUMERATEs to prove the trustlet survived it, which is the part a bare return code cannot say. |
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| implementations | ||
| interfaces | ||
| packaging | ||
| tests | ||
| .gitignore | ||
| LICENSE | ||
| lint-rules.h | ||
| project.cpp | ||
| README.md | ||
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, sopam_fprintd, the Plasma fingerprint KCM andfprintd-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.